Ferroelectricity of hafnium oxide-based materials:Current status and future prospects from physical mechanisms to device applications

The finding of the robust ferroelectricity in HfO_(2)-based thin films is fantastic from the view point of both the fundamentals and the applications.In this review article,the current research status of the future prospects for the ferroelectric HfO_(2)-based thin films and devices are presented from fundamentals to applications.The related issues are discussed,which include:1)The ferroelectric characteristics observed in HfO_(2)-based films and devices associated with the factors of dopant,strain,interface,thickness,defect,fabrication condition,and more;2)physical understanding on the observed ferroelectric behaviors by the density functional theory(DFT)-based theory calculations;3)the characterizations of microscopic and macroscopic features by transmission electron microscopes-based and electrical properties-based techniques;4)modeling and simulations,5)the performance optimizations,and 6)the applications of some ferroelectric-based devices such as ferroelectric random access memory,ferroelectric-based field effect transistors,and the ferroelectric tunnel junction for the novel information processing systems.

Feasibility study on determining the conventional true value of gamma-ray air kerma in a minitype reference radiation

A minitype reference radiation(MRR) with dimensions of only 1 m × 1 m × 1 m has been developed for the in situ calibration of photon dosimeters.The present work conducts a feasibility study on determining the conventional true value of gamma-ray air kerma at the point of test in the MRR.Owing to its smaller dimensions,the scattered gamma-rays in the MRR are expected to induce a non-negligible interference with the radiation field compared with conditions in the standard reference radiation stipulated by ISO4037-1 or GB/T12162.1.A gamma-ray spectrometer was employed to obtain the spectra of scattered gamma-rays within the MRR,and the feature components of the spectra were extracted by principal component analysis to characterize the interference of a dosimeter probe in the radiation field.A prediction model of the CAK at the point of test was built by least squares support vector machine based on the feature component data obtained from nine sample dosimeters under five different dose rates.The mean prediction error of the CAK prediction model was within ±4.5%,and the maximum prediction error was about ±10%.

Improvement of tunnel compensated quantum well infrared detector

To reduce the difficulty of the epitaxy caused by multiple quantum well infrared photodetector(QWIP)with tunnel compensation structure,an improved structure is proposed.In the new structure,the superlattices are located between the tunnel junction and the barrier as the infrared absorption region,eliminating the effect of doping concentration on the well width in the original structure.Theoretical analysis and experimental verification of the new structure are carried out.The experimental sample is a two-cycle device,each cycle contains a tunnel junction,a superlattice infrared absorption region and a thick barrier.The photosurface of the detector is 200×200μm^2 and the light is optically coupled by 45°oblique incidence.The results show that the optimal operating voltage of the sample is-1.1 V,the dark current is 2.99×10^-8A,and the blackbody detectivity is1.352×10^8 cm·Hz^1/2·W^-1at 77 K.Our experiments show that the new structure can work normally.

Thermal Analysis of Implant-Defined Vertical Cavity Surface Emitting Laser Array

一个三维的电热的联合模型基于有限元素方法被使用学习射出激光(VCSEL ) 的垂直的洞表面穿的植入定义的热性质。包括内部元素的间距，规模，注射当前的密度和底层温度的几个参数被考虑。通过实验获得的实际温度在对计算结果的优秀同意，它证明模型的精确性。由于严重热问题，设计数组是必要的低自我加热。分析能为以后设计 VCSEL 数组提供一个基础。

Spectra of spontaneous Raman scattering in taper-drawn micro/nano-fibers

We study the spontaneous Raman scattering （RS） in taper-drawn micro/nano-fibers （MNFs） by employing the photon counting technique. The spectra of RS in five MNFs, which are fabricated by using different heating flames （hydrogen flame or butane flame） and with different diameters, are measured within a frequency shift range of 1435 cm- 1_3200 cm- 1. From the measured spectra, we observe the RS peaks originated from silica and a unique RS peak with a frequency shift of - 2905 cm-1 （- 87.2 THz）. Unlike the former ones, the latter one is not observable in conventional optical fibers. Furthermore, the unique peak becomes obvious and starts to rapidly increase with the decrease of the diameter of MNFs when the diameter is smaller than 2 μm, and the intensity of the unique peak significantly depends on the heating flame used in the fabricating process. Our investigation is useful for the entanglement generation or optical sensing using taper-drawn MNFs.

Photon statistics of pulse-pumped four-wave mixing in fiber with weak signal injection

We study the photon statistics of pulse-pumped four-wave mixing in fibers with weak coherent signal injection by measuring the intensity correlation functions of individual signal and idler fields. The experimental results show that the intensity correlation function of individual signal（idler） field g_（s（i））^（2） decreases with the intensity of signal injection. After applying narrow band filter in signal（idler） band, the value of g_（s（i））^（2） decreases from 1.9 ± 0.02（1.9 ± 0.02） to 1.03 ± 0.02（1.05 ± 0.02） when the intensity of signal injection varies from 0 to 120 photons/pulse. The results indicate that the photon statistics changes from Bose–Einstein distribution to Poisson distribution. We calculate the intensity correlation functions by using the multi-mode theory of four-wave mixing in fibers. The theoretical curves well fit the experimental results.Our investigation will be useful for mitigating the crosstalk between quantum and classical channels in a dense wavelength division multiplexing network.

Simulation of Far-Field Properties of Coherent Vertical Cavity Surface Emitting Laser Array

Far-field properties dependent on array scale, separation, element width and emitted wavelength are system atically analyzed theoretically and experimentally. An array model based on the finite-difference method is established to simulate the far-field profile of the coherent arrays. Some important conclusions are obtained. To achieve a higher quality beam, it is necessary to decrease separation between elements, or to increase the element width. Higher brightness can be achieved in the array with larger scale. Emitted wavelength also has an influence on the far-field profile. These analyses can be extended to the future design of coherent vertical cavity surface emitting laser arrays.

Simulation of gamma minitype reference radiation for calibration of personal dosimeter

An investigation using Monte Carlo simulation on a minitype reference radiation(MRR) for the calibration of gamma personal dosimeters is reported. The distributions of dose rate and scattering gamma spectrum are the main simulation objects with the variable physical structures of MRR and the dosimeters as parameters that are to be calibrated. Further, the influences on the reference radiation caused by these parameters are analyzed in detail.This work provides a theoretical basis for better understanding of MRR used for calibration of gamma personal dosimeters. This analysis can help in the development of a calibration technology for such tools based on MRR.

Photo-responsive droplet manipulation slippery lubricant-infused porous surface with ultra-high durability

Photo-responsive slippery lubricant-infused porous surface(SLIPS) for droplet manipulation is flexible, noncontact and non-destructive in droplet manipulation, which has promising applications in flexible robotics, microfluidics,biomedicine, and chemical analysis. However, the repeated manipulations for droplets of SLIPSs are quite limited in the works reported so far, the poor durability of droplet manipulation severely limits the practical application of the surfaces. In this paper, an Fe3O4-doped polydimethylsiloxane(PDMS)-based SLIPS is proposed and implemented to achieve ultra-high repeated droplet manipulation numbers under near-infrared ray(NIR) laser irradiation. Firstly, a micron columnar array structure with micro-pits on the top side, as well as, a wall structure out of the array is designed on SLIPS to reserve the lubricant. Secondly, the prototype of the SLIPS is fabricated by a 3-step ultraviolet(UV) lithography, and subsequently immersed in silicone oil for more than 96 h to obtain the ultra-high durability slippery lubricant-infused porous surface(UD-SLIPS). With a power of 25 m W–85 m W NIR laser, the repeated manipulation of microdroplets(≤ 5 μL) in the scale of 1 cm can exceed more than 3000 times which is far beyond that in previous reports. Finally, the droplet manipulation performance of this photo-responsive UD-SLIPS and the influence of infusion time on durability are investigated. The mechanism of the PDMS swelling effect is found to be the key factor in improving the droplet manipulation durability of SLIPS. The findings of this work would be of great significance for the development of highly durable photo-responsive functional surfaces for droplet manipulation.

Single fundamental mode photonic crystal VCSEL with high power and low threshold current optimized by modal loss analysis

The characteristics of the photonic crystal vertical cavity surface emitting lasers（PhC-VCSELs） were investigated by using the full vector finite-difference time-domain（FDTD） method through the transverse mode loss analysis. PhC-VCSELs with different photonic crystal structures were analyzed theoretically and experimentally. Through combining the dual mode confinement of oxide aperture and seven-point-defect photonic crystal structure, the PhC-VCSELs with low threshold current of 0.9 mA and maximum output power of 3.1 mW operating in single fundamental mode were demonstrated. Mode loss analysis method was proven as a reliable and useful way to analyze and optimize the PhC-VCSELs.

Broad bandwidth interference filter-stabilized external cavity diode laser with narrow linewidth below 100kHz

Interference filter-stabilized external cavity diode lasers （ECDLs） have properties of simple configurations, high sta- bilities, and narrow linewidths. However, the interference filter used in common ECDL designs requires an ultra-narrow bandwidth （about 0.3 nm） to achieve mode selection, that is considerably expensive and not yet available for a wide range of wavelengths. In this paper, a robust ECDL using an available broad bandwidth （about 4 nm） interference filter as the wavelength discriminator is constructed and tested. The ECDL demonstrated a narrow Lorentzian fitted linewidth of 95 kHz and a spectral purity of 2.9 MHz. The long-term frequency stability of the ECDL reaches 5.59 x 10 12.

Recent advances in flexible photodetectors based on 1D nanostructures

Semiconductor nanowires have demonstrated excellent electronic and optoelectronic properties.When integrated into photodetectors,excellent device performance can be easily attained.Apart from the exceptional performance,these nanowires can also enable robust and mechanically flexible photodetectors for various advanced utilizations that the rigid counterparts cannot perform.These unique applications include personal healthcare,next-generation robotics and many others.In this review,we would first discuss the nanowire fabrication techniques as well as the assembly methods of constructing largescale nanowire arrays.Then,the recent development of flexible photodetectors based on these different nanowire material systems is evaluated in detail.At the same time,we also introduce some recent advancement that allows individual photodetectors to integrate into a more complex system for advanced deployment.Finally,a short conclusion and outlook of challenges faced in the future of the community is presented.

Solution processable transition metal dichalcogenides-based hybrids for photodetection

Transition metal dichalcogenides(TMDs), as one of the most promising two-dimensional(2D) materials, have attracted considerable attention for use in photodetection applications over the past few years due to their distinct properties, such as atomic-scale thickness, tunable direct bandgaps, and decent carrier mobilities at room temperature. Compared with pure 2D TMDs, the construction of hybrids consisting of TMDs and other low-dimensional materials can further improve the performance of photodetectors including their spectral range, responsivity and detectivity, which significantly boosts interest in the development of TMDs-based photodetectors. On the other hand, solution-phase synthesis methods provide a facile strategy for the scalable production of TMD hybrids, opening an exciting avenue to develop low-cost devices. In this review, we summarize the material synthesis, characterizations, and photodetection applications of the solution processable TMDs-based hybrids, as well as provide insights into their prospects.

Beam Steering Analysis in Optically Phased Vertical Cavity Surface Emitting Laser Array

Beam steering in implant defined coherently coupled vertical cavity surface emitting laser （VCSEL） arrays is simulated using the FDTD solution software. Angular deflection dependent on relative phase differences among elements, inter-element spacing, element size and emitted wavelength is analyzed detailedly and systematically. We design and fabricate 1×2 implant defined VCSEL arrays for optimum beam steering performance. Electroni- cally controlled beam steering with a maximum deflection angle of 1.6° is successfully achieved in the 1 × 2 VCSEL arrays. The percentage of the power in the central lobe is above 39% when steering. The results show that the steering is controllable. Compared with other beam steering methods, the fabrication process is simple and of low cost.

New assembly route for three-dimensional metamaterials obtained through effective medium theory

In this study, we illustrate the effective medium theories in the designs of three-dimensional composite metama- terials of both negative permittivity and negative permeability. The proposed metamaterial consists of random coated spheres with sizes smaller compared to the wavelength embedded in a dielectric host. Simple design rules and formulas following the effective medium models are numerically and analytically presented. We demonstrate that the revised Maxwell-Garnett effective medium theory enables us to design three-dimensional composite metamaterials through the assembly of coated spheres which are random and much smaller than the wavelength of the light. The proposed ap- proach allows for the precise control of the permittivity and the permeability and guides a facile, flexible, and versatile way for the fabrication of composite metamaterials.

Vertical cavity surface emitting laser transverse mode and polarization control by elliptical hole photonic crystal

The polarization of traditional photonic crystal（PC） vertical cavity surface emitting laser（VCSEL） is uncontrollable,resulting in the bit error increasing easily.Elliptical hole photonic crystal can control the transverse mode and polarization of VCSEL efficiently.We analyze the far field divergence angle,and birefringence of elliptical hole PC VCSEL.When the ratio of minor axis to major axis b/a = 0.7,the PC VCSEL can obtain single mode and polarization.According to the simulation results,we fabricate the device successfully.The output power is 1.7 mW,the far field divergence angle is less than 10°,and the side mode suppression ratio is over 30 dB.The output power in the Y direction is 20 times that in the X direction.

Polarization Stable Vertical Cavity Surface Emitting Laser Array Based on Proton Implantation

Polarization stable characteristic of vertical cavity surface emitting laser array with rectangular element structure based on proton implantation is achieved. Arrays with different aperture sizes and different array scales are investigated. The maximum orthogonal polarization suppression ratio of a 6 × 6 array with 6μm × 4μm elements is as high as 16.1 dB at 51 mA. The x-polarization dominates y-polarization in the operation current range from the threshold to the saturated current. The full widths at half maximum in far-field profiles for x-polarization and y-polarization are 8.8° and 10.7°, respectively. These results suggest that stable polarization is available in the implant-defined VCSEL array.

Reconfigurable logic and in-sensor encryption operations in an asymmetrically tunable van der Waals heterostructure

Reconfigurable devices can be used to achieve multiple logic operation and intelligent optical sensing with low power consumption,which is promising candidates for new generation electronic and optoelectronic integrated circuits.However,the versatility is still limited and need to be extended by the device architectures design.Here,we report an asymmetrically gate two-dimensional(2D)van der Waals heterostructure with hybrid dielectric layer SiO_(2)/hexagonal boron nitride(h-BN),which enable rich function including reconfigurable logic operation and in-sensor information encryption enabled by both volatile and non-volatile optoelectrical modulation.When the partial gate is grounded,the non-volatile light assisted electrostatic doping endowed partially reconfigurable doping between n-type and p-type,which allow the switching of logic XOR and not implication(NIMP).When the global gate is grounded,additionally taking the optical signal as another input signal,logic AND and OR is realized by combined regulation of the light and localized gate voltage.Depending on the high on/off current ratio approaching 105 and reliable&switchable logic gate,in-sensor information encryption and decryption is demonstrated by manipulating the logic output.Hence,these results provide strong extension for current reconfigurable electronic and optoelectronic devices.

Growth of centimeter scale Nb_(1−x)W_(x)Se_(2) monolayer film by promoter assisted liquid phase chemical vapor deposition

Two-dimensional(2D)transition-metal dichalcogenide materials(TMDs)alloys have a wide range of applications in the field of optoelectronics due to their capacity to achieve wide modulation of the band gap with fully tunable compositions.However,it is still a challenge for growing alloys with uniform components and large lateral size due to the random distribution of the crystal nucleus locations.Here,we applied a simple but effective promoter assisted liquid phase chemical vapor deposition(CVD)method,in which the quantity ratio of promoter to metal precursor can be controlled precisely,leading to tiny amounts of transition metal oxide precursors deposition onto the substrates in a highly uniform and reproducible manner,which can effectively control the uniform distribution of element components and nucleation sites.By this method,a series of monolayer Nb_(1−x)W_(x)Se_(2)alloy films with fully tunable compositions and centimeter scale have been successfully synthesized on sapphire substrates.This controllable approach opens a new way to produce large area and uniform 2D alloy film,which has the potential for the construction of optoelectronic devices with tailored spectral responses.

Design of GaAs/Al_xGa_(1-x)As asymmetric quantum wells for THz-wave by difference frequency generation

The energy levels, wave functions and the second-order nonlinear susceptibilities are calculated in GaAs/Al0.2Ga0.8As/Al0.5Ga0.5As asymmetric quantum well (AQW) by using an asymmetric model based on the parabolic and non-parabolic band. The influence of non-parabolicity can not be neglected when analyzing the phenomena in narrow quantum wells and in higher lying subband edges in wider wells. The numerical results show that under double resonance (DR) conditions, the second-order difference frequency generation (DFG) and optical rectification (OR) generation susceptibilities in the AQW reach 2.5019 mm/V and 13.208 mm/V, respectively, which are much larger than those of the bulk GaAs. Besides, we calculate the absorption coefficient of AQW and find out the two pump wavelengths correspond to the maximum absorption, so appropriate pump beams must be selected to generate terahertz (THz) radiation by DFG.