Evolution of polarization in an inhomogeneous isotropic medium

The depolarization and rotation of the polarization plane of radiation propagating in a two-dimensional graded-index medium is investigated on the basis of the quantum-mechanical method of coherent states. It is shown that the degree of polarization of both linearly and circularly polarized radiation decreases with increasing distance as a result of interaction between the polarization (spin) and the path (orbital angular momentum) of the beam. The wave nature of the depolarization is emphasized. The depolarization decreases as the radiation wavelength decreases. It is found that the degree of polarization exhibits oscillations of pure diffraction origin during the propagation of light in a single-mode optical fiber. It is shown that the rotation of the polarization plane is nonuniform in character and depends on the offset and the tilt angle of the incident-beam axis relative to the fiber axis. The Berry phase is found to undergo oscillations of a wave nature during the propagation of radiation in an inhomogeneous medium. It is shown that the spread in the angle of rotation of the polarization plane increases with distance and can be determined from measurements of the degree of polarization of the radiation. Zh. éksp. Teor. Fiz. 112, 1985–2000 (December 1997)     

Linear polarization of radiation from ZnCdSe/ZnSe quantum wells induced by the anisotropic profile of interfaces

The low-temperature spectra of the exciton luminescence of the ZnCdSe/ZnSe quantum well grown in the [100] direction have been analyzed. It has been found that the observed radiation is linearly polarized in the well plane along either the [011] axis or the orthogonal $[0\bar 11]$ axis depending on the energy of exciting light. Polarization along the [011] axis is associated with the anisotropy of the geometric profiles of interfaces and corresponds to the polarization of excitons localized in the terraces elongated along the [011] axis, which are regions corresponding to an increase in the well thickness by a depth of one or several monolayers. The terraces appear due to the existence of growth steps on the interfaces. Anisotropy in the distribution of growth steps, i.e., a longer length of the steps along the [011] axis, is also responsible for the appearance of polarization along the $[0\bar 11]$ axis. Such a polarization appears due to the found specific channel of anisotropic exciton absorption. As a result of such absorption, free excitons with large wave vectors are excited. The excitation of excitons by light is indirect, because it involves additional processes of the elastic scattering of excitons on the interface growth steps. Prevailing scattering on an ensemble of longer growth steps (along the [011] axis) leads to the alignment of the wave vectors of excited excitons. A difference between the absorption probabilities for the polarizations of excitation along the [011] and $[0\bar 11]$ axes is caused by a difference between the probabilities of scattering on steps for excitons whose dipole moments and, correspondingly, the axes of the P-type wave functions for the holes involved in the process are oriented along and across the direction of the alignment of wave vectors of excitons.     

Optical properties of excitons in semiconductor (InP)-insulator quantum wires

Features in the spectra of absorption, luminescence, and luminescence efficiency obtained under sample excitation with differently polarized laser radiation, and the nonlinear dependence of the luminescence intensity on the excitation level are explained as due to excitonic transitions in semiconductor (InP)-insulator (chrysotile asbestos) quantum wires. The measured excitonic-transition energies in the quantum wires are in quantitative agreement with calculations. The calculations took into account both the size quantization in a quasi-one-dimensional structure and the “dielectric enhancement” of excitons (the noticeable increase of the exciton binding energy and of the excitonic-transition oscillator strength associated with the increased attraction between the electron and the hole due to the large difference between the dielectric permittivities of the semiconductor and the dielectric matrix).     

调制n型掺杂ZnSe/BeTe Ⅱ型量子阱结构的发光特性

报道了调制n型掺杂ZnSe/BeTe/ZnSe Ⅱ型量子阱(type-II QW)在极低温至室温(14—296K)条件下的各种光学性质. 反射光谱显示了对于非掺杂样品,激子(X)的跃迁起着支配作用,而只有在掺杂样品的光谱里展示了一个典型的负的带电激子(X^-)的跃迁特征. PL光谱及其直线偏振度P_l都显著地依赖于n型掺杂量和平行于QW生长方向的外加电场. 这个特征被认为是由n型掺杂导致了内秉电场(built 关键词： 光致发光 二维电子气 带电激子 Ⅱ型量子阱     

Polarization of Čerenkov radiation in anisotropic media

Using the method of Stokes parameters, we examine the polarization of erenkov radiation in anisotropic media. The study reveals that the radiation is totally polarized and that circular polarization is purely a quantum effect. We examine two cases: when the particle initially moves along the optical axis and when the particle initially moves perpendicular to the optical axis.     

The self-activated luminescence of ZnS1−xSex

The self-activated luminescence (SA) of ZnS : I, ZnSe : Cl, ZnSe : A1, and some ZnS_1?xSe_x : Cl single crystals has been investigated at LHeT under excitation with light. The SA-luminescence band has been identified in each case by the temperature dependence and by a direct excitation band resulting in polarized emission. Symmetry C_3v in case of ZnS : I and ZnSe : Cl and symmetry C_s in case of ZnSe : A1 for the SA-centre have been unambiguously revealed from angular dependences of polarization. Thus the nearest neigbour-compensated cation-vacancy has been identified as complex responsible for SA-luminescence.     

Study of polarized luminescence in erbium-doped laser glasses

A partial polarization of luminescence in laser phosphate and silicate erbium-doped glasses was found to take place for the fundamental laser transition ⁴ I _13/2→⁴ I _15/2 (λ=1.55 μm) under excitation by linearly polarized laser radiation (532 and 790–990 nm). The shape of the luminescence spectrum depends on the wavelength of the exciting light and on the composition of the glass matrix. The degree of polarization of the luminescence depends on the spectral range of both the excitation and the detection, attaining a maximum of ～1%. The concentration dependence of the degree of polarization is studied.     

Linear polarization of luminescence in Bose-Einstein condensation of indirect excitons and spontaneous symmetry breaking

The linear polarization of luminescence from the Bose-Einstein condensate of dipolar (indirect) excitons accumulated in the ring lateral traps in GaAs/AlGaAs Schottky-diode heterostructures with a wide single quantum well has been observed. Luminescence from direct excitons remains unpolarized under the same experimental conditions. It has been shown that the linear polarization of the exciton condensate may arise from the anisotropic electron-hole (e–h) exchange interaction associated with the lateral anisotropy of the confining potential. The interaction mixes and splits the ground state of optically active excitons on heavy holes (with angular momentum projections of m=±1). The split spectral components from the corresponding angular momentum projections are linearly polarized in mutually orthogonal directions. Under this e–h exchange, the condensate component of excitons should appear in the lowest of the split states and luminescence from the Bose-Einstein condensate of excitons in such a split state becomes linearly polarized along the 〈110〉 crystallographic direction in the quantum well plane. The observed effect is a manifestation of spontaneous symmetry breaking in Bose-Einstein condensation of excitons.     

Luminescence study of self-trapped excitons in CdMoO4

Luminescence properties of CdMoO₄ crystals have been investigated in a wide temperature range of T=5–300 K. The luminescence-excitation spectra are examined by using synchrotron radiation as a light source. A broad structureless emission band appears with a maximum at nearly 550 nm when excited with photons in the fundamental absorption region (<350 nm) at T=5 K. This luminescence is ascribed to a radiative transition from the triplet state of a self-trapped exciton (STE) located on a (MoO₄)^2? complex anion. Time-resolved luminescence spectra are also measured under the excitation with 266 nm light from a Nd:YAG laser. It is confirmed that triplet luminescence consists of three emission bands with different decay times. Such composite nature is explained in terms of a Jahn–Teller splitting of the triplet STE state. The triplet luminescence at 550 nm is found to be greatly polarized in the direction along the crystallographic c axis at low temperatures, but change the degree of polarization from positive to negative at T>180 K. This remarkable polarization is accounted for by introducing further symmetry lowering of tetrahedral (MoO₄)^2? ions due to a uniaxial crystal field, in addition to the Jahn–Teller distortion. Furthermore, weak luminescence from a singlet state locating above the triplet state is time-resolved just after the pulse excitation, with a polarization parallel to the c axis. The excited sublevels of STEs responsible for CdMoO₄ luminescence are assigned on the basis of these experimental results and a group-theoretical consideration.     

A method for studying the multipolarity and orientation of elementary oscillators in cubic crystals on the basis of axially periodic dependence of the luminescence intensity

A new method for determining the multipolarity and orientation of elementary quantum oscillators in cubic crystals is proposed. This method is based on studying the spatially periodic dependences of the intensity of luminescence observed in crystals with an induced anisotropy. The method was experimentally verified on LiF crystals containing F ₂ and F ₃ ⁺ color centers. It is shown that the observed spatially periodic dependences are in good agreement with the calculated dependences. The proposed method supplements the known methods for studying polarized luminescence (the methods of azimuthal dependences and polarization ratios and their various modifications). However, the new method, in contrast to the known ones, does not require measurements of the polarization ratios (for example, the degree of luminescence polarization), which in some cases significantly simplifies the experiment. Another advantage of the proposed method is its universality; i.e., it can be applied with equal success to both anisotropic and cubic crystals.     

Collective behavior of interwell excitons in GaAs/AlGaAs double quantum wells

Photoluminescence spectra of interwell excitons in double GaAs/AlGaAs quantum wells (n-i-n structures) have been investigated (an interwell exciton in these systems is an electron-hole pair spatially separated by a narrow AlAs barrier). Under resonance excitation by circularly polarized light, the luminescence line of interwell excitons exhibits a significant narrowing and a drastic increase in the degree of circular polarization of photoluminescence with increasing exciton concentration. It is found that the radiative recombination rate significantly increases under these conditions. This phenomenon is observed at temperatures lower than the critical point and can be interpreted in terms of the collective behavior of interwell excitons.     

Exciton dynamics in ZnCdSe/ZnSe quantum-well structures

Exciton dynamics in ZnCdSe/ZnSe quantum-well structures have been studied from luminescence spectra obtained at T=2 K. The energy and phase relaxation times of localized exciton states have been determined from a study of the destruction of exciton optical alignment by an external magnetic field and direct measurements of the polarized-radiation decay kinetics in the picosecond range. The exciton polarization lifetimes measured by two independent techniques are found to be in a good agreement. Fiz. Tverd. Tela (St. Petersburg) 40, 809–810 (May 1998)     

Relaxation of electronic excitations in beryllium oxide: A time-resolved vacuum-UV spectroscopy study

Beryllium oxide crystals are studied by time-resolved optical and luminescence vacuum-UV spectroscopy. The low-temperature luminescence spectra and the luminescence decay kinetics (2.5–10 eV, 1–500 ns) upon selective photoexcitation, and also the luminescence excitation and reflectivity spectra (8–35 eV), are analyzed for BeO crystals with the optic axis aligned parallel and perpendicular to the electric vector of exciting polarized synchrotron radiation. It is found that the radiative relaxation of electronic excitations proceeds through a large number of channels. The excited states of self-trapped excitons are characterized by different multiplicity depending on the excitation energy and the sample orientation.     

Heterostructures with InGaAs/GaAs quantum dots doped with transition elements: II. Study of the circularly polarized luminescence

Optical and transport properties of the heterostructure with the InAs/GaAs quantum dots doped with manganese or chromium atoms in the course of growth using metal-organic vapor phase epitaxy (MOVPE) are analyzed. Circularly polarized luminescence was obtained due to doping of quantum dots with the Mn or Cr atoms. The sign of the degree of circular polarization was found to depend on the dopant. The effect is interpreted using specific features of the radiative recombination in quantum dots in the presence of resident electrons and holes.     

Highly luminescent (ZnSe)ZnS core-shell quantum dots for blue to UV emission: synthesis and characterization

We synthesized nearly monodisperse bare ZnSe nanocrystallites having luminescence which ranges in wavelength from 340 to 430 nm via nucleation due to supersaturation and growth followed by size selective precipitation. Bare ZnSe dots' outermost surface is passivated with organic HDA/TOP. In order to enhance the radiative emission from the semiconductor nanocrystals, we capped the bare ZnSe quantum dots with ZnS semiconductor materials of a wider band gap and 5% of lattice mismatch and produced highly luminescent core-shell (ZnSe)ZnS quantum dots. The core-shell (ZnSe)ZnS nanocrystals show 20 times or more as greatly enhanced luminescence quantum yields as those of bare ZnSe nanocrystals. The ZnSe bare dots and the (ZnSe)ZnS core-shell dots have cubic zinc blende structures as expected from the bulk structure. The observed shapes of bare ZnSe and core-shell (ZnSe)ZnS dots are nearly spherical or ellipsoidal with the aspect ratios of 1.2 and 1.4, respectively. They are not faceted.     

Spin-polarized Mn2+ emission from Mn-doped colloidal nanocrystals

We report magnetophotoluminescence studies of strongly quantum-confined 0D diluted magnetic semiconductors (DMS), realized in Mn(2+)-doped ZnSe/CdSe core-shell colloidal nanocrystals. In marked contrast to their 3D (bulk), 2D (quantum well), 1D (quantum wire), and 0D (self-assembled quantum dot) DMS counterparts, the ubiquitous yellow emission band from internal d-d ((4)T(1)→(6)A(1)) transitions of the Mn(2+) ions in these nanocrystals is not suppressed in applied magnetic fields and does become circularly polarized. This polarization tracks the Mn(2+) magnetization, and is accompanied by a sizable energy splitting between right- and left-circular emission components that scales with the exciton-Mn sp-d coupling strength (which, in turn, is tunable with nanocrystal size). These data highlight the influence of strong quantum confinement on both the excitation and the emission mechanisms of magnetic ions in DMS nanomaterials.     

Charge and energy transfer in double asymmetric quantum wells with quantum dots

The luminescence and luminescence excitation spectra of CdSe/ZnSe quantum dots are studied in a set of double quantum wells with the ZnSe barrier of width 14 nm, the same amount of a deposited CdSe layer forming a deep well and shallow wells with different depths. It is found that for a certain relation between the depths of shallow and deep wells in this set, conditions are realized under which the exciton channel in the luminescence excitation spectrum of a shallow well dominates in the region of kinetic exciton energies exceeding 10 longitudinal optical phonons above the bottom of the exciton band of the ZnSe barrier. A model is developed for the transfer of electrons, holes, and excitons between the electronic states of shallow and deep quantum wells separated by wide enough barriers. It is shown that the most probable process of electronic energy transfer between the states of shallow and deep quantum wells is indirect tunneling with the simultaneous excitation of a longitudinal optical phonon in the lattice. Because the probability of this process for single charge carriers considerably exceeds the exciton tunneling probability, a system of double quantum wells can be prepared in which, in the case of weak enough excitation, the states of quantum dots in shallow quantum wells will be mainly populated by excitons, which explains experimental results obtained.     

Polarization photosensitivity of Schottky barriers on monoclinic ZnAs<Subscript>2</Subscript> crystals

Pioneering experiments on creating Cu(In)/p-ZnAs₂ photosensitive Schottky barriers on monoclinic ZnAs₂ single crystals grown by directional crystallization from a near-stoichiometric melt are described. Photosensitivity spectra are taken from the structures in natural and linearly polarized radiations. The structures are found to be sensitive to polarization. From the photoactive absorption spectra of the monoclinic ZnAs₂, it follows that most minimal direct band-to-band transitions in these crystals are allowed in the E ∥ z axis polarization and can be used, specifically, in designing novel devices-polarization—controlled switches of spectral ranges of incident radiation photorecording.     

Optical properties of excitons in CdS semiconductor-insulator quantum wires

The characteristic features of the luminescence spectra of CdS semiconductor nanocrystals, crystallized in hollow channels in a dielectric template, are explained in terms of excitonic transitions in semiconductor-insulator quantum wires. The excitonic transition energies agree with the values calculated taking into account the effects of size quantization and the “dielectric enhancement of excitons” — the large increase in the electron-hole attraction as a result of the difference between the permittivities of the semiconductor and insulator. The theoretically computed binding energies of excitons in CdS quantum wires with a diameter of 10 nm reach 170 meV. It is shown that the excitonic transition energy is constant for a wide range of wire diameters. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 3, 216–220 (10 August 1999)     

Inelastic interaction of ultrashort pulses of polarized light with an ensemble of isolated quantum dots

A system of equations is formulated describing the evolution of a slowly varying envelope of an arbitrarily polarized ultrashort pulse of electromagnetic radiation in a medium with its resonant properties determined by an ensemble of isolated quantum dots. It is assumed that the concentration of quantum dots is small and that the whole system is equivalent to a gas of resonant four-level atoms. Particular solutions are found that correspond to the propagation of a stationary optical pulse. It is shown by numerical solution of the generalized truncated Maxwell-Bloch equations that steady-state propagation is possible only for circularly polarized light pulses, whereas the pulses of arbitrary polarization either decay and experience the dispersion-related broadening or are converted into circularly polarized solitary waves.