二维黑磷的光学性质

黑磷是继石墨烯、过渡金属硫族化合物(TMDCs)之后又一个备受关注的二维材料.黑磷从单层到块材都是直接带隙半导体,且带隙从单层的1.7 eV一直随着层数的增加而减小,到块材则变为0.3 eV,涵盖了可见光到中红外波段,恰好填补了石墨烯和过渡金属硫族化合物的带隙在该波段的空白.同时,黑磷还具有很高的载流子迁移率、良好的调...     

Exciton absorption of the GaAs semiconductor crystals under optical pumping to the conduction band

Modifications of the exciton structure of the fundamental absorption edge in the GaAs crystals is experimentally studied at T = 1.7 K using the optical pumping at a photon energy that is significantly greater than the band gap. An increase in the amplitude of the fundamental state of the exciton is observed at a stable maximum energy. The dependence of the integral absorption on the pump intensity is interpreted in the framework of the concept of the excitonic polariton using the dissipative scattering of the exciton by free electrons that are generated by the pumping radiation. The constant of the electron-exciton interaction can be estimated with the aid of the solution to the inverse problem for initial pump levels. The integral absorption of the fundamental exciton state at liquid-helium temperatures can be used to characterize the purity of an epitaxial layer. The reasons for the lower saturation level of the integral absorption that is significantly less than the calculated level determined by the exciton oscillator strength need to be further studied.     

Laser field induced interband absorption in a strained GaAs/GaAlAs double quantum well system

Effect of laser field intensity on exciton binding energies is investigated in a GaAs/ GaAlAs double quantum well system. Calculations have been carried out with the variational technique within the single band effective mass approximations using a two parametric trial wave function. The interband emission energy as a function of well width is calculated in the influence of laser field. The laser field induced photoionization cross-section for the exciton placed at the centre of the quantum well is computed as a function of normalized photon energy. The dependence of the photoionization cross-section on photon energy is carried out for the excitons. The resulting spectra are brought out for light polarized along and perpendicular to the growth direction. The intense laser field dependence of interband absorption coefficient is investigated. The results show that the exciton binding energy, interband emission energy, the photoionization cross-section and the interband absorption coefficient depend strongly on the well width and the laser field intensity. Our results are compared with the other existing literature available.     

Excitons in coupled quantum dots

Properties of excitons in vertically coupled GaAs/AlGaAs quantum dots were investigated using the variational method within the envelope function and effective mass approximations. It was found that when the thickness of the spacer layer becomes less than about one exciton Bohr radius, both the exciton binding energy and the fundamental optical transition energy are reduced compared to those in isolated quantum dots. This is a result of increased space extension of exciton due to the penetration of carrier wave functions into the spacer layer and corresponding reduction in confinement energy which dominates over the Coulomb interaction between the electron and the hole.     

应变纤锌矿ZnO/Mg_xZn_(1-x)O柱形量子点中离子受主束缚激子的光学性质

在有效质量近似下,考虑内建电场效应,采用变分法详细研究了受限于纤锌矿Mg_xZn_(1-x)O/ZnO/Mg_xZn_(1-x)O圆柱形应变量子点中离子受主束缚激子(A~-,X)的带间光跃迁吸收系数随量子点尺寸、Mg含量和离子受主杂质中心位置的变化情况,并和离子施主束缚激子(D~+,X)及自由激子进行了比较.结果表明:随着量子点尺寸的减小,(A~-,X)的光跃迁吸收强度增强,吸收曲线向高能方向移动,出现蓝移现象.随着Mg含量增加,(A~-,X)的光跃迁吸收曲线蓝移,且吸收强度减弱.随着离子受主杂质从量子点的左界面沿材料生长方向移至量子点的右界面,光跃迁吸收曲线向低能方向移动,出现红移现象.此外,与离子施主束缚激子(D~+,X)相比,随着沿材料生长方向掺入杂质位置的变化,光跃迁吸收曲线移动的方向相反.但不管是掺入离子受主杂质还是离子施主杂质,当离子杂质从量子点的左异质界面沿材料生长方向移至右异质界面时,光跃迁吸收峰的移动量大致相同.     

EFFECTIVE-MASS THEORY FOR InAs/GaAs STRAINED SUPERLATTICES

By using the recently developed exact effective-mass envelope function theory, the electronic structures of InAs/GaAs strained superlattices grown on GaAs (100) oriented substrates are studied. The electron and hole subband structures, distribution of electrons and holes along the growth direction, optical transition matrix elements, exciton states, and absorption spectra are calculated. In our calculations, the effects due to the different effective masses of electrons and holes in different materials and the strain are included. Our theoretical results are in agreement with the available experimental data.     

Double band-inversions of bilayer phosphorene under strain and their effects on optical absorption

Strain is a powerful tool to engineer the band structure of bilayer phosphorene.The band gap can be decreased by vertical tensile strain or in-plane compressive strain.At a critical strain,the gap is closed and the bilayer phosphorene is turn to be a semi-Dirac semimetal material.If the strain is stronger than the criterion,a band-inversion occurs and it re-happens when the strain is larger than another certain value.For the zigzag bilayer phosphorene ribbon,there are two edge band dispersions and each dispersion curve represents two degenerate edge bands.When the first band-inversion happens,one of the edge band dispersion disappears between the band-cross points while the other survives,and the latter will be eliminated between another pair of band-cross points of the second band-inversion.The optical absorption of bilayer phosphorene is highly polarized along armchair direction.When the strain is turn on,the optical absorption edge changes.The absorption rate for armchair polarized light is decreased by gap shrinking,while that for zigzag polarized light increases.The bandtouch and band-inversion respectively result in the sublinear and linear of absorption curve versus light frequency in low frequency limit.     

Size-dependent electronic and optical properties of an exciton in CdSe/CdS/CdSe/CdS multilayer spherical quantum dot

The electronic and optical properties of a single exciton in a CdSe/CdS/CdSe/CdS quantum dot is studied by using effective mass approximation with parabolic confinement. The Coloumbic interaction between electron and hole is included by Hartree potential. A self-consistent technique is used to calculate the energy eigenvalue and wavefunction of exciton. Based on this approximation we investigate the effect of core size, shell thickness, well width on exciton binding energy, absorption spectra, and oscillator strength. The results provide the tuning possibility of electronic and optical properties of multilayer quantum dot with layer thickness.     

层数调控磷烯能带与光学性质的研究

近年来,黑磷作为兼具石墨烯和过渡金属硫化物之长的新型二维材料而倍受关注.本文基于密度泛函理论,研究了不同厚度黑磷的电子结构与光学性质.结果表明,黑磷的性质与其厚度密切相关,可通过厚度调整实现能带与光学性质的可调控性.层间相互作用导致费米能级附近价带和导带的劈裂,是造成黑磷带隙随层数减小的根本原因.黒磷的静态折射率和静态反射率的大小均随层数的增大有增大的趋势,并且各层黑磷的反射峰均位于紫外光波段.黑磷对光的吸收涵盖了可见光到紫外光区域,对光的损失范围小于4eV.本文基于能带图和分波态密度图,从电子跃迁的角度分析了黑磷各项光学性质的变化情况,旨在为黑磷的带隙及光学性质层数可调控性提供理论依据.     

Optical bistability of a layered semiconductor in the exciton absorption region

The effect of specific features of the exciton and phonon spectra of layered semiconductors on the conditions of realization of optical bistability in the exciton absorption region has been theoretically investigated by the Green’s function method. By the example of the 2H polytype of PbI₂, it is shown that effective exciton scattering from low-energy bending-wave vibrations leads to a blue shift of the frequency range of bistability realization and its narrowing; expansion of the temperature range of bistability observation; and a shift of the hysteresis loop to higher intensities, accompanied by a decrease in its height and width. The possibility of observing polarization optical bistability, which is related to the dependence of the exciton energy spectrum on the propagation direction of a light wave in a crystal, is also substantiated.     

Exciton optical absorption coefficients and refractive index changes in a strained InAs/GaAs quantum wire: The effect of the magnetic field

Magnetic field induced exciton binding energy is investigated in a strained InAs/GaAs quantum wire within the framework of single band effective mass approximation. The strain contribution to the potential is determined through deformation potentials. The interband emission energy of strained InAs/GaAs wire is investigated in the influence of magnetic field with the various structural parameters. Magnetic field induced photoionization cross section of the exciton is studied. The total optical absorption and the refractive index changes as a function of normalized photon energy between the ground and the first excited state in the presence of magnetic field are analyzed. The optical absorption coefficients and the refractive index changes strongly depend on the incident optical intensity and the magnetic field. The occurred blueshift of the resonant peak due to the magnetic field will give the information about the variation of two energy levels in the quantum well wire. The optical absorption coefficients and the refractive index changes are strongly dependent on the incident optical intensity and the magnetic field.     

Excitons in carbon nanotubes: an ab initio symmetry-based approach

The optical absorption spectrum of the carbon (4,2) nanotube is computed using an ab initio many-body approach which takes into account excitonic effects. We develop a new method involving a local basis set which is symmetric with respect to the screw-symmetry of the tube. Such a method has the advantages of scaling faster than plane-wave methods and allowing for a precise determination of the symmetry character of the single-particle states, two-particle excitations, and selection rules. The binding energy of the lowest, optically active states is approximately 0.8 eV. The corresponding exciton wave functions are delocalized along the circumference of the tube and localized in the direction of the tube axis.     

Excitons in tubular molecular aggregates

We present a brief overview of recent work on the optical properties of molecular aggregates with a tubular (cylindrical) shape. The exciton states responsible for these properties can be distinguished with regard to a transverse wave number, which directly relates to optical selection rules and polarization direction of the associated absorption line. We discuss two types of analytical solutions for the exciton wave functions and the associated absorption and dichroism spectra.     

Analysis of quantum well size alteration effects on slow light device based on excitonic population oscillation

In this paper we investigate the effects of quantum well size changes on slow light device properties. The principle properties such as center frequency and slow down factor of a slow light device are affected by changing the size of quantum well. In this way, the effects of quantum well size on Oscillator Strength and binding energy of exciton are considered separately. First, we investigate the variations in oscillator strength of exciton due to different quantum well size. Second, exciton binding energy level shift due to size of quantum well is investigated. According to this analysis, we have developed a new method for tuning slow light device bandwidth center frequency and slow down factor. Analysis and simulation of a basic GaAs/AlGaAs quantum wells optical slow light device based on excitonic population oscillation shows that size of quantum wells could tune both of the frequency properties and slow down factor of an optical slow light device. Simulation results show that slow down factor and oscillation strength of exciton are proportional to each other in direct manner. Moreover, decreasing the quantum well width, causes enhancement in binding energy of excitons. These achievements are useful in optical nonlinearity enhancements, all-optical signal processing applications and optical communications.     

Bleaching in the region of exciton resonance of layered GaSe crystals

Light absorption in the region of exciton resonance of GaSe crystal is studied experimentally at high levels of optical excitation. A picosecond YAG:Nd³⁺ laser emitting 30-ps light pulses and a dye laser with a pulse width of ～3 ns tunable within the range 594–643 nm were used as light sources. It was found that, at high levels of optical excitation, the exciton absorption line of the GaSe crystal disappeared, which was attributed to increasing exciton density with arising mechanisms of their decay: exciton-exciton interactions and screening of excitons by the free charge-carrier plasma. It is shown that these mechanisms are also responsible for the arising new emission band in the long-wavelength region of the photoluminescence spectrum.     

Exciton optical absorption in disordered,strained InGaAs/GaAs single quantum wells

The absorption coefficient spectrum of undoped, disordered InGaAs/GaAs single quantum wells is calculated within the parabolic band approximation in this compressively strained structure. Results are presented for light propagating normal to and along the plane of the quantum well, taking into consideration the 1S-like exciton and all bound states, including the 2D enhancement Sommerfeld factor. The results presented here show that the exciton peaks in TE polarization remain constant with disordering and exhibit a larger wavelength shift than in TM polarization. The absorption edge in disordered InGaAs/GaAs, which is a function of the strain and interdiffusion, can be tailored to the desired wavelength around 1.0 μm. These results can be of interest in the design of photonic devices on a single substrate.     

Exciton distribution on single-walled carbon nanotube

In this paper we calculate the binding energy of an exciton using the tight-binding model and discuss the exciton distribution in detail. We analytically explain the dependence of the distribution direction of exciton on the chiral angle, and the distribution localization along the tube axis and oscillating along the tube circumference. The size of exciton is estimated to be slightly larger than the diameter of the nanotube and it shows two family patterns versus the inverse of tube diameter as similar as in the exciton binding energy.     

Determination of excitonic binding energies in symmetrically strained (GaIn)As/Ga(AsP) multiple quantum wells using quantum beat spectroscopy

We have performed sub-picosecond four-wave mixing experiments on a series of symmetrically strained (GaIn)As/Ga(AsP) multiple quantum wells. We show that these measurements allow a precise determination of exciton binding energies. One of the advantages of this quantum beat method as compared to linear optical methods is that a determination of the exciton binding energy is possible even in the presence of considerable inhomogeneous broadening. In addition, the dependence of the exciton binding energy on the strain in the (GaIn)As quantum well layers suggests that the reduced electron-hole effective mass is not influenced by the increase in strain.     

Exciton absorption of light in a smooth random field

A study is made of interband absorption of light in a smooth random field that varies slowly in space. When the exciton energy in the random field is much less than the exciton binding energy, the influence of the random field on the discrete spectrum of the exciton is expressed through the appearance of an exponential “tail” to every exciton line. But if the exciton energy in the random field is much greater than the exciton binding energy, the exciton effects are unimportant because the random field breaks up the excitons.     

Coherent optical writing and reading of the exciton spin state in single quantum dots

We demonstrate a one-to-one correspondence between the polarization state of a light pulse tuned to neutral exciton resonances of single semiconductor quantum dots and the spin state of the exciton that it photogenerates. This is accomplished using two variably polarized and independently tuned picosecond laser pulses. The first "writes" the spin state of the resonantly excited exciton. The second is tuned to biexcitonic resonances, and its absorption is used to "read" the exciton spin state. The absorption of the second pulse depends on its polarization relative to the exciton spin direction. Changes in the exciton spin result in corresponding changes in the intensity of the photoluminescence from the biexciton lines which we monitor, obtaining thus a one-to-one mapping between any point on the Poincaré sphere of the light polarization to a point on the Bloch sphere of the exciton spin.