Interlayer sensitized van der Waals heterojunction photodetector with enhanced performance

Although photodetection based on two-dimensional(2D)van der Waals(vdWs)P-N heterojunction has attracted extensive attention recently,their low responsivity(R)due to the lack of carrier gain mechanism in reverse bias or zero bias operation hinders their applications in advanced photodetection area.Here,a black phosphorus/rhodamine 6G/molybdenum disulfide(BP/R6G/MoS_(2))photodiode with high responsivity at reverse bias or zero bias has been achieved by using interfacial charge transfer of R6G molecules assembled between heterojunction layers.The formed vdWs interface achieves high performance photoresponse by efficiently separating the additional photogenerated electrons and holes generated by R6G molecules.The devices sensitized by the dye molecule R6G exhibit enhanced photodetection performance without sacrificing the photoresponse speed.Among them,the R increased by 14.8-20.4 times,and the specific detectivity(D^(*))increased by 24.9-34.4 times.The strategy based on interlayer assembly of dye molecules proposed here may pave a new way for realizing high-performance photodetection based on 2D vdWs heterojunctions with high responsivity and fast response speed.

Giant photonic spin Hall effect in momentum space in a structured metamaterial with spatially varying birefringence

The photonic spin Hall effect(SHE)in the reflection and refraction at an interface is very weak because of the weak spin-orbit interaction.Here,we report the observation of a giant photonic SHE in a dielectric-based metamaterial.The metamaterial is structured to create a coordinate-dependent,geometric Pancharatnam–Berry phase that results in an SHE with a spin-dependent splitting in momentum space.It is unlike the SHE that occurs in real space in the reflection and refraction at an interface,which results from the momentum-dependent gradient of the geometric Rytov–Vladimirskii–Berry phase.We theorize a unified description of the photonic SHE based on the two types of geometric phase gradient,and we experimentally measure the giant spin-dependent shift of the beam centroid produced by the metamaterial at a visible wavelength.Our results suggest that the structured metamaterial offers a potential method of manipulating spin-polarized photons and the orbital angular momentum of light and thus enables applications in spin-controlled nanophotonics.

Cylindrical vector beam multiplexer/demultiplexer using off-axis polarization control

The emergence of cylindrical vector beam(CVB)multiplexing has opened new avenues for high-capacity optical communication.Although several configurations have been developed to couple/separate CVBs,the CVB multiplexer/demultiplexer remains elusive due to lack of effective off-axis polarization control technologies.Here we report a straightforward approach to realize off-axis polarization control for CVB multiplexing/demultiplexing based on a metal–dielectric–metal metasurface.We show that the left-and right-handed circularly polarized(LHCP/RHCP)components of CVBs are independently modulated via spin-to-orbit interactions by the properly designed metasurface,and then simultaneously multiplexed and demultiplexed due to the reversibility of light path and the conservation of vector mode.We also show that the proposed multiplexers/demultiplexers are broadband(from 1310 to 1625 nm)and compatible with wavelength-division-multiplexing.As a proof of concept,we successfully demonstrate a four-channel CVB multiplexing communication,combining wavelength-division-multiplexing and polarization-division-multiplexing with a transmission rate of 1.56 Tbit/s and a bit-error-rate of 10^(−6) at the receive power of−21.6 dBm.This study paves the way for CVB multiplexing/demultiplexing and may benefit high-capacity CVB communication.

Orbital angular momentum mode logical operation using optical diffractive neural network

Optical logical operations demonstrate the key role of optical digital computing,which can perform general-purpose calculations and possess fast processing speed,low crosstalk,and high throughput.The logic states usually refer to linear momentums that are distinguished by intensity distributions,which blur the discrimination boundary and limit its sustainable applications.Here,we introduce orbital angular momentum(OAM)mode logical operations performed by optical diffractive neural networks(ODNNs).Using the OAM mode as a logic state not only can improve the parallel processing ability but also enhance the logic distinction and robustness of logical gates owing to the mode infinity and orthogonality.ODNN combining scalar diffraction theory and deep learning technology is designed to independently manipulate the mode and spatial position of multiple OAM modes,which allows for complex multilight modulation functions to respond to logic inputs.We show that few-layer ODNNs successfully implement the logical operations of AND,OR,NOT,NAND,and NOR in simulations.The logic units of XNOR and XOR are obtained by cascading the basic logical gates of AND,OR,and NOT,which can further constitute logical half-adder gates.Our demonstrations may provide a new avenue for optical logical operations and are expected to promote the practical application of optical digital computing.

Improved common-path spectral interferometer for single-shot terahertz detection

To seek high signal-to-noise ratio(SNR) is critical but challenging for single-shot intense terahertz(THz)coherent detection. This paper presents an improved common-path spectral interferometer for single-shot THz detection with a single chirped pulse as the probe for THz electro-optic(EO) sampling. Here, the spectral interference occurs between the two orthogonal polarization components with a required relative time delay generated with only a birefringent plate after the EO sensor. Our experiments show that this interferometer can effectively suppress the noise usually suffered in a non-common-path interferometer. The measured single-shot SNR is up to 88.85, and the measured THz waveforms are independent of the orientation of the used Zn Te EO sensor, so it is easy to operate and the results are more reliable. These features mean that the interferometer is quite qualified for applications where strong THz pulses, usually with single-shot or low repetition rate, are indispensable.

Controllable photonic spin Hall effect with phase function construction

Photonic spin Hall efect(SHE)provides new opportunities for achieving spin-based photonics applications.However,flexibly manipulating the spin-dependent sltting(SDS)of photonic SHE and imposing extra phase modulation on the two spin components are always a challenge.Here,a controllable SHE mechanism based on phase function construction is reported.It is conduded that the phases with specific functional structures performing a coordinate translation are equivalent to integrating a gradient phase to the original phases.Hence,the original phase can be used for independent phase modulation,and the gradient phase originating from the co-ordinate translation is capable of manipulating the SDS.A metasurface with Pancharatnam-Berry phase that can impose conjugate phases to the two spin components of light is fabricated to verify this mechanism.By shifing the light position,the SDS is continuously manipulated in the visible region,which is successfully used for detecting the polarization llipticity.The extra phase modulation is also performed with the original phase and thus enables measuring singular beams.It is anticipated that the controllable SHE manipulation method may open new avenues in the fields of spin photonics,optical sensing,optical communications,etc.

Experimental realization to efficiently sort vector beams by polarization topological charge via Pancharatnam–Berry phase modulation

This paper reports the experimental realization of efficiently sorting vector beams by polarization topological charge(PTC). The PTC of a vector beam can be defined as the repetition number of polarization state change along the azimuthal axis, while its sign stands for the rotating direction of the polarization. Here, a couple of liquid crystal Pancharatnam–Berry optical elements(PBOEs) have been used to introduce conjugated spatial phase modulations for two orthogonal circular polarization states. Applying these PBOEs in a 4-f optical system,our experiments show the setup can work for PTC sorting with a separation efficiency of more than 58%. This work provides an effective way to decode information from different PTCs, which may be interesting in many fields, especially in optical communication.

Forty-five terawatt vortex ultrashort laser pulses from a chirped-pulse amplification system

We report on a vortex laser chirped-pulse amplification(CPA)system that delivers pulses with a peak power of 45 TW.A focused intensity exceeding 1019 W/cm2 has been demonstrated for the first time by the vortex amplification scheme.Compared with other schemes of strong-field vortex generation with high energy flux but narrowband vortex-converting elements at the end of the laser,an important advantage of our scheme is that we can use a broadband but size-limited q-plate to realize broadband mode-converting in the front end of the CPA system,and achieve high-power amplification with a series of amplifiers.This method is low cost and can be easily implemented in an existing laser system.The results have verified the feasibility to obtain terawatt and even petawatt vortex laser amplification by a CPA system,which has important potential applications in strong-field laser physics,for example,generation of vortex particle beams with orbital angular momentum,fast ignition for inertial confinement fusion and simulation of the extreme astrophysical environment.

Ultrafast nonlinear absorption and nonlinear refraction in few-layer oxidized black phosphorus

We experimentally investigated the nonlinear optical response in few-layer oxidized black phosphorus(OBP) by the femtosecond Z-scan measurement technique, and found that OBP not only possesses strong ultrafast saturable absorption but also a nonlinear self-defocusing effect that is absent in black phosphorus(BP). The saturable absorption property originates mainly from the direct band structure, which is still maintained in OBP. The emergence of self-defocusing might originate from the combined consequences of the oxygen-induced defects in BP. Our experimental findings might constitute the first experimental evidence on how to dynamically tune its nonlinear property, offering an inroad in tailoring its optical properties through chemical modification(oxidation, introducing defects, etc.). The versatile ultrafast nonlinear optical properties(saturable absorption and self-defocusing) imply a significant potential of the layered OBP in the development of unprecedented optoelectronic devices, such as mode lockers, optical switches, laser beam shapers, and wavelength converters.

Broadband nonlinear optical resonance and all-optical switching of liquid phase exfoliated tungsten diselenide

As a kind of two-dimensional transition metal dichalcogenide material, tungsten diselenide(WSe_2) has attracted increasing attention, owing to its gapped electronic structure, relatively high carrier mobility, and valley pseudospin, all of which show its valuable nonlinear optical properties. There are few studies on the nonlinear optical properties of WSe_2 and correlation with its electronic structure. In this paper, the effects of spatial self-phase modulation(SSPM) and distortion influence of WSe_2 ethanol suspensions are systematically studied, namely,the nonlinear refractive index and third-order nonlinear optical effect. We obtained the WSe_2 dispersions SSPM distortion formation mechanism, and through it, we calculated the nonlinear refractive index n_2,nonlinear susceptibility χ^(3), and their wavelength dependence under the excitation of 457 nm, 532 nm, and671 nm lasers. Moreover, by use of its strong and broadband nonlinear optical response, all-optical switching of two different laser beams due to spatial cross-phase modulation has been realized experimentally. Our results are useful for future optical devices, such as all-optical switching and all-optical information conversion.

In-situ neutron-transmutation for substitutional doping in 2D layered indium selenide based phototransistor

Neutron-transmutation doping(NTD)has been demonstrated for the first time in this work for substitutional introduction of tin(Sn)shallow donors into two-dimensional(2D)layered indium selenide(InSe)to manipulate electron transfer and charge carrier dynamics.Multidisciplinary study including density functional theory,transient optical absorption,and FET devices have been carried out to reveal that the field effect electron mobility of the fabricated phototransistor is increased 100-fold due to the smaller electron effective mass and longer electron life time in the Sn-doped InSe.The responsivity of the Sn-doped InSe based phototransistor is accordingly enhanced by about 50 times,being as high as 397 A/W.The results show that NTD is a highly effective and controllable doping method,possessing good compatibility with the semiconductor manufacturing process,even after device fabrication,and can be carried out without introducing any contamination,which is radically different from traditional doping methods.

Generation of ultrafast radially polarized pulses through chirp-assisted femtosecond optical parametric amplification

Radially polarized beams characterized by an axially symmetric polarization distribution can be sharply focused to produce strong longitudinal fields in the vicinity.Future applications of these beams will be facilitated by the availability of higher powers and shorter durations.Currently,the ultrafast radially polarized pulse is typically generated via wavefront reconstruction from conventional linearly polarized states.Achievable pulse duration and intensity limits are strictly dependent on extra-cavity optics.Herein,a chirp-assisted near-degenerate type-II parametric process is presented as a pulse-energy-scalable method of accessing ultrafast radially polarized pulses.In a proof-of-principle experiment,the broadband gain balance between the orthogonally polarized signal components was realized via controlling the chirp of the pump pulse.Through an analogous pulseduration transfer effect,the radially polarized signal inherited the temporal and spectral characteristics of the pump pulse and maintained the radial polarization state of each frequency component of the signal.With a shorter pump pulse,the generation of few-cycle radially polarized pulses should be achievable,which may facilitate a wide range of ultrafast applications such as vacuum electron acceleration and high-harmonic generation.

Efficient idler broadening via oppositely dual-chirped difference frequency generation

Dual-chirped difference frequency generation(DFG)is an advantageous technique for generating the broadband midinfrared(IR)idler wave,which is inaccessible by a population-inversion-based laser system.In principle,the generated idler wave may even suffer a spectrum broadening compared with the driving pulsed lasers if the pump and signal waves are oppositely chirped.However,broadband phase-matching is always the determining factor for the resulting efficiency and the bandwidth of the generated idler wave.In this study,specific to an oppositely dual-chirped DFG scheme,we derive the precondition to realize broadband frequency conversion,wherein a negative(1/υp-1/υi)/(1/υs-1/υi),in terms of the correlation coefficient of the group velocity(σ),is necessary.However,most birefringence bulk crystals can only provide the required material dispersions in limited spectral regions.We show that the periodically poled lithium niobate crystal that satisfies an inactive Type-II(eo-o)quasi-phase-matching condition has a stable negativeσand exerts the expected broadband gain characteristic across an ultra-broad idler spectral region(1.7-4.0µm).Finally,we propose and numerically verify a promising DFG configuration to construct a tunable mid-IR spectrum broader based on the broadband phase-matched oppositely dual-chirped DFG scheme.

Frequency-domain parametric downconversion for efficient broadened idler generation

An opposite-chirped frequency-domain optical parametric amplification(OC-FOPA) design is demonstrated and numerically verified. This scheme combines both an ultrabroad seeding generation and the subsequent effective amplification in one single optical parametric amplification stage. Based on a slightly asymmetrical 4-f optical system, the spectral contents of both pump and signal waves are spectrally dispersed with opposite spatial chirps,to broaden the initial idler seeding. Via a properly designed fan-out periodically poled LiNbO_3 chip, nearly perfect quasi phase matching can be realized across the full spectrum, whereby each individual spectral pair precisely maps to its required grating period. Full-dimensional simulations based on commercial ~110 fs(FWHM) nearinfrared(near-IR) lasers at 790 and 1030 nm are quantitatively discussed, and few-cycle mid-IR laser pulses(~60 fs at 3.4 μm) plus a high conversion efficiency exceeding 50% are theoretically predicted. By means of a high-power pump source, the OC-FOPA scheme can be also applied to directly produce high-intensity carrier-envelope-phase-stabilized mid-IR idler pulses.

Neural network-based surrogate model for inverse design of metasurfaces

Metasurfaces composed of spatially arranged ultrathin subwavelength elements are promising photonic devices for manipulating optical wavefronts,with potential applications in holography,metalens,and multiplexing communications.Finding microstructures that meet light modulation requirements is always a challenge in designing metasurfaces,where parameter sweep,gradient-based inverse design,and topology optimization are the most commonly used design methods in which the massive electromagnetic iterations require the design computational cost and are sometimes prohibitive.Herein,we propose a fast inverse design method that combines a physicsbased neural network surrogate model(NNSM)with an optimization algorithm.The NNSM,which can generate an accurate electromagnetic response from the geometric topologies of the meta-atoms,is constructed for electromagnetic iterations,and the optimization algorithm is used to search for the on-demand meta-atoms from the phase library established by the NNSM to realize an inverse design.This method addresses two important problems in metasurface design:fast and accurate electromagnetic wave phase prediction and inverse design through a single phase-shift value.As a proof-of-concept,we designed an orbital angular momentum(de)multiplexer based on a phase-type metasurface,and 200 Gbit/s quadrature-phase shift-keying signals were successfully transmitted with a bit error rate approaching 1.67×10^(-6).Because the design is mainly based on an optimization algorithm,it can address the“one-to-many”inverse problem in other micro/nano devices such as integrated photonic circuits,waveguides,and nano-antennas.

Superior optical Kerr effects induced by two-dimensional excitons

Materials with strong optical Kerr effects(OKEs)are crucial for a broad range of applications,such as all-optical data processing and quantum information.However,the underlying OKE mechanism is not clear in 2D materials.Here,we reveal key insights of the OKE associated with 2D excitons.An admirably succinct formalism is derived for predicting the spectra and the magnitude of the nonlinear refractive index(n_(2))of 2D materials.The predicted n_(2)spectra are consistent with reported experimental data and exhibit pronounced excitonic resonances,which is distinctively different from bulk semiconductors.The n_(2) value is predicted to be 3×10^(−10)cm^(2)/W for a 2D layered perovskite at low temperature as 7 K,which is four orders of magnitude larger than those of bulk semiconductors.The superior OKE induced by 2D excitons would give rise to a narrow refractive index-near-zero region for intense laser light.Furthermore,we demonstrate that the 2D layered perovskite should exhibit the best OKE efficiency(W_(FOM)=1.02,T_(FOM)=0.14)at 1550 nm,meeting the material requirements for all-optical switching.Our findings deepen the understanding of the OKE of 2D semiconducting materials and pave the way for highly efficient all-optical excitonic devices.

High-gain amplification for femtosecond optical vortex with mode-control regenerative cavity

Ultra-intense femtosecond vortex pulses can provide an opportunity to investigate the new phenomena with orbital angular momentum(OAM)involved in extreme cases.This paper reports a high gain optical vortex amplifier for intense femtosecond vortex pulses generation.Traditional regeneration amplifiers can offer high gain for Gaussian mode pulses but cannot amplify optical vortex pulses while maintaining the phase singularity because of mode competition.Here,we present a regeneration amplifier with a ring-shaped pump.By controlling the radius of the pump,the system can realize the motivation of the Laguerre–Gaussian[LG0,1(−1)]mode and the suppression of the Gaussian mode.Without seeds,the amplifier has a donut-shaped output containing two opposite OAM states simultaneously,as our prediction by simulation.If seeded by a pulse of a topologic charge of 1 or−1,the system will output an amplified LG0,1(−1)mode pulse with the same topologic charge as the seed.To our knowledge,this amplifier can offer the highest gain as 1.45×106 for optical vortex amplification.Finally,we obtain a 1.8 mJ,51 fs compressed optical vortex seeded from a 2 nJ optical vortex.