Unconventional ferroelectricity in half-filling states of antiparallel stacking of twisted WSe_(2)

We report on emergence of an abnormal electronic polarization in twisted double bilayer WSe_(2) in antiparallel interface stacking geometry,where local centrosymmetry of atomic registries at the twist interface does not favor the spontaneous electronic polarizations as recently observed in the parallel interface stacking geometry.The unconventional ferroelectric behaviors probed by electronic transport measurement occur at half filling insulating states at 1.5 K and gradually disappear at about 40 K.Single band Hubbard model based on the triangular moirélattice and the interlayer charge transfer controlled by insulating phase transition are proposed to interpret the formation of electronic polarization states near half filling in twisted WSe_(2) devices.Our work highlights the prominent role of many-body electronic interaction in fostering novel quantum states in moiré-structured systems.

Nonlinear terahertz devices utilizing semiconducting plasmonic metamaterials

The development of responsive metamaterials has enabled the realization of compact tunable photonic devices capable of manipulating the amplitude,polarization,wave vector and frequency of light.Integration of semiconductors into the active regions of metallic resonators is a proven approach for creating nonlinear metamaterials through optoelectronic control of the semiconductor carrier density.Metal-free subwavelength resonant semiconductor structures offer an alternative approach to create dynamic metamaterials.We present InAs plasmonic disk arrays as a viable resonant metamaterial at terahertz frequencies.Importantly,InAs plasmonic disks exhibit a strong nonlinear response arising from electric field-induced intervalley scattering,resulting in a reduced carrier mobility thereby damping the plasmonic response.We demonstrate nonlinear perfect absorbers configured as either optical limiters or saturable absorbers,including flexible nonlinear absorbers achieved by transferring the disks to polyimide films.Nonlinear plasmonic metamaterials show potential for use in ultrafast terahertz(THz)optics and for passive protection of sensitive electromagnetic devices.

Revealing the intrinsic nonlinear optical response of a single MoS_(2) nanosheet in a suspension based on spatial self-phase modulation

The reorientation of 2D materials caused by nonlocal electron coherence is the formation mechanism of 2D material spatial self-phase modulation under laser irradiation,which is widely known as the“wind-chime”model.Here,we present a method that provides strong evidence for the reorientation of 2D-material-induced spatial self-phase modulation.The traditional“wind-chime”model was modified by taking into account the attenuation,i.e.,damping of the incident light beam in the direction of the optical path.Accordingly,we can extract the nonlinear refractive index of a single MoS2 nanosheet,instead of simply obtaining the index from an equivalent MoS2 film that was constructed by all nanosheets.Our approach introduces a universal and accurate method to extract intrinsic nonlinear optical parameters from 2D material systems.

Effect of concentration on the formation time of diffraction rings in spatial self-phase modulation

A new unsaturated wind-chime model is proposed for calculating the formation time of the diffraction rings induced by spatial self-phase modulation(SSPM) in molybdenum disulfide suspension.To optimize the traditional wind-chime model,the concentration variable of 2 D materials was introduced.The results of the unsaturated wind-chime model match quite well with the SSPM experimental results of molybdenum disulfide.Based on this model,the shortest formation time of diffraction rings and their corresponding concentration and light intensity can be predicted using limited data.Theoretically,by increasing the viscosity coefficient of the solution,the response time of the diffraction ring,to reach the maximum value,can be significantly reduced.It has advanced significance in shortening the response time of photonic diodes.

Voltage-tunable dual-layer terahertz metamaterials

This paper presents the design,fabrication,and characterization of a real-time voltage-tunable terahertz metamaterial based on microelectromechanical systems and broadside-coupled split-ring resonators.In our metamaterial,the magnetic and electric interactions between the coupled resonators are modulated by a comb-drive actuator,which provides continuous lateral shifting between the coupled resonators by up to 20μm.For these strongly coupled split-ring resonators,both a symmetric mode and an anti-symmetric mode are observed.With increasing lateral shift,the electromagnetic interactions between the split-ring resonators weaken,resulting in frequency shifting of the resonant modes.Over the entire lateral shift range,the symmetric mode blueshifts by~60 GHz,and the anti-symmetric mode redshifts by~50 GHz.The amplitude of the transmission at 1.03 THz is modulated by 74%;moreover,a 180°phase shift is achieved at 1.08 THz.Our tunable metamaterial device has myriad potential applications,including terahertz spatial light modulation,phase modulation,and chemical sensing.Furthermore,the scheme that we have implemented can be scaled to operate at other frequencies,thereby enabling a wide range of distinct applications.

Nonlinear terahertz metamaterial perfect absorbers using GaAs[Invited]

We investigate the nonlinear response of terahertz(THz) metamaterial perfect absorbers consisting of electric split ring resonators on GaAs integrated with a polyimide spacer and gold ground plane. These perfect absorbers on bulk semi-insulating GaAs are characterized using high-field THz time-domain spectroscopy. The resonance frequency redshifts 20 GHz and the absorbance is reduced by 30% as the incident peak field is increased from 30 to 300 kV/cm. The nonlinear response arises from THz field driven interband transitions and intervalley scattering in the GaAs. To eliminate the Fresnel losses from the GaAs substrate, we design and fabricate a flexible metamaterial saturable perfect absorber. The ability to create nonlinear absorbers enables appealing applications such as optical limiting and self-focusing.