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A periodic metagate is designed on top of a boron nitride-graphene heterostructure to modulate the local carrier density distribution on the monolayer graphene. This causes the bandgaps of graphene surface plasmon polaritons to emerge because of either the interaction between the plasmon modes, which are mediated by the varying local carrier densities, or their interaction with the metal gates. Using the example of a double-gate graphene device, we discuss the tunable band properties of graphene plasmons due to the competition between these two mechanisms. Because of this, a bandgap inversion, which results in a Zak phase switching, can be realized through electrostatic gating. Here we also show that an anisotropic plasmonic topological edge state exists at the interface between two graphene gratings of different Zak phases. While the orientation of the dipole moments can differentiate the band topologies of each graphene grating, the angle of radiation remains a tunable property. This may serve as a stepping stone toward active control of the band structures of surface plasmons for potential applications in optical communication, wave steering, or sensing.

The nature of trions and their interaction with light has remained a puzzle. The composition and dispersion of polaritons involving trions provide insights into this puzzle. Trions and excitons in doped two-dimensional (2D) materials are not independent excitations but are strongly coupled as a result of Coulomb interactions. When excitons in doped 2D materials are also strongly coupled with light inside an optical waveguide, the resulting polariton states are coherent superpositions of exciton, trion, and photon states. We realize these exciton-trion-polaritons by coupling an electron-doped monolayer of two-dimensional material MoSe2 to the optical mode in a photonic crystal waveguide. Our theoretical model, based on a many-body description of these polaritons, reproduces the measured polariton energy band dispersion and Rabi splittings with excellent accuracy. Our work sheds light on the structure of trion states in 2D matrials and also on the indirect mechanism by which they interact with light., Comment: 12 Pages, 5 Figures, and one Supplementary Material Section. Slightly modified from the previous version

When light-matter polaritons travel in periodic media, they acquire properties akin to Bloch quasi-particles in crystals., Efficient control of photons is enabled by hybridizing light with matter. The resulting light-matter quasi-particles can be readily programmed by manipulating either their photonic or matter constituents. Here, we hybridized infrared photons with graphene Dirac electrons to form surface plasmon polaritons (SPPs) and uncovered a previously unexplored means to control SPPs in structures with periodically modulated carrier density. In these periodic structures, common SPPs with continuous dispersion are transformed into Bloch polaritons with attendant discrete bands separated by bandgaps. We explored directional Bloch polaritons and steered their propagation by dialing the proper gate voltage. Fourier analysis of the near-field images corroborates that this on-demand nano-optics functionality is rooted in the polaritonic band structure. Our programmable polaritonic platform paves the way for the much-sought benefits of on-the-chip photonic circuits.

Optical absorption and emission spectra of doped two-dimensional (2D) materials exhibit sharp peaks that are often identified with pure excitons and pure trions (or charged excitons), but both peaks have been recently attributed to superpositions of 2-body exciton and 4-body trion states and correspond to the approximate energy eigenstates in doped 2D materials. In this paper, we present the radiative lifetimes of these exciton-trion superposition energy eigenstates using a many-body formalism that is appropriate given the many-body nature of the strongly coupled exciton and trion states in doped 2D materials. Whereas the exciton component of these superposition eigenstates are optically coupled to the material ground state, and can emit a photon and decay into the material ground state provided the momentum of the eigenstate is within the light cone, the trion component is optically coupled only to the excited states of the material and can emit a photon even when the momentum of the eigenstate is outside the light cone. In an electron-doped 2D material, when a 4-body trion state with momentum outside the light cone recombines radiatively, and a photon is emitted with a momentum inside the light cone, the excess momentum is taken by an electron-hole pair left behind in the conduction band. The radiative lifetimes of the exciton-trion superposition states, with momenta inside the light cone, are found to be in the few hundred femtoseconds to a few picoseconds range and are strong functions of the doping density. The radiative lifetimes of exciton-trion superposition states, with momenta outside the light cone, are in the few hundred picoseconds to a few nanoseconds range and are again strongly dependent on the doping density., 12 pages, 9 figures, some typos corrected from the previous version

We point out that in non-Hermitian systems, Jordan decomposition should replace eigendecomposition so that all ``good approximate eigenstates'' of the system are identified. These states can be resonantly excited. As a concrete example, we study the location and field distribution of zero-energy corner states in a non-Hermitian quadrupole insulator (QI) and split the parameter space into three distinct regimes according to properties of the corner states: near-Hermitian QI, intermediate phase, and trivial insulator. In the newly discovered intermediate phase, the Hamiltonian becomes defective, and the counterintuitive and delicate response of the system to external drives can be perfectly explained by the Jordan decomposition.

We present a many-body theory of exciton-trion polaritons (ETPs) in doped two-dimensional semiconductor materials. ETPs are robust coherent hybrid excitations involving excitons, trions, and photons. In ETPs, the 2-body exciton states are coupled to the material ground state via exciton-photon interaction, and the 4-body trion states are coupled to the exciton states via Coulomb interaction. The trion states are not directly optically coupled to the material ground state. The energy-momentum dispersion of ETPs exhibit three bands. We calculate the energy band dispersions and the compositions of ETPs at different doping densities using Green's functions. The energy splittings between the polariton bands, as well as the spectral weights of the polariton bands, depend on the strength of the Coulomb coupling between the excitons and the trions, which in turn depends sensitively on the doping density. The doping density dependence of the ETP bands and the charged nature of the trion states could enable novel electrical and optical control of ETPs.

Polarization is one of the important properties of light, and its detection is of significant interest for various fundamental and practical applications. We demonstrate a mid-infrared polarimetry device using a gate-tunable graphene-integrated anisotropic metasurface. The Stokes parameters of the incident light are extracted by sweeping the gate voltage applied to the device and subsequent fitting of the measured reflected intensities. Considering subpicosecond carrier relaxation times in graphene, the polarization measurement rate of our device is governed only by the speed of the gate voltage sweep. Thus, our work serves as a proof-of-principle demonstration for high-speed microscale polarimetric devices.

We demonstrate that the presence of a localized state at the corner of an insulating domain is not always a predictor of a certain non-trivial higher-order topological invariant, even though they appear to co-exist in the same Hamiltonian parameter space. Our analysis of $C^n$-symmetric crystalline insulators and their multi-layer stacks reveals that topological corner states are not necessarily correlated with other well-established higher-order boundary observables, such as fractional corner charge or filling anomaly. In a $C^3$-symmetric breathing Kagome lattice, for example, we show that the bulk polarization, which successfully predicts the fractional corner anomaly, fails to be the relevant topological invariant for zero-energy corner states; instead, these corner states can be exactly explained by the decoration of topological edges. Also, while the zero-energy corner states in $C^4$-symmetric topological crystalline insulators have long been conjectured to be the result of the bulk polarization at quarter-filling, we correct this misconception by introducing a proper bulk invariant at half-filling and establishing a precise bulk-corner correspondence. By refining several bulk-corner correspondences in two-dimensional topological crystalline insulators, our work motivates further development of rigorous theoretical grounds for associating the existence of corner states with higher-order topology of host materials., Comment: 10 pages, 10 figures, lastly re-uploaded on 4/6/21

Photonic crystals are commonly implemented in media with periodically varying optical properties. Photonic crystals enable exquisite control of light propagation in integrated optical circuits, and also emulate advanced physical concepts. However, common photonic crystals are unfit for in-operando on/off controls. We overcome this limitation and demonstrate a broadly tunable two-dimensional photonic crystal for surface plasmon polaritons. Our platform consists of a continuous graphene monolayer integrated in a back-gated platform with nano-structured gate insulators. Infrared nano-imaging reveals the formation of a photonic bandgap and strong modulation of the local plasmonic density of states that can be turned on/off or gradually tuned by the applied gate voltage. We also implement an artificial domain wall which supports highly confined one-dimensional plasmonic modes. Our electrostatically-tunable photonic crystals are derived from standard metal oxide semiconductor field effect transistor technology and pave a way for practical on-chip light manipulation., Traditional photonic crystals consist of periodic media with a pre-defined optical response. Here, the authors combine nanostructured back-gate insulators with a continuous layer of graphene to demonstrate an electrically tunable two-dimensional photonic crystal suitable for controlling the propagation of surface plasmon polaritons.

Color digital holography (DH) has been researched in various fields such as the holographic camera and holographic microscope because it acquires a realistic color object wave by measuring both amplitude and phase. Among the methods for color DH, the phase-shifting DH has an advantage of obtaining a signal wave of objects without the autocorrelation and conjugate noises. However, this method usually requires many interferograms to obtain signals for all wavelengths. In addition, the phase-shift algorithm is sensitive to the phase-shift error caused by the instability or hysteresis of the phase shifter. In this paper, we propose a new method of color phase-shifting digital holography with monitoring the phase-shift. The color interferograms are recorded by using a focal plane array (FPA) with a Bayer color filter. In order to obtain the color signal wave from the interferograms with unexpected phase-shift values, we devise a generalized phase-shifting DH algorithm. The proposed method enables the robust measurement in the interferograms. Experimentally, we demonstrate the proposed algorithm to reconstruct the object image with negligibly small conjugate noises.

An energy-area efficient IVN-connected software execu tion architecture in ECU processor is proposed. ECU processor is commonly implemented using the conventional embedded process or only providing the fixed services, which includes statically com piled embedded software in on-chip flash memory. Instead of con ventional on–chip flash memory for an instruction code area, we a dopt a virtually mapped internal memory concept to realize IVN- connected software execution, in where the storage area of VCU i s indirectly mapped onto the physical address space of the instruc tion memory using a binary translation. The proposed IVN-conne cted architecture of the system enables on-demand code execution for the instructions, which are fetched from the IVN-side storage area of VCU in the runtime, instead of using a directly-connected on-chip instruction bus. The proposed storage-less approach may be adopted to reduce the high access current and large chip area o verhead by in order to retrieve the requested instruction, a small- sized RAM scratch pad is adopted for retaining the hot-spot instr uction code and early filled with pre-estimated instruction sector. As the result of this paper, we show that the proposed technique r educes the energy consumption and packet delay of ECU for exec uting the embedded software, as well as the reduced chip area by realizing a storage-less ECU architecture.

A valley plasmonic crystal for graphene surface plasmons is proposed. We demonstrate that a designer metagate, placed within a few nanometers of graphene, can be used to impose a periodic Fermi energy landscape on graphene. For specific metagate geometries and bias voltages, the combined metagate-graphene structure is shown to produce complete propagation band gaps for the plasmons, and to impart them with nontrivial valley-linked topological properties. Sharply curved domain walls between differently patterned metagates are shown to guide highly localized plasmons without any reflections owing to suppressed intervalley scattering. Our approach paves the way for nonmagnetic and dynamically reconfigurable topological nanophotonic devices.

This paper proposes the development of cylindrical diffusing optical fiber probe (CDOFP), which possesses a flexible structure and solidity apt to treat cancer during photo-dynamic therapy (PDT). The cylindrical diffusing tip of CDOFP is made from a laser processing system with homogeneity and precision. We fabricated and characterized CDOFP to investigate its clinical feasibility and other possible applications of light therapy using the diffusing optical fiber.

We propose a electrically tunable plasmonic platform for valleytronics—a mono-layer graphene with its chemical potential controlled by a periodically structured metagate. Topologically protected edge states show reflection-free propagation around sharp corners.

We introduce an electromagnetic metamaterial - a spin-valley photonic topological insulator (SV-PTI) - that emulates a wide class of theoretically predicted gapped two-dimensional materials with antiferromagnetic order coupled to the valley degree of freedom. First-principles electromagnetic simulations and an analytic model reveal that a single parameter controls the propagation properties of the chiral states at the domain wall between two SV-PTIs: their group velocity, polarization, and the existence/absence of topological protection. The latter is determined by the geometry of the line of the least frequency separation between the bandgap-forming propagation bands. Topologically-protected compression of electromagnetic energy via group velocity control is enabled by SV-PTIs.