Optical Metamaterials and Metasurfaces

Metamaterials had enormous promise for realizing science fantasy and changing the world. The current demonstration of metamaterials is based on high-resolution nanofabrication, which requires costly processes and restricted resources. We are undertaking material research on low-loss hydrogenated amorphous silicon, printable nanoparticle embedded resin, and anodic aluminum oxide to overcome the constraints of conventional nanofabrication. We are also investigating multilayer deposition for experimental demonstration and hyperlens applications. Furthermore, we are doing cutting-edge bottom-up nanofabrication employing self-assembly, nanoparticles, block co-polymers, and other materials to apply metamaterials and plasmonics applications.

Flat Optics and Metaphotonics

Metasurfaces are two-dimensional networks of subwavelength optical antenna arrays that can be modulated by the shape, geometry, size, orientation, and arrangement of each antenna to manipulate the output amplitude, phase, and polarization of light. Metasurfaces provide unprecedented opportunities to overcome the limitations of conventional optics and have demonstrated promising applications such as ultrathin achromatic metalens and high-resolution multicolor holograms for AR/VR devices, full-space structural light metasurface for wide field-of-view three-dimensional imaging (LiDAR), spin-Hall effect enhancing metasurface for spin-dependent control, and fiber integrated metalens for endoscopy.

Active Nanophotonics and Plasmonics

Many intriguing dynamic mechanisms and active materials bring up a new avenue for tunable nanophotonic devices that change their optical responses in reaction to external stimuli such as chemicals, heat, or electricity. We are attempting to make more practical and functional photonics devices using a variety of tunable mechanisms ranging from conventional liquid crystals to state-of-the-art 2D materials. Electric tuning mechanisms, in particular, will bring metasurfaces closer to a commercially feasible platform.

Acoustic, Elastic, Mechanical and Seismic Metamaterials

Inspired by intriguing phenomena from solid state physics and photonics, the classical waves, i.e., mechanical waves can be much developed through coupling applied physics in a new way. Mechanical waves based on the stress field, in particular, exhibit unique coupling phenomena in elasticity governed by tensor fields. This gives diverse polarization and wave features ranging from linear to highly nonlinear regimes. We are especially interested in the analytic lattice dynamics of wave dynamics of periodic structures made up of discrete particles or continuums.

Topological Photonics and Non-Hermitian Photonics

Topology is a mathematical branch dealing with the conserved quantities which examine the classification of a manifold. Combining with topology in mathematics and band theory in electronics, topological photonics become a powerful methodology to understand new phases of matters in photonics. The periodic array of 2D or 3D artificial engineered metamaterials with topologically non-trivial dispersion can exhibit hitherto unknown optical properties such as robustness to optical loss and defect or spin-dependent light propagation. We are studying and engineering the new phase of photonic matter in a topological fashion. On the other hand, optical loss was regarded as a fatal impediment in device engineering, and it was often overlooked in theoretical analyses. However, with lossy materials, the photonic system can exhibit unfamiliar phenomena such as unidirectional invisibility and eigenstates coalescing at a singular point known as the exceptional point (EP). These are driven by the non-Hermitian features of the Hamiltonian or scattering matrix in a lossy system. We are developing novel photonic devices such as parity-time gratings and non-Hermitian metasurfaces by employing non-Hermitian physics.

Quantum Photonics

Recently, the integration of quantum emitters with nanoresonators has emerged in quantum photonics. Optical phenomena from quantum emitters such as single-photon emission and photoluminescence can be modulated by nanoresonators. We are investigating the interaction between quantum emitters and nanoresonators for promising applications like FRET, lasing and directional photoluminescence.

Nanofabrication and Nanomanufacturing

While metamaterials are frequently used in advanced optical devices, fabrication processes have down sides such as poor throughput, small fabrication areas, and difficult post-processing. Thus, we are probing alternative nanofabrication techniques such as electron beam lithography (EBL), focused ion beam (FIB), particle-embedded resin nanoimprint (PER-NIL), atomic layer deposition (ALD), two-photon polymerization, and three-dimensional aerosol nanoprinting for high-precision or scalable realization of metamaterials. In addition, we are investigating conventional EBL for ultrahigh-precision nanostructures and vertically stacked metamaterials.

Design and Optimization (Artificial Intelligence, Machine Learning, Deep Learning)

Metasurfaces are sub-wavelength periodic structures made of metal/dielectrics for various optical applications. The wave-modulating characteristics of these materials are generally determined by their structural features. Optimization strategies based on gradient descent methods have been suggested to obtain the desired optical phenomena. Deep learning-based strategies are the most recent ways for bridging structural features and optical phenomena. Our members are primarily interested in developing deep learning models for the purpose of designing core-shell structures, meta-antennas, and free-shaped meta-reflectors/absorbers, and more.

Nanophotonic Device Applications

Our group has demonstrated hyperlens imaging beyond the diffraction limit by using the features of abnormally high wavevector access. Its superior capabilities, which include super-resolution, real-time imaging, and a non-vacuum working environment, open up a new horizon for practical nanoscale biological imaging. We are also interested in working on anti-counterfeiting, daytime radiative cooling, structural colors, and optical sensors. In this manner, nanophotonics devices are being used in a wide range of real-world applications, and the range of applications is growing. Such efforts to make metamaterials and plasmonics more feasible continue to realize metadevices. Another interest is integrating metamaterials into the MEMS/NEMS devices to realize reconfigurable and actively controllable metadevices.