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Research Group of Prof. Yongmin Liu
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Publication Statistics (from Google Scholar)
Total citations: >11000
H-index: 51

Publication Statistics (from Researcher ID)
Total citations: > 8600
H-index: 45
Selected Journal Publications
– Y. H. Xu, L Li, H Jeong, S. Kim, I. Kim, J. Rho, and Y. M. Liu, "Subwavelength control of light transport at the exceptional point by non-Hermitian metagratings," Science Advances 9, eadf3510 (2023) [PDF]
– B. Xiong, Y. Liu, Y. H. Xu, L. Deng, C. W. Chen, J. N. Wang, R. W. Peng, Y. Lai, Y. M. Liu, and M. Wang, "Breaking the limitation of polarization multiplexing in optical metasurfaces with engineered noise", Science 379, 294 (2023) [PDF]
– W. Ma, Y. H. Xu, B. Xiong, L. Deng, R. W. Peng, M. Wang, and Y. M. Liu, "Pushing the Limits of Functionality-Multiplexing Capability in Metasurface Design Based on Statistical Machine Learning", Advanced Materials 34, 2110022 (2022) [PDF]
– R.C. Jin, Y. H. Xu, Z. G. Dong and Y. M. Liu, "Optical Pulling Forces Enabled by Hyperbolic Metamaterials", Nano Letters 21, 10431 (2021) [PDF]
– L. Zhang, W. Lee, X. H. Li, Y. H. Jiang, N. X. Fang, G. H. Dai, and Y. M. Liu, "3D direct printing of mechanical and biocompatible hydrogel meta-structures", Bioactive Materials 10, 48 (2021) [PDF]
– F. Z. Shu, J. N. Wang, R. W. Peng, B. Xiong, R. H. Fan, Y. J. Gao, Y. M. Liu, D. X. Qi, and M. Wang, "Electrically Driven Tunable Broadband Polarization States via Active Metasurfaces Based on Joule-Heat-Induced Phase Transition of Vanadium Dioxide", Laser & Photonics Reviews 33, 2100155 (2021) [PDF]
– B. Xiong, Y. H. Xu, J. N. Wang, L. Li, L. Deng, F. Cheng, R. W. Peng, M. Wang, and and Y. M. Liu, "Realizing Colorful Holographic Mimicry by Metasurfaces", Advanced Materials 33, 2005864 (2021) [PDF]
– W. Ma, Z. C. Liu, Z. A. Kudyshev, A. Boltasseva, W. S. Cai and Y. M. Liu, "Deep learning for the design of photonic structures", Nature Photonics 15, 77 (2021) [PDF]
– L. Li, K. Yao, Z. J. Wang and Y. M. Liu, "Harnessing Evanescent Waves by Bianisotropic Metasurfaces", Laser & Photonics Reviews 14, 1900244 (2020) [PDF]
– F. Cheng, C. T. Wang, Z. X. Su, X. J. Wang, Z. Q. Cai, N. X. Sun, and Y. M. Liu, "All-Optical Manipulation of Magnetization in Ferromagnetic Thin Films Enhanced by Plasmonic Resonances", Nano Letters, online publication (2020) [PDF]
– W. Ma, F. Cheng, and Y. M. Liu, "Probabilistic Representation and Inverse Design of Metamaterials Based on a Deep Generative Model with Semi-Supervised Learning Strategy", Advanced Materials 31, 1901111 (2019) [PDF]
– W. Ma, F. Cheng, and Y. M. Liu, "Deep-Learning-Enabled On-Demand Design of Chiral Metamaterials", ACS Nano 12, 6326 (2018) [PDF]
– T. T. Zhu, Y. Y. Cao, L. Wang, Z. Q. Nie, T. Cao, F. K. Sun, Z. H. Jiang, M. Nieto-Vesperinas, Y. M. Liu, C. W. Qiu, and W. Q. Ding, "Self-Induced Backaction Optical Pulling Force", Physical Review Letters 120, 123901 (2018) [PDF]
– L. Kang, S. P. Rodrigues, M. Taghinejad, S. F. Lan, K. Lee, Y. M. Liu, D. H. Werner, A. Urbas and W. S. Cai, "Preserving Spin States upon Reflection: Linear and Nonlinear Responses of a Chiral Meta-Mirror", Nano Letters, 17, 7102 (2017) [PDF]
– W. Wang, L. Wang, R. Xue, H. Chen, R. Guo, Y. M. Liu, and J. Chen, "Unidirectional Excitation of Radiative-Loss-Free Surface Plasmon Polaritons in Parity-Time Symmetric Systems", Physical Review Letters 119, 077401 (2017) [PDF]
– Z. J. Wang, L. Q. Jing, K. Yao, Y. H. Yang, B. Zheng, C. M. Soukoulis, H. S. Chen, and Y. M. Liu, "Origami-Based Reconfigurable Metamaterials for Tunable Chirality", Advanced Materials 29, 1700472 (2017) [PDF]
– Z. J. Wang, K. Yao, M. Chen, H. S. Chen and Y. M. Liu, "Manipulating Smith-Purcell Emission with Babinet Metasurfaces", Physical Review Letters 117, 157401 (2016) [PDF]
– W. L. Gao, F. Z. Fang, Y. M. Liu, and S. Zhang, "Chiral Surface Waves Supported by Biaxial Hyperbolic Metamaterials", Light: Science and Applications 4, e328 (2015) [PDF]
– H. Cang, Y. M. Liu, Y. Wang, X. Yin, and X. Zhang, "Giant suppression of photobleaching for single molecule detection via the Purcell effect", Nano Letters 13, 5945 (2013) [PDF]
– C. L. Zhao*, Y. M. Liu*, Y. H. Zhao, N. Fang, and T. J. Huang, "A reconfigurable plasmofluidic lens", Nature Communications 4:2350 (2013) [PDF]
– Y. M. Liu*, S. Palomba*, Y. Park, T. Zentgraf, X. B. Yin and X. Zhang, "Compact magnetic antennas for directional excitation of surface plasmons", Nano Letters 12, 4853 (2012) [PDF]
– Y. M. Liu and X. Zhang, "Recent advances in transformation optics" (invited review paper), Nanoscale 4, 5277 (2012) [PDF]
– S. C. Kehr*, Y. M. Liu*, L. W. Martin, P. Yu, M. Gajek, S.-Y. Yang, C.-H. Yang, M. T. Wenzel, R. Jacob, H.-G. von Ribbeck, M. Helm, X. Zhang, L. M. Eng and R. Ramesh, "Near-field examination of perovskite-based superlenses and superlens-enhanced probe-object coupling", Nature Communications 2, 249 (2011) [PDF]
– T. Zentgraf*, Y. M. Liu*, M H. Mikkelsen*, J. Valentine and X. Zhang, "Plasmonic Luneburg and Eaton lenses", Nature Nanotechnology 6, 151 (2011) [PDF]
– Y. M. Liu and X. Zhang, "Metamaterials: a new frontier of science and technology" (invited review paper), Chemical Society Reviews 40, 2494 (2011) [PDF]
– H. Cang, A. Labno, C. G. Lu, X. B. Yin, M. Liu, C. Glandden, Y. M. Liu and X. Zhang, "Probing the electromagnetic field of a 15-nanometre hotspot by single molecule imaging", Nature 469, 385 (2011) [PDF]
– Y. M. Liu, T. Zentgraf, G. Bartal and X. Zhang, "Transformational plasmon optics", Nano Letters 10, 1991(2010) [PDF]. See News and Views by Nature Nanotechnlogy.
– M. Liu, T. Zentgraf, Y. M. Liu, G. Bartal and X. Zhang, "Light-driven nanoscale plasmonic motors", Nature Nanotechnology 5, 570 (2010) [PDF]. See Research Hilight by Nature, and News and Views by Nature Nanotechnlogy.
– J. Yao*, Z. W. Liu*, Y. M. Liu*, Y. Wang, C. Sun, G. Bartal, A. Stacy and X. Zhang, "Optical negative refraction in bulk metamaterials of nanowires", Science 321, 930 (2008) [PDF]. Ranked among Top 10 Scientific Discoveries and 50 Best Inventions of 2008 by TIME magazine, and Top 100 Stories of 2008 by DISCOVER magazine.
– Y. M. Liu, G. Bartal, D. A. Genov and X. Zhang, "Subwavelength discrete solitons in nonlinear metamaterials", Physical Review Letters 99, 153901 (2007) [PDF]. See Research Highligh by Nature Photonics.

Full List of Journal Publications
[121] L. Zhang, H. F. Du, X. Sun, F. Cheng, W. H. Lee, J. H. Li, G. H. Dai, N. X. Fang and Y. M. Liu, "3D Printing of Interpenetrating Network Flexible Hydrogels with Enhancement of Adhesiveness,"ACS Applied Materials & Interfaces, 2300236 (2023) [PDF]
[120] Z. Liao, X. Peng, L. L. Liu, Y. H. Xu, K. D. Xu, B. C. Pan, G. Q. Luo and Y. M. Liu, "Microwave Plasmonic Exceptional Points for Enhanced Sensing," Laser & Photonics Reviews, 2300236 (2023) [PDF]
[119] L. Deng, R. C. Jin, Y. H. Xu, and Y. M. Liu, "Structured Light Generation Using Angle-Multiplexed Metasurfaces,"Advanced Optical Materials 11, 2300299 (2023) [PDF]

Due to the intrinsic limitation of a planar achiral structure, the maximum number of independent, complex elements in one Jones matrix is three, giving rise to up to three-channel amplitude and phase control. Here, we propose to add more Jones matrices corresponding to different angles of incidence are proposed, so that the degrees of freedom in the amplitude and phase control can be further increased. The supercell of the designed metasurfaces consists of three dielectric nanoblocks with predefined rotation angles and displacements in the 2D space, which can be inversely determined with the help of the genetic algorithm. Empowered by the ability to realize four- or even eight-channel amplitude and phase control, the generation of multiple structured lightis numerically demonstrated. Such novel designs are expected to benefit the development of modern optical applications, including optical communications, quantum information, and signal encryption.

[118] Y. H. Xu, L Li, H Jeong, S. Kim, I. Kim, J. Rho, and Y. M. Liu, "Subwavelength control of light transport at the exceptional point by non-Hermitian metagratings," Science Advances 9, eadf3510 (2023) [PDF]

Interfacing non-Hermitian photonics and nanoplasmonics, here, we demonstrate unidirectional excitation and reflection of surface plasmon polaritons by elaborately designing the permittivity profile of non-Hermitian metagratings, in which the eigenstates of the system can coalesce at an exceptional point. Continuous tuning of the excitation or reflection ratios is also possible through altering the geometry of the metagrating. The controllable directionality and robust performance are attributed to the phase transition near the exceptional point, which is fully confirmed by the theoretic calculation, numerical simulation, and experimental characterization. Our work pushes non-Hermitian photonics to the nanoscale regime and paves the way toward high-performance plasmonic devices with superior controllability, performance, and robustness by using the topological effect associated with non-Hermitian systems.

[117] F. Cheng, C. T. Wang, Y. H. Xu, W. Ma, and Y. M. Liu, "Multiphysics Modeling of Plasmon-Enhanced All-Optical Helicity-Dependent Switching," ACS Photonics 10, 1259 (2023) [PDF]

In this work, we propose a multiphysics approach to simulate all-optical helicity-dependent switching induced by the local hot spots of plasmonic nanostructures. Due to the plasmonic resonance of an array of gold nanodisks, strong electromagnetic fields are generated within the magnetic recording media underneath the gold nanodisks. We construct a multiphysics framework
considering the opto-magnetic and opto-thermal effects, and then model the magnetization switching using the Monte Carlo method. Our approach bridges the gap between plasmonic nanostructure design and magnetization switching modeling, allowing for the simulation of helicity-dependent, nanoscale magnetization switching in the presence of localized surface plasmons.

[116] Y. H. Xu, B. Xiong, W. M, and Y. M. Liu, "Software-defined nanophotonic devices and systems empowered by machine learning," Progress in Quantum Electronics 89, 100469 (2023) [PDF]
[115] B. Xiong, Y. Liu, Y. H. Xu, L. Deng, C. W. Chen, J. N. Wang, R. W. Peng, Y. Lai, Y. M. Liu, and M. Wang, "Breaking the limitation of polarization multiplexing in optical metasurfaces with engineered noise", Science 379, 294 (2023) [PDF]

Noise is usually undesired yet inevitable in science and engineering. However, by introducing the engineered noise to the precise solution of Jones matrix elements, we break the fundamental limit of polarization multiplexing capacity of metasurfaces that roots from the dimension constraints of the Jones matrix. We experimentally demonstrate up to 11 independent holographic images using a single metasurface illuminated by visible light with different polarizations. To the best of our knowledge, it is the highest capacity reported for polarization multiplexing. Combining the position multiplexing scheme, the metasurface can generate 36 distinct images, forming a holographic keyboard pattern. This discovery implies a new paradigm for high-capacity optical display, information encryption, and data storage.

[114] R. C. Jin, L. Deng, L. L. Tang, Y. Cao, Y. M. Liu, and Z. G. Dong, "Decoupled Phase Modulation for Circularly Polarized Light via Chiral Metasurfaces", ACS Photonics 10, 155 (2023) [PDF]
[113] A. R. Will-Cole, C. T. Wang, N. Bhattacharjee, Y. M. Liu, and N. X. Sun, "Electric field tuning of ultrafast demagnetization in a magnetoelectric heterostructure", Physical Review B 106, 174401 (2022) [PDF]
[112] L. Deng, Y. H. Xu, and Y. M. Liu, "Hybrid inverse design of photonic structures by combining optimization methods with neural networks", Photonics and Nanostructures - Fundamentals and Applications 52, 101073 (2022) [PDF]
[111] L. Deng, Y. H. Xu, R. C. Jin, Z. Q. Cai, and Y. M. Liu, "On-Demand Mode Conversion and Wavefront Shaping via On-Chip Metasurfaces", Advanced Optical Materials 10, 2200910 (2022) [PDF]

In this work, mode conversion and wavefront shaping by integrating a metallic metasurface on top of a planar waveguide are proposed and demonstrated. The metasurface consists of C-shaped nanoantennas. By controlling the orientation of each nanoantenna, mode conversion and focusing effect for the cross-polarized electric fields inside the waveguide are achieved. The design and simulation results of 16 scenarios of wideband transversemagnetic to transverse-electric mode converters with the mode purity up to 98%, and on-chip lenses at the wavelength of 1550 nm are reported. It is worth noting that the dimension of the devices along the propagation direction is only 9.6 μm. This work manifests the potential application of mode division multiplexing systems and on-chip optical interconnections based on metasurfaces.

[110] Z. Liao, Y. Che, L. L. Liu, B. C. Pan, B. G. Cai, J. N. Zhou, G. Q. Luo, and Y. M. Liu, "Reconfigurable Vector Vortex Beams Using Spoof Surface Plasmon Ring Resonators", IEEE Transactions on Antennas and Propagation (2022) [PDF]
[109] W. Ma, Y. H. Xu, B. Xiong, L. Deng, R. W. Peng, M. Wang, and Y. M. Liu, "Pushing the Limits of Functionality-Multiplexing Capability in Metasurface Design Based on Statistical Machine Learning", Advanced Materials 34, 2110022 (2022) [PDF]

In this work, we propose to embed machine-learning models in both gradient based and nongradient optimization loops for the automatic implementation of multifunctional metasurfaces. Fundamentally different from the traditional two-step approach that separates phase retrieval and meta-atom structural design, the proposed end-to-end framework facilitates full exploitation of the prescribed design space and pushes the multifunctional design capacity to its physical limit. With a single-layer structure that can be readily fabricated, metasurface focusing lenses and holograms are experimentally demonstrated in the near-infrared region. They show up to eight controllable responses subjected to different combinations of working frequencies and linear polarization states, which are unachievable by the conventional physics-guided approaches.

[108] Z. Q. Cai and Y. M. Liu, "Near-Infrared Reflection Modulation Through Electrical Tuning of Hybrid Graphene Metasurfaces", Advanced Optical Materials 10, 2102135 (2022) [PDF]

Graphene has recently attracted intense interest for reconfigurable metasurfaces. So far, the working wavelength of graphene-based or hybrid graphene metasurfaces has been limited in the mid-infrared and terahertz spectra. In this paper, by combining graphene with Au nanostructures, the authors demonstrate a near-infrared tunable metasurface with decent modulation efficiency, weak dependence on graphene's carrier mobility, and small gate voltages, attributing to the unique interband transition of graphene. The experimental results agree well with numerical simulations. It is also shown that by properly designing the structural parameters of Au nanostructures, the hybrid graphene metasurface can be tunable in both near-infrared and mid-infrared regions.

[107] F. Farhangdoust, F. Cheng, W. T. Liang, Y. M. Liu, and M. Wanunu, "Rapid Identification of DNA Fragments through Direct Sequencing with Electro-Optical Zero-Mode Waveguides", Advanced Materials 34, 2108479 (2022) [PDF]
[106] R.C. Jin, Y. H. Xu, Z. G. Dong and Y. M. Liu, "Optical Pulling Forces Enabled by Hyperbolic Metamaterials", Nano Letters (2021) [PDF]

We propose a novel approach to generating optical pulling forces on a gold nanowire, which are placed inside or above a hyperbolic metamaterial and subjected to plane wave illumination. Two mechanisms are found to induce the optical pulling force, including the concave isofrequency contour of the hyperbolic metamaterial and the excitation of directional surface plasmon polaritons. We systematically study the optical forces under various conditions, including the wavelength, the angle of incidence of light, and the nanowire radius. It is shown that the optical pulling force enabled by hyperbolic metamaterials is broadband and insensitive to the angle of incidence. The mechanisms and results reported here open a new avenue to manipulating nanoscale objects.

[105] L. Zhang, W. Lee, X. H. Li, Y. H. Jiang, N. X. Fang, G. H. Dai, and Y. M. Liu, "3D direct printing of mechanical and biocompatible hydrogel meta-structures", Bioactive Materials 10, 48 (2021) [PDF]

In this work, we report the implementation of hydrogel meta-structures using DIW at room temperature, which seamlessly integrate large specific surface areas, interconnected porous characteristics, mechanical toughness, biocompatibility, and water absorption and retention capabilities. Robust but hydrophobic polymers and weakly crosslinked nature-origin hydrogels form a balance in the self-supporting ink, allowing us to directly print complex meta-structures without sacrificial materials and heating extrusion. Mechanically, the mixed bending or stretching of symmetrical re-entrant cellular lattices and the unique curvature patterns are combined to provide little lateral expansion and large compressive energy absorbance when external forces are applied on the printed meta-structures. We anticipate that the reported 3D meta-structured hydrogel would offer a new strategy to develop functional biomaterials for tissue engineering applications in the future.

[104] F. Z. Shu, J. N. Wang, R. W. Peng, B. Xiong, R. H. Fan, Y. J. Gao, Y. M. Liu, D. X. Qi, and M. Wang, "Electrically Driven Tunable Broadband Polarization States via Active Metasurfaces Based on Joule-Heat-Induced Phase Transition of Vanadium Dioxide", Laser & Photonics Reviews 33, 2100155 (2021) [PDF]

Here, the electrically tunable broadband polarization states by combining phase-change material (vanadium dioxide) and dispersion-free metasurface are demonstrated for the first time. The polarization states are modulated through the electrically driven, Joule-heat-induced phase transition of vanadium dioxide, where the output polarization state can be continuously tuned from horizontal one to vertical one, or from circular polarization to linear polarization. With accurate on-chip control of the phase transition, continuous and reversible modulation of polarization is verified in a scanning display. Moreover, a proof-of-concept demonstration for dynamically independent control of multiple polarization display is carried out. Different images are produced by applying electrical currents in N separate channels to generate a dynamic multiplexing polarization display with 2N encoding states.

[103] G. X. Su, H. Su, M. H. Lu, Z. F. Qin, X. P. Shen, J. P. Ding, F. X. Liu, M. H. Lu, P. Zhan, and Y. M. Liu, "Demonstration of microwave plasmonic-like vortices with tunable topological charges by a single metaparticle", Applied Physics Letters 118, 241106 (2021) [PDF]
[102] B. Xiong, Y. H. Xu, J. N. Wang, L. Li, L. Deng, F. Cheng, R. W. Peng, M. Wang, and and Y. M. Liu, "Realizing Colorful Holographic Mimicry by Metasurfaces", Advanced Materials 33, 2005864 (2021) [PDF]

Mimicry is a biological camouflage phenomenon whereby an organism can change its shape and color to resemble another object. Herein, the idea of biological mimicry and rich degrees of freedom in metasurface designs are combined to realize holographic mimicry devices. A general mathematical method, called phase matrix transformation, to accomplish the holographic mimicry process is proposed. Based on this method, a dynamic metasurface hologram is designed, which shows an image of a "bird" in the air, and a distinct image of a "fish" when the environment is changed to oil. Furthermore, to make the mimicry behavior more generic, holographic mimicry operating at dual wavelengths is also designed and experimentally demonstrated. The work potentially opens a new research paradigm interfacing bionics with nanophotonics, which may produce novel applications for optical information encryption, virtual/augmented reality (VR/AR), and military camouflage systems.

[101] Y. H. Xu, X. Z. Zhang, Y. Fu, and Y. M. Liu,"Interfacing photonics with artificial intelligence: an innovative design strategy for photonic structures and devices based on artificial neural networks"(invited review), Photonics Researchs 9, B135 (2021) [PDF]
[100] H. Gao*, Y. H. Xu*, K. Yao, and Y. M. Liu,"Self-Assembly of Silica−Gold Core−Shell Microparticles by Electric Fields Toward Dynamically Tunable Metamaterials", ACS Applied Materials & Interfaces 13, 14417 (2021) [PDF]
[99] L. Q. Jing, X. Lin, Z. J. Wang, I. Kaminer, H. Hu, E. P. Li, Y. M. Liu, M. Chen, B. L. Zhang, and H. S. Chen, "Polarization Shaping of Free-Electron Radiation by Gradient Bianisotropic Metasurfaces", Laser & Photonics Reviews 15, 2000426 (2021) [PDF]
[98] C. T. Wang and Y. M. Liu, "Ultrafast optical manipulation of magnetic order in ferromagnetic materials", Nano Convergence 7, 35 (2020) [PDF]
[97] W. Ma, Z. C. Liu, Z. A. Kudyshev, A. Boltasseva, W. S. Cai and Y. M. Liu, "Deep learning for the design of photonic structures" (invited review), Nature Photonics 15, 77 (2021) [PDF]

Innovative approaches and tools play an important role in shaping design, characterization and optimization for the field of photonics. As a subset of machine learning that learns multilevel abstraction of data using hierarchically structured layers, deep learning offers an efficient means to design photonic structures, spawning data-driven approaches complementary to conventional physics- and rule-based methods. Here, we review recent progress in deep-learning-based photonic design by providing the historical background, algorithm fundamentals and key applications, with the emphasis on various model architectures for specific photonic tasks. We also comment on the challenges and perspectives of this emerging research direction.

[96] L. Li, K. Yao, Z. J. Wang and Y. M. Liu, "Harnessing Evanescent Waves by Bianisotropic Metasurfaces", Laser & Photonics Reviews 14, 1900244 (2020) [PDF]

The exponentially decaying nature of evanescent waves renders it difficult to capture, extract and engineer the wealth of energy and information that they can carry. Utilizing the out-of-plane electric dipoles and in-plane magnetic diploes produced by a bianisotropic C-aperture metasurface, in this work, it is shown that evanescent waves can be effectively molded. More specifically, it is demonstrated that the phase, polarization state or beam profile of the emission from evanescent waves can be controlled via the orientation of C-aperture nanostructures. The work opens a new avenue for metasurfaces to work in the critical near-field region to efficiently harness evanescent waves, and promises many potential applications, including on-chip free-electron light sources, tabletop particle detectors and near-field energy harvesting.

[95] W. Ma and Y. M. Liu, "A data-efficient self-supervised deep learning model for design and characterization of nanophotonic structures", Science China Physics, Mechanics & Astronomy 63, 284212 (2020) [PDF]
[94] F. Cheng, C. T. Wang, Z. X. Su, X. J. Wang, Z. Q. Cai, N. X. Sun, and Y. M. Liu, "All-Optical Manipulation of Magnetization in Ferromagnetic Thin Films Enhanced by Plasmonic Resonances", Nano Letters 9, 6437 (2020) [PDF]

In this paper, we report all-optical manipulation of magnetization in ferromagnetic Co/Pt thin films enhanced by plasmonic resonances. By annealing a thin Au layer, we fabricate large-area Au nanoislands on top of the Co/Pt magnetic thin films, which show plasmonic resonances around the wavelength of 606 nm. Using a customized magneto-optical Kerr effect setup, we experimentally observe an 18.5% decrease in the minimum laser power required to manipulate the magnetization, comparing the on- and off-resonance conditions. The results are in very good agreement with numerical simulations. Our research findings demonstrate the possibility to achieve an all-optical magnetic recording with low energy consumption, low cost, and high areal density by integrating plasmonic nanostructures with magnetic media.

[93] F. Cheng, Z. D. Du, X. J. Wang, Z. Q. Cai, L. Li, C. T. Wang, A. Benabbas, P. Champion, N. X. Sun, L. Pan and Y. M. Liu, "All-Optical Helicity-Dependent Switching in Hybrid Metal–Ferromagnet Thin Films", Advanced Optical Materials 8, 2000379 (2020) [PDF]

In this paper, all-optical helicity-dependent switching (AO-HDS) in hybrid metal–ferromagnet thin films, which consist of Co/Pt multilayers with perpendicular magnetic anisotropy and an Au film capping layer on the top, is experimentally demonstrated. The switching behaviors of the hybrid Co/Pt–Au material, with various laser repetition rates, scanning speeds, and fluencies, are systematically studied. In comparison with bare Co/Pt multilayers, the hybrid metal–ferromagnet thin films show pronounced AO-HDS when the number of laser pulses per μm along the scanning direction gradually increases. In addition, the AO-HDS effect is very robust against laser fluences. A possible mechanism is further proposed based on numerical simulations of the optomagnetic coupling model. Our findings promise a new material system that exhibits stable AO-HDS phenomena, and hence can transform future magnetic storage devices, especially with the addition of plasmonic nanostructures.

[92] Z. Liao, J. N. Zhou, G. Q. Luo, M. Wang, S. Sun, T. Zhou, H. F. Ma, T. J. Cui, and Y. M. Liu, "Microwave-Vortex-Beam Generation Based on Spoof-Plasmon Ring Resonators", Physical Review Applied 13, 054013 (2020) [PDF]
[91] X. P. Zhou, S. K. Gupta, X. Y. Zhu, G. X. Su, P. Zhan, Y. M. Liu, Z. Chen, M. H. Lu, and Z. L. Wang, "Nonreciprocal Isolation and Wavelength Conversion via a Spatiotemporally Engineered Cascaded Cavity", Physical Review Applied 13, 044037 (2020)[PDF]
[90] R. C. Jin, L. L. Tang, J. Q. Li, J. Wang, Q. J. Wang, Y. M. Liu, and Z. G. Dong, "Experimental Demonstration of Multidimensional and Multifunctional Metalenses Based on Photonic Spin Hall Effect", ACS Photonics 7, 512 (2020) [PDF]
[89] C. N. Niu, Z. J. Wang, J. Zhao, L. G. Du, N. Liu, Y. M. Liu and Xun Li, "Photonic Heterostructures for Spin-Flipped Beam Splitting", Physical Review Applied 12, 044009 (2019) [PDF]
[88] Z. Q. Cai*, Y. H. Xu*, C. T. Wang and Y. M. Liu, "Polariton Photonics Using Structured Metals and 2D Materials" (invited review), Advanced Optical Materials 5, 1901090 (2019) [PDF]

Polaritons are quasiparticles originating from strong interactions between photons and elementary excitations that could enable high tunability, tight electromagnetic field confinement, and large density of photonic states, making it possible to achieve novel and otherwise inaccessible functionalities. In recent years, the explosive growth of research in graphene and other 2D van der Waals materials is witnessed because they provide a new platform that substantially complements conventional metals, dielectrics, and semiconductors to investigate different polariton modes. This review highlights the works published in recent years on the topic of polariton photonics based on structured metals, graphene, and transition-metal dichalcogenides (TMDs). The exotic optical properties of the polaritons in metallic structures and 2D van der Waals materials offer bright prospects for the development of high-performance photonic and optoelectronic devices.

[87] B. Xiong, L. Deng, R. W. Peng and Y. M. Liu, "Controlling the degrees of freedom in metasurface designs for multi-functional optical devices" (invited review), Nanoscale Advances 1, 3786 (2019) [PDF]

This review focuses on the control over the degrees of freedom (DOF) in metasurfaces, which include the input DOF (the polarization, wavelength and incident angle of the input light and some dynamic controls), parameter DOF (the complex geometric design of metasurfaces) and output DOF (the phase, polarization and amplitude of the output light). This framework could clearly show us the development process of metasurfaces, from single-functional to multi-functional ones. Advantages of the multi-functional metasurfaces are discussed in the context of various applications, including 3D holography, broadband achromatic metalenses and multi-dimensional encoded information. By combining all the input and output DOF together, we can realize ideal optical meta-devices with deep subwavelength thickness and striking functions beyond the reach of traditional optical components.

[86] D. C. Abeysinghe, N. Nader, J. Myers,J. R. Hendrickson, J. W. Cleary, D. E. Walker, K. H. Chen, Y. M. Liu, and S. Mou, "Edge Doping Effect to the Surface Plasmon Resonances in Graphene Nanoribbons", The Journal of Physical Chemistry C 123, 19820 (2019) [PDF]
[85] L. Wehmeier, D. Lang, Y. M. Liu, X. Zhang, S. Winner, L. M. Eng and S. C. Kehr, "Polarization-dependent near-field phonon nanoscopy of oxides: SrTiO3, LiNbO3, and PbZr0.2Ti0.8O3", Physical Review B 100, 035444 (2019) [PDF]
[84] Z. X. Su, F. Cheng, L. Li and Y. M. Liu, "Complete Control of Smith-Purcell Radiation by Graphene Metasurfaces", ACS Photonics 6, 1947 (2019) [PDF]

In this work, we report the on-demand control of Smith-Purcell radiation by rationally designed graphene metasurfaces. Not only can we strongly enhance the efficiency of Smith-Purcell radiation, but also the amplitude, phase, and polarization state of the radiated wave can be fully manipulated by tuning the structure and Fermi level of the graphene metasurface. Through designing the geometric parameters of each unit cell of the metasurface, the intensity of the radiated wave from each unit cell can be changed from zero to maximum. Meanwhile, the phase of the radiated wave at any position of the metasurface can change within a range of 2π by adjusting the displacement of the patterned graphene structures. Utilizing these two properties, we demonstrate that we can steer the direction of the Smith-Purcell radiation and focus the radiated wave with dual focal points. Furthermore, a circularly polarized wave with an arbitrary phase can also be realized via introducing cross-polarization. Our findings provide a new way to design electron-beam-induced light sources as well as particle detectors with high efficiency and a compact footprint.

[83] W. Ma, F. Cheng, Y. H. Xu, Q. L. Wen and Y. M. Liu, "Probabilistic Representation and Inverse Design of Metamaterials Based on a Deep Generative Model with Semi-Supervised Learning Strategy", Advanced Materials 31, 1901111 (2019) [PDF]

The research of metamaterials has achieved enormous success in the manipulation of light in a prescribed manner using delicately designed subwavelength structures, so-called meta-atoms. Even though modern numerical methods allow for the accurate calculation of the optical response of complex structures, the inverse design of metamaterials, which aims to retrieve the optimal structure according to given requirements, is still a challenging task owing to the nonintuitive and nonunique relationship between physical structures and optical responses. We propose to represent metamaterials and model the inverse design problem in a probabilistically generative manner, enabling to elegantly investigate the complex structure–performance relationship in an interpretable way, and solve the one-to-many mapping issue that is intractable in a deterministic model. Moreover, to alleviate the burden of collecting data, a semisupervised learning strategy is developed that allows the model to utilize unlabeled data in addition to labeled data in an end-to-end training.

[82] Z. X. Su, B. Xiong, Y. H. Xu, Z. Q. Cai, J. B. Yin, R. W. Peng and Y. M. Liu, "Manipulating Cherenkov Radiation and Smith–Purcell Radiation by Artificial Structures" (invited review), Advanced Optical Materials 7, 1801666 (2019) [PDF]

A moving charged particle, such as an electron, can radiate light due to the interaction between its Coulomb field and surrounding matter. This phenomenon has spawned great interest in the fields of physics, electron microscopy, optics, biology, and materials science. Since the radiation generated by the charged particles strongly depends on the surrounding matter, artificially engineered materials with exotic electromagnetic and optic properties, including metamaterials and metasurfaces, provide an unprecedented opportunity to tailor the interaction between the charged particle and matter, and ultimately enable to manipulate the radiated light. In this review, the fundamentals of Cherenkov radiation and Smith–Purcell radiation are presented. Subsequently, the recent advances in the control of Cherenkov radiation and Smith–Purcell radiation based on metamaterials and metasurfaces are summarized. Finally, the applications using these two physical phenomena, including electron-driven photon sources and electron accelerators, are discussed in this review.

[81] Z. Cheng, H. L. Ye, F. Cheng, H. Y. Li, Y. Ma, Q. Zhang, A. Natan, A. Mukhopadhyay, Y. C. Jiao, Y. L, Y. M. Liu and H. L. Zhu, "Tuning Chiral Nematic Pitch of Bioresourced Photonic Films via Coupling Organic Acid Hydrolysis", Advanced Materials Interface 6, 1802010 (2019) [PDF]
[80] Z. Cheng, Y. Ma, L. Yang, F. Cheng, Z. J. Huang, A. Natan, H. Y. Li, Y. Chen, D. X. Cao, Z. F. Huang, Y. H. Wang, Y. M. Liu, R. D. Yang and H. L. Zhu, "Plasmonic-Enhanced Cholesteric Films: Coassembling Anisotropic Gold Nanorods with Cellulose Nanocrystals", Advanced Optical Materials 7, 1801816 (2019) [PDF]
[79] Y. M. Liu, "A Metamaterial for Superscattering Light", Physics 12, 14 (2019) [PDF]
[78] R. C. Jin, J. Q. Li, L. Li, Z. G. Dong, and Y. M. Liu, "Dual-mode subwavelength trapping by plasmonic tweezers based on V-type nanoantennas", Optics Letters 44, 319 (2019) [PDF]

We propose novel plasmonic tweezers based on silver V-type nanoantennas placed on a conducting ground layer, which can effectively mitigate the plasmonic heating effect and thus enable subwavelength plasmonic trapping in the near-infrared region. Using the centroid algorithm to analyze the motion of trapped spheres, we can experimentally extract the value of optical trapping potential. The result confirms that the plasmonic tweezers have a dual-mode subwavelength trapping capability when the incident laser beam is linearly polarized along two orthogonal directions. We have also performed full-wave simulations, which agree with the experimental data very well in terms of spectral response and trapping potential. It is expected that the dual-mode subwavelength trapping can be used in non-contact manipulations of a single nanoscale object, such as a biomolecule or quantum dot, and find important applications in biology, life science, and applied physics.

[77] H. Su, X. P. Shen, G. X. Su, L. Li, J. P. Ding, F. X. Liu, P. Zhan, Y. M. Liu, and Z. L. Wang, "Efficient Generation of Microwave Plasmonic Vortices via a Single Deep-Subwavelength Meta-Particle", Laser & Photonics Reviews 12, 6326 (2018) [PDF]

Light beams carrying orbital angular momentum (OAM) in the form of optical vortices have attracted great interest due to their capability for providing a new dimension and approach to manipulate light–matter interactions. Recently, plasmonics has offered efficient ways to focus vortex beams beyond the diffraction limit. However, unlike in the visible and near-infrared regime, it is still a big challenge to realize plasmonic vortices at far-infrared and even longer wavelengths. An effective strategy to create deep-subwavelength near-field electromagnetic (EM) vortices operating in the low frequency region is proposed. Taking advantage of the asymmetric spatial distribution of EM field supported by a metallic comb-shaped waveguide, plasmonic vortex modes that are strongly confined in a well-designed deep-subwavelength meta-particle with desired topological charges can be excited. Such unique phenomena are confirmed by the microwave experiments. This spoof-plasmon assisted focusing of EM waves with OAM may find potentials for functional integrated devices operating in the microwave, terahertz, and even far-infrared regions.

[76] W. Ma, F. Cheng, and Y. M. Liu "Deep-Learning-Enabled On-Demand Design of Chiral Metamaterials", ACS Nano 12, 6326 (2018) [PDF]

We report a deep-learning-based model, comprising two bidirectional neural networks assembled by a partial stacking strategy, to automatically design and optimize three-dimensional chiral metamaterials with strong chiroptical responses at predesignated wavelengths. The model can help to discover the intricate, nonintuitive relationship between a metamaterial structure and its optical responses from a number of training examples, which circumvents the timeconsuming, case-by-case numerical simulations in conventional metamaterial designs. This approach not only realizes the forward prediction of optical performance much more accurately and efficiently but also enables one to inversely retrieve designs from given requirements. Our results demonstrate that such a data-driven model can be applied as a very powerful tool in studying complicated light−matter interactions and accelerating the on-demand design of nanophotonic devices, systems, and architectures for real world applications.

[75] Z. Huang, K. Yao, G. X. Su, W. Ma, L. Li, Y. M. Liu, P. Zhan, and Z. L. Wang, "Graphene–metal hybrid metamaterials for strong and tunable circular dichroism generation", Optics Letters 43, 2636 (2018) [PDF]

A strong and dynamically controlled circular dichroism (CD) effect has aroused great attention due to its desirable applications in modern chemistry and life sciences. In this Letter, we propose a graphene–metal hybrid chiral metamaterial to generate mid-infrared CD with an intensity of more than 10%, which can be actively controlled over a wide wavelength range. In addition to the strong tunability, the CD signal intensity of our nanostructure is drastically larger than that of the purely graphene-based chiroptical nanostructures. Our design offers a new strategy for developing tunable chiral metadevices, which could be used in various applications, such as biochemical detection and information processing.

[74] K. Yao and Y. M. Liu, "Enhancing Circular Dichroism by cChiral Hotspots in Silicon Nanocube Dimers", Nanoscale 10, 8779 (2018) [PDF]

Circular dichroism (CD) spectroscopy, which measures the differential absorption of circularly polarized light with opposite handedness, is an important technique to detect and identify chiral molecules in chemistry, biology and life sciences. However, CD signals are normally very small due to the intrinsically weak chirality of molecules. Here we theoretically investigate the generation of chiral hotspots in silicon nanocube dimers for CD enhancement. Up to 15-fold enhancement of the global optical chirality is obtained in the dimer gap, which boosts the CD signal by one order of magnitude without reducing the dissymmetry factor. This chiral hotspot originates from the simultaneous enhancement of magnetic and electric fields and their proper spatial overlap. Our findings could lead to integrated devices for CD spectroscopy, enantioselective sensing, sorting and synthesis.

[73] T. T. Zhu, Y. Y. Cao, L. Wang, Z. Q. Nie, T. Cao, F. K. Sun, Z. H. Jiang, M. Nieto-Vesperinas, Y. M. Liu, C. W. Qiu, W. Q. Ding, "Self-Induced Backaction Optical Pulling Force", Physical Review Letters 120, 123901 (2018) [PDF]

In this Letter, we propose a novel approach based on the self-induced backaction-generated gradient force that achieves long-range optical pulling in a periodic photonic structure, namely, in a photonic crystal (PC). Interestingly, the pulling force arising in the PC is not predominantly contributed by the scattering force as reported before but induced by the intensity gradient force generated by the dynamic interaction of the object with the self- collimation (SC) mode of the PC. The SC mode is a unique kind of Bloch mode with a finite transversal size that can propagate infinitely long without diffraction. Hence, our results are sharply distinct to the widely held conception that the optical pulling force should be a scattering force, and shed new insightful concepts concerning the optical force and momentum physics and technologies.

[72] F. Z. Shu, F. F. Yu, R. W. Peng, Y. Y. Zhu, B. Xiong, R. H. Fan, Z. H. Wang, Y. M. Liu, and M. Wang, "Dynamic Plasmonic Color Generation Based on Phase Transition of Vanadium Dioxide", Advanced Optical Materials, online publication (2018) [PDF]

Plasmonic color filtering and color printing have attracted considerable attention in recent years due to their supreme performance in display and imaging technologies. In this article, dynamic color generation is demonstrated by integrating plasmonic nanostructures with vanadium dioxide based on its tunable optical properties through insulator–metal transition. Periodic arrays of silver nanodisks on a vanadium dioxide film are fabricated to realize different colors, relying on the excitation of localized and propagating surface plasmons, and Wood's anomaly. By tuning spatial periodicity of the arrays and diameter of the silver nanodisks, various colors can be achieved across the entire visible spectrum. Further, using insulator–metal transition of vanadium dioxide, the colors can be actively tuned by varying temperature. The approach of dynamic color generation based on the phase transition of vanadium dioxide can easily realize diverse color patterns, which makes it beneficial for display and imaging technology with distinct advantages of multifunctionality, flexibility, and high efficiency.

[71] Z. Y. Jia, F. Z. Shu, Y. J. Gao, F. Cheng, R. W. Peng, R. H. Fan, Y. M. Liu, and M. Wang, "Dynamically Switching the Polarization State of Light Based on the Phase Transition of Vanadium Dioxide", Physical Review Applied, 9, 034009 (2018) [PDF]

There have been great endeavors devoted to manipulating the polarization state of light by plasmonic nanostructures in recent decades. However, the topic of active polarizers has attracted much less attention. We present a composite plasmonic nanostructure consisting of vanadium dioxide that can dynamically modulate the polarization state of the reflected light through a thermally induced phase transition of vanadium dioxide. Under a particular linearly polarized incidence, the polarization state of the reflected light changes at room temperature, and reverts to its original polarization state above the phase-transition temperature. The composite structure can also be used to realize a dynamically switchable infrared image, wherein a pattern can be visualized at room temperature while it disappears above the phase-transition temperature. The composite structure could be potentially used for versatile optical modulators, molecular detection, and polarimetric imaging.

[70] K. Yao and Y. M. Liu, "Infrared Plasmonic Resonators Based on Self-Assembled Core−Shell Particles", ACS Photonics, 5, 844 (2018) [PDF]

Infrared resonators emerge as key elements of infrared optical devices demanded for widespread applications. Traditional resonators based solely on noble metals suffer from considerable ohmic losses, limited tunability, and low excitation efficiency of surface plasmons in the infrared region, while the alternatives made of high-refractive-index dielectrics encounter fabrication challenges and relatively weak optical coupling. This work presents the experimental demonstration of self-assembled plasmonic core−shell particle clusters exhibiting pronounced and spectrally tunable resonances at infrared, especially mid-infrared, wavelengths. It is shown that the reflection spectra of individual clusters are unique, which are determined by the number and configuration of the constituent particles. These findings would leadto a substantial step toward reconfigurable plasmonics and metamaterials with potential applications in sensing, imaging, spectroscopy, surveillance, and energy harvesting.

[69] L. Kang, S. P. Rodrigues, M. Taghinejad, S. F. Lan, K. Lee, Y. M. Liu, D. H. Werner, A. Urbas and W. S. Cai, "Preserving Spin States upon Reflection: Linear and Nonlinear Responses of a Chiral Meta-Mirror", Nano Letters 17, 7102 (2017) [PDF]

Conventional metallic mirrors flip the spin of a circularly polarized wave upon normal incidence by inverting the direction of the propagation vector. Altering or maintaining the spin state of light waves carrying data is a critical need to be met at the brink of photonic information processing. In this work, we report a chiral metamaterial mirror that strongly absorbs a circularly polarized wave of one spin state and reflects that of the opposite spin in a manner conserving the circular polarization. A circular dichroic response in reflection as large as 0.5 is experimentally observed in a near-infrared wavelength band. Beyond the linear regime, the chiral resonances enhance light-matter interaction under circularly polarized excitation, greatly boosting the ability of the metamaterial to perform chiral-selective signal generation and optical imaging in the nonlinear regime.

[68] L. Kang, S. P. Rodrigues, M. Taghinejad, S. F. Lan, K. Lee, Y. M. Liu, D. H. Werner, A. Urbas and W. S. Cai, "Prediction of Deterministic All-Optical Switching of Ferromagnetic Thin Film by Ultrafast Optothermal and Optomagnetic Couplingsr" Scientific Reports 7: 13513 (2017) [PDF]

All-optical switching (AOS) of magnetization induced by ultrafast laser pulses is fundamentally interesting and promises unprecedented speed for magnetic data storage that is three orders of magnitudes faster than the current techniques. Since the magnetic field needed to flip the ferromagnetic magnetization within femtosecond timescale is unphysically high, some theories hypothesized that there exists a prolonged magnetic field beyond the pulse duration in the switching process. This is intuitively inconsistent with the phenomenological explanation based on the light-induced magnetic field arising from the inverse Faraday effect (IFE). Here, we numerically study the AOS process and provide new insights into the long-standing paradox of the duration of the induced magnetic field. We show that the prolonged magnetic field duration originates from the ultrafast optothermal and optomagnetic coupling. Moreover, we numerically studied both single- and multiplepulse AOS under different coupling strength between spins and the thermal bathThis numerical model may provide a guide to find suitable ferromagnetic materials for AOS.

[67] L. Chang, Y. Li and Y. M. Liu, "Controlling the Bidirectional Circular Polarization States Using Ultrathin Back-to-Back Quarter- Wave Plates Cavity", Scientific Reports 7: 15257 (2017) [PDF]
[66] Q. Hu, D. F. Jin, J. Xiao, S. H. Nam, X. Z. Liu, Y. M. Liu, X. Zhang, and N. Fang, "Ultrafast Fluorescent Decay Induced by Metal-Mediated Dipole–Dipole Interaction in Two-Dimensional Molecular Aggregates", Proceedings of the National Academy of Sciences 114, 10017 (2017) [PDF]

Two-dimensional molecular aggregate (2DMA), a thin sheet of strongly interacting dipole molecules self-assembled at close distance on an ordered lattice, is a fascinating fluorescent material. In this paper, we verify that when a 2DMA is placed at a nanometric distance from a metallic substrate, the strong and coherent interaction between the dipoles inside the 2DMA dominates its fluorescent decay at a picosecond timescale. Our streak-camera lifetime measurement and interacting lattice–dipole calculation reveal that the metalmediated dipole–dipole interaction shortens the fluorescent lifetime to about one-half and increases the energy dissipation rate by 10 times that expected from the noninteracting single-dipole picture. Our finding can enrich our understanding of nanoscale energy transfer in molecular excitonic systems and may designate a unique direction for developing fast and efficient optoelectronic devices.

[65] W. Wang, L. Wang, R. Xue, H. Chen, R. Guo, Y. M. Liu, and J. Chen, "Unidirectional Excitation of Radiative-Loss-Free Surface Plasmon Polaritons in Parity-Time Symmetric Systems", Physical Review Letters 119, 077401 (2017) [PDF]

We investigate the excitation and propagation of surface plasmon polaritons (SPPs) at a geometrically flat metal-dielectric interface with a parity-time symmetric modulation on the permittivity of the dielectric medium. We show that two striking effects can be simultaneously achieved thanks to the nonreciprocal nature of the Bloch modes in the system. Firstly, SPPs can be unidirectionally excited when light is normally incident on the interface. Secondly, the backscattering of SPPs into the far field is suppressed, producing a radiative-loss-free effect on the unidirectional SPPs. As a result, the life time and propagation distance of SPPs can be significantly improved. These results show that parity-time symmetry can be employed as a new approach to designing transformative nanoscale optical devices, such as low-loss plasmonic routers and isolators for efficient optical computation, communication and information processing.

[64] M. Kim*, Kan Yao*, G. Yoon, I. Kim, Y. M. Liu and Junsuk Rho, "A Broadband Optical Diode for Linearly Polarized Light Using Symmetry-breaking Metamaterials", Advanced Optical Materials, 5, 1700600 (2017) [PDF]

As an analogue of electrical diodes, optical diodes enable asymmetric transmission, or one-way transmission of light. Here, we experimentally demonstrate a thin bi-layer metamaterial supporting asymmetric transmission for linearly polarized light but not for circularly polarized light over a broad bandwidth up to 50 terahertz in the near-infrared region. We provide a simple and intuitive working principle based on the symmetry inherent in the metamaterial design, along with full-wave simulations that agree well with the experimental results. We also prove that our design is extremely insensitive to spatial misalignment, which might occur during the nanofabrication process. These prominent features promise a wide range of applications, such as ultrafast optical computing, information processing and suppressing undesired interactions of light in integrated micro- and nano-devices.

[63] W. Ma, Z. Huang, X. K. Bai, P. Zhan and Y. M. Liu, "Dual-Band Light Focusing Using Stacked Graphene Metasurfaces", ACS Photonics 4, 1770 (2017) [PDF]

We theoretically study the optical properties of graphene metasurfaces consisting of layered graphene ribbons, and their potential applications as multi-functional optical devices in the long-wavelength infrared region. By engineering the plasmonic resonance in graphene ribbons with different widths, the phase of reflected light can be tuned over a range of nearly 2π while the reflectivity is kept relatively high at the target frequencies. Owing to the weak light-graphene interaction in the off-resonance region, independent control of the reflected light can be achieved by stacking multiple layers of graphene ribbons. We have developed a transmission line based uncoupled model as a physical interpretation of the stacked metasurface, which show excellent agreement with numerical simulations. As a proof-of-principle demonstration, we have designed and demonstrated a graphene metasurface as flat, dual-band focusing reflector operating at 25 THz and 16 THz.

[62] L. Q. Jing, Z. J. Wang, Y. H. Yang, B. Zheng, Y. M. Liu and H. S. Chen, "Chiral Metamirrors for Broadband Spin-Selective Absorption", Applied Physics Letters 110, 231103 (2017) [PDF]
[61] Z. J. Wang, L. Q. Jing, K. Yao, Y. H. Yang, B. Zheng, C. M. Soukoulis, H. S. Chen and Y. M. Liu, "Origami-Based Reconfigurable Metamaterials for Tunable Chirality", Advanced Materials 29, 1700472 (2017) [PDF]

Origami is the art of folding two-dimensional (2D) materials, such as a flat
sheet of paper, into complex and elaborate three-dimensional (3D) objects. This study reports origami-based metamaterials whose electromagnetic responses are dynamically controllable via switching the folding state of Miura-ori split-ring resonators. The deformation of the Miura-ori unit along the third dimension induces net electric and magnetic dipoles of split-ring resonators parallel or anti-parallel to each other, leading to the strong chiral responses. Circular dichroism as high as 0.6 is experimentally observed while the chirality switching is realized by controlling the deformation direction and kinematics. In addition, the relative density of the origami metamaterials can be dramatically reduced to only 2% of that of the unfolded structure. These results open a new avenue toward lightweight, reconfigurable, and deployable metadevices with simultaneously customized electromagnetic and mechanical properties.

[60] T. Zhang, M. R. C. Madhy, Y. M. Liu, J. H. Teng, C. T. Lim, Z. Wang and C. W. Qiu, "All-Optical Chirality-Sensitive Sorting via Reversible Lateral Forces in Interference Fields", ACS Nano 11, 4292 (2017) [PDF]
[59] Q. L. Yang, X. Q. Zhang, S. X. Li, Q. Xu, R. Singh, Y. M. Liu, Y. F. Li, S. S. Kruk, J. Q. Gu, J. G. Han and W. L. Zhang, "Near-field Surface Plasmons on Quasicrystal Metasurfaces", Scientific Reports 6:26 (2016) [PDF]
[58] M. Kim, S. So, K. Yao, Y. M. Liu and J. Rho, "Deep Sub-wavelength Nanofocusing of UV-visible Light by Hyperbolic Metamaterials", Scientific Reports 6:38635 (2016) [PDF]
[57] Y. B. Deng, Z. Y. Liu, Y. M. Liu and Y. H. Wu, "Inverse Design of Dielectric Resonator Cloaking Based on Topology Optimization", Plasmonics, Article ASAP (2016) [PDF]
[56] Z. J. Wang, H. Jia, K. Yao, W. S. Cai, H. S. Chen and Y. M. Liu, "Circular Dichroism Metamirrors with Near-Perfect Extinction", ACS Photonics 3, 2096 (2016) [PDF]

In nature, the beetle Chrysina gloriosa derives its iridescence by selectively reflecting left-handed circularly polarized light only. Here, an optical analogue is suggested based on an ultrathin metamaterial, which is termed circular dichroism metamirror. A general method to design the circular dichroism metasmirror is presented under the framework of Jones calculus. It is analytically shown that the building block of such a metamirror needs to simultaneously break the n-fold rotational (n > 2) symmetry and mirror symmetry. By combining two layers of anisotropic metamaterial structures, a circular dichroism metamirror is designed in the mid-infrared region, which shows perfect reflectance for left-handed circularly polarized light without reversing its handedness, while it almost completely absorbs right-handed circularly polarized light. These findings offer a new methodology to implement novel photonic devices for a polarimetric imaging, molecular spectroscopy, and quantum information processing.

[55] Z. J. Wang, K. Yao, M. Chen, H. S. Chen and Y. M. Liu, "Manipulating Smith-Purcell Emission with Babinet Metasurfaces", Physical Review Letters 117, 157401 (2016) [PDF]

Swift electrons moving closely parallel to a periodic grating produce far-field radiation of light, which is known as the Smith-Purcell effect. In this letter, we demonstrate that designer Babinet metasurfaces composed of C-aperture resonators offer a powerful control over the polarization state of the Smith-Purcell emission, which can hardly be achieved via traditional gratings. By coupling the intrinsically nonradiative energy bound at the source current sheet to the out-of-plane electric dipole and in-plane magnetic dipole of the C--aperture resonator, we are able to excite cross-polarized light thanks to the bianisotropic nature of the metasurface. The polarization direction of the emitted light is aligned with the orientation of the C-aperture resonator. Furthermore, the efficiency of the Smith-Purcell emission from Babinet metasurfaces is significantly increased by 84%, in comparison with the case of conventional gratings. These findings o promise compact, tunable, and efficient light sources and particle detectors.

[54] Z. J. Wang, F. Chen, T. Winsor and Y. M. Liu, "Optical chiral metamaterials: a review of the fundamentals, fabrication methods and applications" (invited review), Nanotechnology 27, 412001(2016) [PDF]
[53] K. Yao and Y. M. Liu, "Controlling Electric and Magnetic Resonances for Ultracompact Nanoantennas with Tunable Directionality", ACS Photonics 3, 953 (2016) [PDF]

We design and numerically demonstrate an ultracompact plasmonic nanoantenna with tunable high directionality. The antenna consists of a metallic trimer that can support a highly spectrally tunable magnetic dipole mode with its amplitude comparable to that of an electric dipole mode. Superior forward radiation is achieved when these modes satisfy the Kerker conditions, leading to a very low side lobe level of −22 dB. It is shown that by moving one of the three particles by less than 10 nm the resonance wavelength of the magnetic mode will shift dramatically, resulting in the change of the interference conditions and hence the radiation characteristics. From the evolution of the resonant modes, we find optimized designs that reverse the radiation direction at the same wavelength. Meanwhile, the enhancement of spontaneous emission and radiated power of a nanoemitter adjacent to the antenna can reach approximately 4 and 3 orders of magnitude, respectively. Analyses based on a simple dipole model are performed, and the reconstructed radiation patterns agree well with the simulation results.

[52] X. Q. Zhang, Q. Xu, Q. Li, Y. H. Xu, J. Q. Gu, Z. Tian, C. M. Ouyang, Y. M. Liu, S. Zhang, X. X. Zhang, J. G. Han and W. L. Zhang, "Asymmetric excitation of surface plasmons by dark mode coupling", Science Advances 2: e1501142 (2016) [PDF]

Control over surface plasmons (SPs) is essential in a variety of cutting-edge applications, such as highly integrated photonic signal processing systems, deep-subwavelength lasing, high-resolution imaging, and ultrasensitive biomedical detection. Recently, asymmetric excitation of SPs has attracted enormous interest. In free space, the analog of electromagnetically induced transparency (EIT) in metamaterials has been widely investigated to uniquely manipulate the electromagnetic waves. In the near field, we show that the dark mode coupling mechanism of the classical EIT effect enables an exotic and straightforward excitation of SPs in a metasurface system. This leads to not only resonant excitation of asymmetric SPs but also controllable exotic SP focusing by the use of the Huygens-Fresnel principle. Our experimental findings manifest the potential of developing plasmonic metadevices with unique functionalities.

[51] Y. B. Deng, Z. Y. Liu, C. Song, P. Hao, Y. Wu, Y. M. Liu and J. G. Korvink, "Topology optimization of metal nanostructures for localized surface plasmon resonances", Structural and Multidisciplinary Optimization 53: 967 (2015) [PDF]
[50] Q. H. Guo, W. L. Gao, J. Chen, Y. M. Liu and S. Zhang, "Line Degeneracy and Strong Spin-Orbit Coupling of Light with Bulk Bianisotropic Metamaterials", Physical Review Letters 115, 067402 (2015) [PDF]

Propagation of light in a medium is dictated by equifrequency surfaces (EFSs), which play a similar role as Fermi surfaces for electrons in crystals. Engineering the equifrequency surface of light through structuring a photonic medium enables superior control over light propagation that goes beyond natural materials. In this Letter, we show that a bulkmetamaterial with a suitably designed bianisotropy can exhibit line degeneracy in its EFSs that consist of two ellipsoids of opposite helicity states intersecting with each other. Very interestingly, light propagating along the direction of the line degeneracy experiences strong spin-dependent photon deflection, or optical spin Hall effect, which may lead to applications in optical signal processing and spinopticalmanipulations. We provide a realistic metamaterial design to showthat the required bianisotropy can be readily obtained.

[49] Z. B. Li, K. Yao, F. N. Xia, S. Shen, J. G. Tian and Y. M. Liu, "Graphene Plasmonic Metasurfaces to Steer Infrared Light", Scientific Reports 5, 12423 (2015) [PDF]

In this work, combining the pronounced plasmonic resonance of patterned graphene structures with a subwavelength-thick optical cavity, we propose and demonstrate novel graphene metasurfaces that manifest the potential to dynamically control the phase and amplitude of infrared light with very high efficiency. It is shown that the phase of the infrared light reflected from a simple graphene ribbon metasurface can span over almost the entire 2π range by changing the width of the graphene ribbons, while the amplitude of the reflection can be maintained at high values without significant variations. We successfully realize anomalous reflection, reflective focusing lenses, and non-diffracting Airy beams based on graphene metasurfaces. Our results open up a new paradigm of highly integrated photonic platforms for dynamic beam shaping and adaptive optics in the crucial infrared wavelength range.

[48] L. Kang, S. F. Lan, Y. H. Cui, S. P. Rodrigues, Y. M. Liu, D. H. Werner and W. S. Cai, "An Active Metamaterial Platform for Chiral Responsive Optoelectronics", Advanced Materials 27, 4377 (2015) [PDF]

In this work we propose and experimentally demonstrate a novel metamaterial confi guration that is composed of a pair of perforated films with angularly shifted elliptical holes. This metamaterial design gives rise to remarkably strong chiralselective light-matter interactions in both the linear and nonlinear domains. With a deeply subwavelength thickness, the metamaterial exhibits a circular dichroism as large as 0.40 around the high frequency end of the near-infrared spectrum. This resonance behavior in turn leads to a pronounced contrast in the second-harmonic signal generated from the metamaterial under circularly polarized fundamental waves of opposite handedness. More importantly, the electrically connected metal parts that form the metamaterial facilitate thecontrol and detection of chiroptical processes via electrical means.

[47] W. L. Gao, F. Z. Fang, Y. M. Liu and S. Zhang, "Chiral Surface Waves Supported by Biaxial Hyperbolic Metamaterials", Light: Science and Applications 4, e328 (2015) [PDF]

We discover a new kind of surface wave on biaxial hyperbolic metamaterial, which, in the k-space, connects the two diabolical points (or conical singularities) of the equifrequency surface. The propagation of such surface wave is found to be sensitive to the refractive index of the surrounding dielectric medium, showing a convex, concave or flat phase front when the refractive index is varied. Furthermore, the surface wave shows an elliptically polarized state, in which helicity is dependent on the propagation direction. This feature can be utilized for the spin-controllable excitation of surface waves, opening a gateway towards integrated photonic circuits with reconfigurable functionalities.

[46] M. Fehrenbacher, S. Winnerl, H. Schneider, J. Döring, S. C. Kehr, L. M. Eng, Y. H. Huo, O. G. Schmidt, K. Yao, Y. M. Liu and M. Helm, "Plasmonic superlensing in doped GaAs", Nano Letters, 15, 1057 (2015) [PDF]

We demonstrate a semiconductor based broadband near-field superlens in the mid-infrared regime. Here, the Drude response of a highly doped n-GaAs layer induces a resonant enhancement of evanescent waves accompanied by a significantly improved spatial resolution at radiation wavelengths around λ = 20 μm, adjustable by changing the doping concentration. In our experiments, gold stripes below the GaAs superlens are imaged with a λ/6 subwavelength resolution by an apertureless near-field optical microscope utilizing infrared radiation from a free-electron laser. The resonant behavior of the observed superlensing effect is in excellent agreement with simulations based on the Drude−Lorentz model. Our results demonstrate a rather simple superlens implementation for infrared nanospectroscopy.

[45] C. L. Zhao, J. S. Zhang and Y. M. Liu, "Light manipulation with encoded plasmonic nanostructures", EPJ Applied Metamaterials 1, 6 (2014) [PDF]
[44] B. A. Liu, Y. M. Liu and S. Sheng, "Thermal plasmonic interconnects in graphene", Physical Review B 90, 195411 (2014) [PDF]

As one emerging plasmonic material, graphene can support surface plasmons at infrared and terahertz frequencies with unprecedented properties due to the strong interactions between graphene and low-frequency photons. Here we show that thermal graphene plasmons can be efficiently excited and have monochromatic and tunable spectra, thus paving a way to harness thermal energy for graphene plasmonic devices. We further demonstrate that "thermal information communication" via graphene surface plasmons can be potentially realized by effectively harnessing thermal energy from various heat sources, e.g., the waste heat dissipated from nanoelectronic devices. These findings open up an avenue of thermal plasmonics based on graphene for different applications ranging from infrared emission control, to information processing and communication, to energy harvesting.

[43] J. W. Xu, Z. D Gao, K. Han, Y. M. Liu and Y. Y. Song, "Synthesis of magnetically separable Ag3PO4/TiO2/Fe3O4
heterostructure with enhanced photocatalytic performance under visible light for photoinactivation of bacteria", ACS Applied Materials & Interfaces 6, 15122 (2014) [PDF]

Silver orthophosphate (Ag3PO4) is a low-bandgap photocatalyst that has received considerable research interest in recent years. In this work, the magnetic Ag3PO4/ TiO2/Fe3O4 heterostructured nanocomposite was synthesized. The nanocomposite was found to exhibit markedly enhanced photocatalytic activity, cycling stability, and long-term durability in the photodegradation of acid orange 7 (AO7) under visible light. Moreover, the antibacterial film prepared from Ag3PO4/TiO2/Fe3O4 nanocomposite presented excellent bactericidal activity and recyclability toward Escherichia coli (E.coli) cells under visible-light irradiation. In addition to the intrinsic cytotoxicity of silver ions, the elevated bactericidal efficiency of Ag3PO4/TiO2/Fe3O4 can be largely attributed to its highly enhanced photocatalytic activity. The photogenerated hydroxyl radicals and superoxide ions on the formed Ag/Ag3PO4/TiO2 interfaces cause considerable morphological changes in the microorganism's cells and lead to the death of the bacteria.

[42] C. L. Zhao, Y. M. Liu, J. Yang and J. S. Zhang, "Single-molecule detection and radiation control in solutions at high concentrations via a heterogeneous optical slot antenna", Nanoscale 6, 9103 (2014) [PDF]

We designed a heterogeneous optical slot antenna (OSA) that is capable of detecting single molecules in solutions at high concentrations, where most biological processes occur. A heterogeneous OSA consists of a rectangular nanoslot fabricated on heterogeneous metallic films formed by sequential deposition of gold and aluminum on a glass substrate. The rectangular nanoslot gives rise to large field and fluorescence enhancement for single molecules. The near-field intensity inside a heterogeneous OSA is 170 times larger than that inside an aluminum zero-mode waveguide (ZMW), and the fluorescence emission rate of a molecule inside the heterogeneous OSA is about 70 times higher than that of the molecule in free space. Our results can be used as a direct guidance for designing high-performance, low-cost plasmonic nanodevices for the study of bio-molecule and enzyme dynamics at the single-molecule level.

[41] K, Yao, H. Y. Chen, Y. M. Liu and X. Y. Jiang, "An analogy strategy for transformation optics", New Journal of Physics 16, 063008 (2014) [PDF]

We introduce an analogy strategy to design transformation optical devices. Based on the similarities between field lines in different physical systems, the trajectories of light can be intuitively determined to curve in a gentle manner, and the resulting materials are isotropic and nonmagnetic. Furthermore, the physical meaning of the analogue problems plays a key role in the removal of dielectric singularities. We illustrate this approach by creating two designs of carpet cloak and a collimating lens as representative examples in two- and three dimensional spaces, respectively. The analogy strategy reveals the intimate connections between different physical disciplines, such as optics, fluid mechanics and electrostatics.

[40] Y. H. Cao, J. Xie, Y. M. Liu and Z. Y. Liu, "Modeling and optimization of photonic crystal devices based on transformation optics method", Optics Express 22, 2725 (2014) [PDF]
[39] K. Yao and Y. M. Liu, "Plasmonic Metamaterials"(invited review), Nanotechnology Review 3, 177(2014) [PDF]

Plasmonics and metamaterials have attracted considerable attention over the past decade, owing to the revolutionary impacts that they bring to both the fundamental physics and practical applications in multiple disciplines. Although the two fields initially advanced along their individual trajectories in parallel, they started to interfere with each other when metamaterials reached the optical regime. The dynamic interplay between plasmonics and metamaterials has generated a number of innovative concepts and approaches, which are impossible with either area alone. This review presents the fundamentals, recent advances, and future perspectives in the emerging field of plasmonic metamaterials, aiming to open up new exciting opportunities for nanoscience and nanotechnology.

[38] H. Cang, Y. M. Liu, Y. Wang, X. Yin and X. Zhang, "Giant suppression of photobleaching for single molecule detection via the Purcell effect", Nano Letters 13, 5945 (2013) [PDF]

We report giant suppression of photobleaching and a prolonged lifespan of single fluorescent molecules via the Purcell effect in plasmonic nanostructures. The plasmonic structures enhance the spontaneous emission of excited fluorescent molecules, reduce the probability of activating photochemical reactions that destroy the molecules, and hence suppress the bleaching. Experimentally, we observe up to a 1000-fold increase in the total number of photons that we can harvest from a single fluorescent molecule before it bleaches. This approach demonstrates the potential of using the Purcell effect to manipulate photochemical reactions at the subwavelength scale.

[37] Y. M. Liu and X. Zhang, "Metasurfaces for manipulating surface plasmons", Appled Physics Letters 103, 141101 (2013) [PDF]

Metasurfaces have recently emerged as an innovative approach to control light propagation with unprecedented capabilities. Different from previous work concentrating on steering far-field propagating waves, here we demonstrate that metallic metasurfaces can efficiently and effectively manipulate surface plasmons in the near-field regime. By engineering the dispersion of surface plasmons on a simple grating structure, we are able to realize normal, non-divergent as well as anomalous diffraction of surface plasmons. In particular, all-angle and broadband negative refraction of surface plasmons is achieved, largely attributed to the uniquely designed hyperbolic constant frequency contour of surface plasmons propagating along the metasurface.

[36] C. L. Zhao*, Y. M. Liu*, Y. H. Zhao, N. Fang and T. J. Huang, "A reconfigurable plasmofluidic lens", Nature Communications 4:2305 (2013) [PDF]

The majority of plasmonic devices are in the solid state and have limited tunability or configurability. Moreover, individual solid-state plasmonic devices lack the ability to deliver multiple functionalities. Here we utilize laser-induced surface bubbles on a metal film to demonstrate, for the first time, a plasmonic lens in a microfluidic environment. Our 'plasmofluidic lens' is dynamically tunable and reconfigurable. We record divergence, collimation and focusing of surface plasmon polaritons using this device. Our results show that the integration of plasmonics and microfluidics allows for new opportunities in developing complex plasmonic elements with multiple functionalities, high-sensitivity and high-throughput biomedical detection systems, as well as on-chip, all-optical information processing techniques.

[35] Y. M. Liu*, S. Palomba*, Y. Park, T. Zentgraf, X. B. Yin and X. Zhang, "Compact magnetic antennas for directional excitation of surface plasmons", Nano Letters 12, 4853 (2012) [PDF]

Bridging the optical antenna theory and the recently developed concept of metamaterials, we demonstrate a subwavelength, highly efficient plasmonic source for directional generation of surface plasmon polaritons (SPPs). The designed device consists of two nanomagnetic resonators with detuned resonant frequencies. By tailoring the relative phase at resonance and the separation between the two nanoresonators, SPPs can be steered to predominantly propagate along one specific direction. This novel magnetic nanoantenna paves a new way to manipulate photons in the near-field, and also could be useful for SPP-based nonlinear applications, active modulations, and wireless optical communications.

[34] Y. M. Liu and X. Zhang, "Recent advances in transformation optics" (invited review), Nanoscale 4, 5277 (2012) [PDF]

Within the past a few years, transformation optics has emerged as a new research area, since it provides a general methodology and design tool for manipulating electromagnetic waves in a prescribed manner. In this paper, we review the most recent advances in transformation optics. We focus on the theory, design, fabrication and characterization of transformation devices such as the carpet cloak, "Janus" lens and plasmonic cloak at optical frequencies, which allow routing light at the nanoscale. We also provide an outlook of the challenges and future directions in this fascinating area of transformation optics.

[33] R. H. Fan, R. W. Peng, X. R. Huang, J. Li, Y. M. Liu, Q. Hu, M. Wang and X. Zhang, "Transparent metals for ultrabroadband electromagnetic waves", Advanced Materials 24, 1980 (2012) [PDF]
[32] S. C. Kehr, P. Yu, Y. M. Liu, M. Parzefall, A. I. Khan, R. Jacob, M. T. Wenzel, H.-G. von Ribbeck, M. Helm, X. Zhang, L. M. Eng and R. Ramesh, "Microspectroscopy on perovskite-based superlenses", Optical Materials Express 1, 1051 (2011) [PDF]
[31] P. Zhang*, S. Wang*, Y. M. Liu*, X. B. Yin, C. G. Lv, Z. G. Chen and X. Zhang, "Plasmonic Airy beams with dynamically controlled trajectories", Optics Letters 36, 3191 (2011) [PDF]
[30] M. Gharghi, C. Gladden, T. Zentgraf, Y. M. Liu, X. B. Yin, J. Valentine and Xiang Zhang, "A carpet cloak for visible light", Nano Letters 11, 2825 (2011) [PDF]

We report an invisibility carpet cloak device, which is capable of making an object undetectable by visible light. The cloak is designed using quasi conformal mapping and is fabricated in a silicon nitride waveguide on a specially developed nanoporous silicon oxide substrate with a very low refractive index (n<1.25). The spatial index variation is realized by etching holes of various sizes in the nitride layer at deep subwavelength scale The fabricated device demonstrates wideband invisibility throughout the visible spectrum with low loss. This silicon nitride on low index substrate can also be a general scheme for implementation of transformation optical devices at visible frequencies.

[29] S. C. Kehr*, Y. M. Liu*, L. W. Martin, P. Yu, M. Gajek, S.-Y. Yang, C.-H. Yang, M. T. Wenzel, R. Jacob, H.-G. von Ribbeck, M. Helm, X. Zhang, L. M. Eng and R. Ramesh, "Near-field examination of perovskite-based superlenses and superlens-enhanced probe-object coupling", Nature Communications 2, 249 (2011) [PDF]

We demonstrate a superlens for electric evanescent fields with low losses using perovskites in the mid-infrared regime. The combination of near-field microscopy with a tunable free-electron laser allows us to address precisely the polariton modes, which are critical for super-resolution imaging. We spectrally study the lateral and vertical distributions of evanescent waves around the image plane of such a lens, and achieve imaging resolution of λ/14 at the superlensing wavelength. Interestingly, at certain distances between the probe and sample surface, we observe a maximum of these evanescent fields. Comparisons with numerical simulations indicate that this maximum originates from an enhanced coupling between probe and object, which might be applicable for multifunctional circuits, infrared spectroscopy and thermal sensors.

[28] T. Zentgraf*, Y. M. Liu*, M H. Mikkelsen*, J. Valentine and X. Zhang, "Plasmonic Luneburg and Eaton lenses", Nature Nanotechnology 6, 151 (2011) [PDF]

We use grey-scale lithography to adiabatically tailor the topology of a dielectric layer adjacent to a metal surface to demonstrate a plasmonic Luneburg lens that can focus surface plasmon polaritons. We also make a plasmonic Eaton lens that can bend surface plasmon polaritons. Because the optical properties are changed gradually rather than abruptly in these lenses, losses due to scattering can be significantly reduced in comparison with previously reported nano plasmonic devices. Our approach opens a new way to achieve low-loss, functional plasmonic elements with a standard fabrication technology based on grey-scale electron-beam lithography and is fully compatible with active plasmonics. Furthermore, this method provides a scheme with which to realize more complex two-dimensional plasmonic elements using transformation optics.

[27] Y. M. Liu and X. Zhang, "Metamaterials: a new frontier of science and technology" (invited review), Chemical Society Reviews 40, 2494 (2011) [PDF]

This critical review focuses on the fundamentals, recent progresses and future directions in the research of electromagnetic metamaterials. An introduction to metamaterials followed by a detailed elaboration on how to design unprecedented electromagnetic properties of metamaterials is presented. A number of intriguing phenomena and applications associated with metamaterials are discussed, such as negative refraction, sub-diffraction-limited imaging, strong optical activities in chiral metamaterials, interaction of meta-atoms and transformation optics. Finally, we offer an perspective of metamaterials research including 3D optical metamaterials, nonlinear metamaterials and "quantum" perspectives of metamaterials.

[26] X. D. Yang, Y. M. Liu, R. F. Oulton, X. B. Yin and X. Zhang, "Optical forces in hybrid plasmonic waveguides", Nano Letters 11, 321 (2011) [PDF]

We demonstrate that in a hybrid plasmonic system the optical force exerted on a dielectric waveguide by a metallic substrate is enhanced by more than 1 order of magnitude compared to the force between a photonic waveguide and a dielectric substrate.. Moreover, the hybridization between the surface plasmon modes and waveguide modes allows efficient optical trapping of single dielectric nanoparticle with size of only several nanometers in the gap region, manifesting various optomechanical applications such as nanoscale optical tweezers.

[25] H. Cang, A. Labno, C. G. Lu, X. B. Yin, M. Liu, C. Glandden, Y. M. Liu and X. Zhang, "Probing the electromagnetic field of a 15-nanometre hotspot by single molecule imaging", Nature 469, 385 (2011) [PDF]
[24] C. W. Chang, M. Liu, S. Nam, S. Zhang, Y. M. Liu, G. Bartal and Xiang Zhang, "Optical Möbius symmetry in metamaterials", Physical Review Letters 105, 235501 (2010) [PDF]
[23] Y. M. Liu*, S. Wang*, Y. Park*, X. B. Yin and X. Zhang, "Fluorescence enhancement by a two-dimensional dielectric annular Bragg resonant cavity", Optics Express 18, 25029 (2010) [PDF]
[22] Y. M. Liu, T. Zentgraf, G. Bartal and X. Zhang, "Transformational plasmon optics", Nano Letters 10, 1991(2010) [PDF]

Taking advantage of transformation optics, we demonstrate that the confinement as well as propagation of surface plasmons can be managed in a prescribed manner by careful control of the dielectric material properties adjacent to a metal. Since the metal properties are completely unaltered, it provides a straightforward way for practical realizations. We show that our approach can assist to tightly bound SPPs over a broad wavelength band at uneven and curved surfaces, where SPPs would normally suffer significant scattering losses. In addition, a plasmonic waveguide bend and a plasmonic Luneburg lens with practical designs are proposed.

[21] M. Liu, T. Zentgraf, Y. M. Liu, G. Bartal and X. Zhang, "Light-driven nanoscale plasmonic motors", Nature Nanotechnology 5, 570 (2010) [PDF]

Photons have both linear and angular momentum. The transfer of linear momentum from photons to an object results in an optical force that can be used for optical trappingand cooling. Also, the angular momentum carried by photons can induce a mechanical torque via scattering or absorbance. The ability to generate strong rotational force at the nanoscale could open up a range of applications in physics, biology and chemistry, including DNA unfolding and sequencing and nanoelectromechanical systems. Here, we demonstrate a nanoscale plasmonic structure that can, when illuminated with linearly polarized light, generate a rotational force that is capable of rotating a silica microdisk that is 4,000 times larger in volume. Furthermore, we can control the rotation velocity and direction by varying the wavelength of incident light to excite different plasmonic modes.

[20] S. Zhang, Y. S. Park, Y. M. Liu, T. Zentgraf and X. Zhang, "Far-field measurement of ultra-small plasmonic mode volume", Optics Express 18, 6048 (2010) [PDF]
[19] Y. Gao, J. P. Huang, Y. M. Liu, L. Gao, K. W. Yu and X. Zhang, "Optical negative refraction in ferrofluids with magnetocontrollability", Physical Review Letters 104, 034501 (2010) [PDF]
[18] H. Liu, Y. M. Liu, T. Li, S. M. Wang, S. N. Zhu and X. Zhang, "Coupled magnetic plasmons in metamaterials" (invited review paper), Physica Status Solidi B 246, 1397 (2009) [PDF]
[17] J. Yao*, Z. W. Liu*, Y. M. Liu*, Y. Wang, C. Sun, G. Bartal, A. Stacy and X. Zhang, "Optical negative refraction in bulk metamaterials of nanowires", Science 321, 930 (2008) [PDF]

Photons have both linear and angular momentum. The transfer of linear momentum from photons to an object results in an optical force that can be used for optical trappingand cooling. Also, the angular momentum carried by photons can induce a mechanical torque via scattering or absorbance. The ability to generate strong rotational force at the nanoscale could open up a range of applications in physics, biology and chemistry, including DNA unfolding and sequencing and nanoelectromechanical systems. Here, we demonstrate a nanoscale plasmonic structure that can, when illuminated with linearly polarized light, generate a rotational force that is capable of rotating a silica microdisk that is 4,000 times larger in volume. Furthermore, we can control the rotation velocity and direction by varying the wavelength of incident light to excite different plasmonic modes.

[16] Y. M. Liu, G. Bartal and X. Zhang, "All-angle negative refraction and imaging in a bulk medium made of metallic nanowires in the visible region", Optics Express 16, 15439 (2008) [PDF]
[15] Y. M. Liu, G. Bartal, D. A. Genov and X. Zhang, "Subwavelength discrete solitons in nonlinear metamaterials", Physical Review Letters 99, 153901 (2007) [PDF]

We present the first study of subwavelength discrete solitons in nonlinear metamaterials: nanoscaled periodic structures consisting of metal and nonlinear dielectric slabs. The solitons supported by such media result from a balance between tunneling of surface plasmon modes and nonlinear self-trapping. The dynamics in such systems, arising from the threefold interplay between periodicity, nonlinearity, and surface plasmon polaritons, is substantially different from that in conventional nonlinear dielectric waveguide arrays. We expect these phenomena to inspire fundamental studies as well as potential applications of nonlinear metamaterials, particularly in subwavelength nonlinear optics.

[14] H. Liu, D. A. Genov, D. M. Wu, Y. M. Liu, Z. W. Liu, C. Sun, S. N. Zhu and X. Zhang, "Magnetic plasmon hybridization and optical activity at optical frequencies in metallic nanostructures", Physical Review B 76, 073101 (2007) [PDF]
[13] R. F. Oulton, D. F. P. Pile, Y. M. Liu and X. Zhang, "Scattering of surface plasmon polaritons at abrupt surface interfaces: Implications for nanoscale cavities", Physical Review B 76, 035408 (2007) [PDF]
[12] W. Wu, E. Kim, E. Ponizovskaya, Y. M. Liu, Z. N. Yu , N. Fang, Y. R. Shen, A. M. Bratkovsky, W. Tong, C. Sun, X. Zhang , S. Y. Wang and R. S. Williams, "Optical metamaterials at near and mid-IR range fabricated by nanoimprint lithography", Applied Physics A 87, 143 (2007) [PDF]
[11] W. Wu*, Y. M. Liu*, E. Kim*, Z. N. Yu, N. Fang, C. Sun, X. Zhang, Y. R. Shen, S. Y. Wang and R. S. Williams, "Midinfrared metamaterials fabricated by nanoimprint lithography", Applied Physics Letters 90, 063107(2007) [PDF]

A metamaterial comprising an ordered array of four metallic L-shaped components designed to operate in the mid-IR frequency regime has been fabricated and characterized. The fourfold rotational symmetry of the unit cell should suppress the undesirable bianisotropy observed for split-ring resonators. Nanoimprint lithography was used to demonstrate scalability for mass production. A dipole plasmon resonance with a negative permittivity and a magnetic resonance with a negative permeability were observed at wavelengths of 3.7 and 5.25 micro-meter, respectively, in agreement with theoretical predictions.

[10] Y. M. Liu, N. Fang, C. Sun and X. Zhang, "Symmetric and antisymmetric modes of electromagnetic resonators", Applied Physics A 87, 171 (2007) [PDF]
[9] H. Liu, D. A. Genov, D. M. Wu, Y. M. Liu, J. M. Steele, C. Sun, S. N. Zhu and X. Zhang, "Magnetic plasmon propagation along a chain of connected subwavelength resonators at infrared frequencies", Physical Review Letters 97, 243902 (2006) [PDF]
[8] X. F. Hu, Z. H. Peng, R. W. Peng, Y. M. Liu, F. Qiu, X. Q. Huang, A. Hu and S. S. Jiang, "Electronic delocalization and persistent currents in nonsymmetric-dimer mesoscopic rings threaded by magnetic flux", Journal of Applied Physics 95, 7545 (2004) [PDF]
[7] R. W. Peng, Y. M. Liu, X. Q. Huang, M. Wang, A. Hu, S. S. Jiang, D. Feng, L. Z. Ouyang and J. Zou, "Dimerlike positional correlation and resonant transmission of electromagnetic waves in aperiodic dielectric multilayers", Physical Review B 69, 165109 (2004) [PDF]
[6] X. Q. Huang, S. S. Jiang, R. W. Peng, Y. M. Liu, F. Qiu and A. Hu, "Characteristic wavefunctions of one-dimensional periodic, quasiperiodic and random lattices", Modern Physics Letters B 17, 1461 (2003) [PDF]
[5] F. Qiu, R. W. Peng, X. Q. Huang, Y. M. Liu, M. Wang, A. Hu and S. S. Jiang, "Resonant transmission and frequency trifurcation of light waves in Thue-Morse dielectric multilayers", Europhysics Letters 63, 853 (2003) [PDF]
[4] R. W. Peng, X. Q. Huang, F. Qiu, Y. M. Liu, A. Hu and S. S. Jiang, "Structural symmetry and optical properties of dielectric multilayers", Surface Review and Letters 10, 311 (2003) [PDF]
[3] Y. M. Liu, R. W. Peng, X. Q. Huang, M. Wang, A. Hu and S. S. Jiang, "Resonant scattering in random polymer chains with inversely symmetric impurities", Physical Review B 67, 205209 (2003) [PDF]
[2] Y. M. Liu, R. W. Peng, X. Q. Huang, M. Wang, A. Hu and S. S. Jiang, "Absence of suppression in the persistent current by delocalization in random-dimer mesoscopic rings", Journal of the Physical Society of Japan 72, 346 (2003) [PDF]
[1] Y. M. Liu, R. W. Peng, G. J. Jin, X. Q. Huang, M. Wang, A. Hu and S. S. Jiang, "Persistent currents in k-component Fibonacci mesoscopic rings threaded by a magnetic flux", Journal of Physics: Condensed Matter 14, 7253 (2002) [PDF]
*First authors with equal contributions.

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