Optical anisotropy of black phosphorus by total internal reflection

Although abundant research on the anisotropy of van der Waals(vd W)materials has been published,we undertake an in-depth study of their optical properties as they have an important guiding role for light control in two-dimensional(2D)nanospace.As an example,we study the reflectance of few-layered black phosphorus(BP)in the total internal reflection(TIR)mode in detail.We demonstrate that its optical anisotropy can be changed on a large scale by varying the incident angles,polarization states,and the in-plane rotation angles of the BP samples.Theoretical analysis indicates that the phenomena observed are common to all the atom-thick biaxial crystals,so these conclusions can be widely applied to other anisotropic 2D materials.This research furthers the current understanding of the properties of BP more comprehensively,and provides guidance for developing new optoelectronic applications,especially when BP and other atom-thick biaxial crystals are integrated with TIR devices.

Strong in-plane optical anisotropy in 2D van der Waals antiferromagnet VOCl

Two-dimensional(2D)van der Waals(vdW)magnetic materials with strong in-plane anisotropy can make possible novel applications such as optospintronics and strain sensors.In this work,the strong in-plane optical anisotropy in 2D vdW antiferromagnet VOCl has been systematically investigated.The optical brightness and absorption coefficient exhibit evident periodic variation with the change of incident polarization,unveiling the strong in-plane anisotropic optical absorption.The Raman intensity in this material shows obvious dependence on the polarization angle of incident laser,demonstrating that the phonon properties possess strong in-plane anisotropy.Besides,we have also realized in-situ visualization of in-plane optical reflection anisotropy in this material.Moreover,the strong second harmonic generation(SHG)signal can only be detected when the incident polarization is along specific in-plane crystal orientations,illustrating the presence of strong in-plane nonlinear optical anisotropy.These findings will benefit the applications of VOCl in the field of polarization-dependent electronics and spintronics.

Broadband strong optical dichroism in topological Dirac semimetals with Fermi velocity anisotropy

Prototypical three-dimensional(3D)topological Dirac semimetals(DSMs),such as Cd3As2 and Na3Bi,contain electrons that obey a linear momentum-energy dispersion with different Fermi velocities along the three orthogonal momentum dimensions.Despite being extensively studied in recent years,the inherent Fermi velocity anisotropy has often been neglected in the theoretical and numerical studies of 3D DSMs.Although this omission does not qualitatively alter the physics of light-driven massless quasiparticles in 3D DSMs,it does quantitatively change the optical coefficients which can lead to nontrivial implications in terms of nanophotonics and plasmonics applications.Here we study the linear optical response of 3D DSMs for general Fermi velocity values along each direction.Although the signature conductivity-frequency scaling,σ(ω)∝ω,of 3D Dirac fermion is well-protected from the Fermi velocity anisotropy,the linear optical response exhibits strong linear dichroism as captured by the universal extinction ratio scaling law,Λi j=(vi/v j)^2(where i=j denotes the three spatial coordinates x,y,z,and vi is the i-direction Fermi velocity),which is independent of frequency,temperature,doping,and carrier scattering lifetime.For Cd3As2 and Na3Bi3,an exceptionally strong extinction ratio larger than 15 and covering a broad terahertz window is revealed.Our findings shed new light on the role of Fermi velocity anisotropy in the optical response of Dirac semimetals and open up novel polarization-sensitive functionalities,such as photodetection and light modulation.

Giant in-plane optical anisotropy of a-plane ZnO on r-plane sapphire

We have measured the in-plane optical anisotropy （IPOA） of （1120） ZnO （a-plane） on （10]-2） sapphire （r-plane） by reflectance difference spectroscopy （RDS） at room temperature. Giant IPOA has been observed be- tween the light polarized direction parallel and perpendicular to the c axis of ZnO, since the symmetry of a-plane is C2v. A sharp resonance has been observed near the fundamental band gap, which is induced by the polarization- depend band gap shift. The sharp line shape is attributed to the exciton transition. The spectra fitting and differential spectra indicate the polarization-depend band energies. The giant IPOA is possible enhanced by anisotropy strain along and perpendicular to the c axis in the a-plane.

Tuning of the Electron Spin Relaxation Anisotropy via Optical Gating in GaAs/AlGaAs Quantum Wells

The carrier-density-dependent spin relaxation dynamics for modulation-doped GaAs/Al0.3 Gao,TAs quantum wells is studied using the time-resolved magneto-Kerr rotation measurements. The electron spin relaxation time and its in-plane anisotropy are studied as a function of the optically injected electron density, Moreover, the relative strength of the Rashba and the Dresselhaus spin-rbit coupling fields, and thus the observed spin relaxation time anisotropy, is further tuned by the additional excitation of a 532nm continuous wave laser, demonstrating an effective spin relaxation manipulation via an optical gating method.

Strain effects on optical polarisation properties in (11■2) plane GaN films

We present the theoretical results of the electronic band structure of wurtzite GaN films under biaxial strains in the （11^-22）-plane. The calculations are performed by the k.p perturbation theory approach through using the effectivemass Hamiltonian for an arbitrary direction. The results show that the transition energies decrease with the biaxial strains changing from -0.5% to 0.5%. For films of （11^-22）-plane, the strains are expected to be anisotropic in the growth plane. Such anisotropic strains give rise to valence band mixing which results in dramatic change in optical polarisation property. The strain can also result in optical polarisation switching phenomena. Finally, we discuss the applications of these properties to the （1132） plane GaN-based light-emitting diode and lase diode.

Optical and electronic anisotropy of a 2D semiconductor SiP

Two-dimensional anisotropic materials have been widely concerned by researchers because of their great application potential in the field of polarized detector devices and optical elements,which is a very important and popular research direction at present.As a IV-V two-dimensional material,silicon phosphide(SiP)has obvious in-plane anisotropy and exhibits excellent optical and electrical anisotropy properties.Herein,the optical anisotropy of SiP is studied by spectrometric ellipsometry measurements and polarization-resolved optical microscopy,and its electrical anisotropy is tested by SiP-based field-effect transistor.In addition,the normal and anisotropic photoelectric performance of SiP is shown by fabricating a photodetector and measuring it.In various measurements,SiP exhibits obvious anisotropy and good photoelectric performance.This work provides basic optical,electrical,and photoelectric performance information of SiP,and lays a foundation for further study of SiP and applications of SiP-based devices.

Photoresponding ionic complex containing azobenzene chromophore for use in birefringent film

A novel photoresponding ionic complex （PANDAZO） was prepared by the ionic self-assembly （1SA） of sodium polyacrylate （PANa） and azobenzene chromophores （NDAZO）. The ionic complex forms an interdigitated lamellar structure with full overlap of the side chains. The optical anisotropy was investigated by using a polarization pulse laser （355 nm）. Furthermore, a high photoinduced birefringence （An = 0.365） was measured by using a continuous 488 nm laser as the pump light.

阴阳离子协同作用在系列准一维氟化草酸锑中实现双折射调控

具有较大光学各向异性的双折射材料可以用来调制光的偏振,在激光科学与技术中发挥着关键作用.然而,由于缺乏理性设计的有效策略,新型高性能双折射材料的探索研究进展极其缓慢.本文通过采用π共轭C_(2)O_(4)2-阴离子和含具有立体化学活性孤对电子的Sb^(3+)阳离子相结合,成功地合成了三例氟化草酸锑盐,分别为Na_(2)Sb_(2)(C_(2)O_(4))F_(6)、K_(2)Sb_(2)-((C_(2)O_(4))F_(6)和Cs_(2)Sb_(2)(C_(2)O_(4))_(2)F_(4)·H_(2)O.它们都具有独特的准一维链状结构,并表现出大的光学各向异性.尤其是Cs_(2)Sb_(2)(C_(2)O_(4))_(2)F_(4)·H_(2)O表现出巨大的双折射率(0.325@546 nm),在所有已报道的锑(Ⅲ)基含氧酸盐中最大.详细的结构分析和理论计算证实,阳离子模版剂和阴离子功能基团的协同作用可以调节化合物的光学各向异性.这项工作为高效设计优良的双折射材料打开了一扇大门,并为新型结构驱动功能材料的进一步发现提供了指导.

Linear dichroism and polarization controllable persistent spin helix in two-dimensional ferroelectric ZrOI_(2) monolayer

Two-dimensional ferroelectric(2D-FE)materials that characterize the spontaneous ferroelectricity down to monolayer limit and rich ferroic properties arising from FE orderings,have been extensively explored as low-dimensional sensor,electric and memory devices.In current work,group-IV transition metal oxide dihalide MOX_(2)(M=Zr and Hf,X=Cl,Br and I)monolayers have been identified as a new group of 2D-FE materials.Using the comprehensive first-principles calculations combined with finite temperature Monte Carlo(MC)and ab initio molecular dynamics(MD)simulations,we investigate the temperature stability of FE polarization and further uncover the unique properties associated with spontaneous ferroelectricity of MOX_(2) monolayers.In particular,ZrOI_(2) monolayer,a promising 2D-FE material with room temperature stable ferroelectricity,semiconducting electronic structure and optoelectronic response under visible light,offers an ideal material platform to investigate the coupling of intrinsic anisotropy,optical absorption selectivity and spin degree of freedom with 2D ferroelectricity.Typically,significant optical absorption anisotropy and giant linear dichroism effect are predicted for a 2D optical polarizer device based on ZrOI_(2) monolayer,where the adsorption of incident monochromatic linearly polarized light(hv=3.23 eV)along two planar directions with a nearly 100%optical selectivity can be achieved.Moreover,the spin–orbit coupling(SOC)induced spin splitting of valence band edges and out-of-plane textured spin configuration occur in ZrOI_(2) monolayer.In the meanwhile,the unidirectional spin–orbit field protected by C2v wave-vector point group can further create the persistent spin helix(PSH)state,leading to the extraordinarily long spin carrier lifetime.More importantly,the nonvolatile control of PSH state via the electric field induced polarization reversal has also been demonstrated for FE-ZrOI_(2) monolayer,which manifests as a great advantage for applications of ZrOI_(2) as the lowdimensional spin-field effect transistor and all-electric spintronics devices.

Pump induced birefringence in dual-polarization fiber grating lasers

Birefringence is critical in dual-polarization fiber-laser-based fiber-optic sensing systems, as it directly determines the beat frequency between the two polarizations. A study of pump induced birefringence in dualpolarization fiber lasers is presented here, which shows that the pump induced birefringence is a result of the interplay among pump induced refractive index change, laser dynamics, and anisotropy inside fiber lasers.For erbium-doped fiber lasers, pumping at 1480 nm is better than pumping at 980 nm in lower pump induced birefringence. Moreover, injection at 532 nm for an adequately long enough time can permanently reduce anisotropy and, hence, reduce pump induced birefringence.