Optical properties of Tl3SbS3 acousto-optic crystals

Optical properties of acousto-optical material Tl₃SbS₃ were investigated at the fundamental absorption edge. The basic (n = 1) and excited (n = 2, 3) exciton states were obtained from the λ-modulated reflection spectra for polarizations E⊥c at 10 K. Taking into account the spatial dispersion we determined the parameters of excitons by calculating the spectra shapes of λ-modulated reflection of line n = 1 and estimated values of the zone-translation masse and the reduced effective masse of excitons, the effective masse of electrons , heavy and light holes.     

Optical spectra and energy band structure of AgAsS2 crystals

Ground and excited states of three exciton series are observed in the region of fundamental absorption edge of AgAsS₂ crystals. The contours of exciton reflection spectra are calculated and the main parameters of excitons and energy bands are determined in the center of Brillouin zone. The optical reflection spectra are investigated at 30 K in E∥c and E⊥c polarizations in AgAsS₂ crystals in the region of 2-6 eV. The optical functions are calculated from the reflection spectra and a scheme of electronic transitions responsible for peculiarities of reflection spectra deep into the absorption band is proposed.     

Interference of resonance luminescence of exciton polaritons in CuGaS2 crystals

The nonmodulated and wavelength-modulated reflection spectra of CuGaS₂ crystals for the polarization EIIc of 10 K are studied. The states n = 1, 2 and 3 of the excitons Γ₄ (A-excitons) and n = 1, n = 2 of B- and C-excitons are found. The nonmodulated absorption spectra for the polarization E⊥c at 10 K have been studied. The states n = 1, 2 and 3 of Γ₅ excitons are found. The main parameters of the A (Γ₄, Γ₅) and B, C exciton series at the energies of the longitudinal and transverse excitons Γ₄ for the states n = 1 and n = 2, the effective masses of electrons and holes are determined. The photoluminescence peaks were observed at n = 3 and n = 4 of the excitons Γ₅ in the luminescence spectra excited by the line 4880 Å of Ar⁺ laser. In the luminescence spectra the interference is found.     

Influence of exciton decay on the polariton luminescence spectra of CdTe crystal

The polariton luminescence spectra of CdTe crystals have been numerically calculated with allowance for the decay of mechanical excitons and compared with the known experimental low-temperature photoluminescence spectra of these crystals. The mechanism of formation of polariton luminescence in dependence of the optical parameters of crystal for the exciton resonance A _n=1 is discussed.     

Spatial dispersion in polariton spectra of CuGaS2 crystals

The unmodulated and wavelength-modulated reflectivity spectra of CuGaS₂ crystals for the polarization E ‖ c, k ⊥ c at 77 and 8 K have been studied. The states n=1, 2, and 3 of A excitons and n=1 and 2 of B and C excitons are established. The luminescence spectra from the surface at k ‖ c and k ⊥ c are obtained. The fine structure of the reflectivity spectra of excitons are analyzed with due regard for the normal and oblique incidence of light onto the crystal surface. The main parameters of the A, B, and C excitonic series are determined such as the energies of the longitudinal and transverse excitons Γ₄ (E ‖ c) for states n=1 and 2, the longitudinal and the transverse mass of excitons in CuGaS 2, and the effective masses of electrons (m _c1*) and holes (m _v1*, m _v2*, m _v3*). It is shown that the mass m _v1* in the upper valence band at k ‖ c equals (0.7–0.8)m ₀ and at k ⊥ c, 1.87m ₀.     

Optical properties of natural quantum-well compounds (C6H5-CnH2n-NH3)2PbBr4 (n=1-4)

This paper describes the synthesis and characterization of self-assembled organic-inorganic layered perovskite compounds, (C₆H₅-C_nH_2n-NH₃)₂PbBr₄ (n=1-4). the effect of the number of carbon atoms of the alkyl chain length (n) on optical properties has been studied. (C₆H₅-C_nH_2n-NH₃)₂PbBr₄ films fabricated by spin-coating are microcrystalline form, single phase and oriented with the c-axis. Crystallinity, the maximum PL intensity and the lifetime of exciton emissions varied with the number of carbon atoms. the lowest-energy exciton splits into a few fine-structure levels at low temperatures. Time-resolved photoluminescence spectra reveal that (C₆H₅-C_nH_2n-NH₃)₂PbBr₄ shows both singlet and triplet excitons. with decreasing temperature, triplet exciton emissions become dominant for (C₆H₅-C_nH_2n-NH₃)₂PbBr₄ (n=1-3), while (C₆H₅-C₄H₈-NH₃)₂PbBr₄ shows mainly singlet exciton emissions. The intersystem crossing from excited singlet state to triplet state plays an important role in the relaxation process of excitons.     

Photovoltaic spectra of undoped GaAsAl0.25Ga0.75As multiple quantum well structures: Correlation with photoluminescence

The photovoltaic spectra of molecular beam epitaxy GaAsAl_0.25Ga_0.75As multiple quantum wells in the vicinity of the n=1 sub-bandgap region and their correlation with the corresponding photoluminescence spectra have been studied in the temperature range 2–297K. Photovoltaic and photoluminescence spectra are found to agree for the transition of the n=1 heavy- and light-hole excitons. The photovoltaic spectra in the Schottky-barrier configuration show as the relative maxima and minima at the corresponding exciton photoluminescence lines. A possible model of the observed coincidence of photoluminescence and photovoltage is discussed.     

Excitons in monoclinic zinc diphosphide: Orthoexciton and polariton effects at n=1 resonance

Quantitative investigations of the hydrogen-like exciton B series in the absorption spectra of the β-ZnP₂ crystal for various wave vector directions and polarization states of radiation are conducted. It is shown that the B spectrum constitutes a single orthoexciton series with S-type envelope functions, and low-energy components in doublet lines belong to the S-type for lines in the series with n≥3. Polariton effects are clearly manifested at the B _n=1 exciton resonance, and Bouguer’s law is violated. The oscillator strength tensor components are determined for transitions to the exciton states of the B series, and the polariton parameters at the B _n=1 exciton resonance are calculated.     

Magnetopolaritons in ZnTe

From an invariant expansion, we construct the exciton Hamiltonian for the Γ₆×Γ₈ excitons in theT _d -type material ZnTe represented by an 8×8 matrix including the influences of a finite wave vector and an external magnetic field. We diagonalize the Hamiltonian matrix to obtain the exciton states. Then the excitons are coupled to the electromagnetic radiation field thus giving the polariton states. The theoretical dispersion curves are fitted to the results of two-photon Raman scattering and reflection experiments in magnetic fields up to 22 T. From this fit we deduce precise values for the eigenergies, exciton masses,g-factors, and diamagnetic shifts.     

Theory of infra-red and optical spectra of antiferromagnets

The contributions of magnons to the optical properties of antiferromagnets having the rutile structure are discussed. The properties considered are electric-dipole active two-magnon absorption in the infra-red, and magnon sidebands on sharp-line exciton transitions in the visible. The discussion is based on a thorough treatment of the properties of excitons and magnons in the antiferromagnetically ordered state. The site-group and space-group symmetries of the magnetic excitations are derived and the selection rules for electric and magnetic dipole transitions are determined. The occurrence of magnetic Davydov splittings of the excitons is investigated, and their symmetry properties throughout the Brillouin zone are derived. The functional dependences of exciton energy on wave vector are calculated. Applications of the theory are made to experimental results on excitons and magnons in MnF₂, FeF₂ and CoF₂.

The possible mechanisms for two-magnon and magnon-sideband absorption are discussed, and the influence of crystal symmetry on these mechanisms is described. The two-magnon state responsible for electric-dipole absorption is identified and selection rules for electric-dipole activity are presented. A spin Hamiltonian for the two-magnon process is set up and used to derive expressions for absorption coefficients for electric vector parallel and perpendicular to the crystal c-axis. Comparison with experiment for MnF₂ yields numerical values for the parameters of the basic coupling mechanism. The exciton-magnon states which give rise to magnon-sideband absorption are explicitly constructed and electric-dipole selection rules are derived for all possible types of sideband. Spin Hamiltonians for the various magnonsideband absorption processes are presented and used to derive expressions for sideband shapes. The results are applied to the experimental spectra for MnF₂ and FeF₂ and the sideband shapes in MnF₂ are calculated numerically. The sideband shapes observable in emission spectra are also briefly discussed.     

Energy band structure and Frenkel excitons in PbGa2S4

Optical reflection spectra are measured and calculated in PbGa₂S₄ crystals in the region of resonances related to excitons with large oscillator strength and binding energy (Frenkel excitons). The splitting of the upper valence band in the center of the Brillouin zone due to crystal field (Δ_cf) and spin orbit (Δ_so) interaction are determined. Optical reflection spectra are measured and calculated according to Kramers-Kronig relations in the region of 3-6 eV in Е⊥с and Е∥с polarizations, and the optical constants n, k, ε₁ and ε₂ are determined. The observed electronic transitions in PbGa₂S₄ crystals are discussed in the frame of theoretical energy band structure calculation for thiogallate crystals.     

Interference of ordinary and extraordinary waves in AgAsS2 crystals

The anisotropy of the near-bandgap absorption is investigated in AgAsS₂ crystals. The refraction indices, n^‖ and n^⊥ respectively for the E‖c and E⊥c polarizations as well as the spectral dependence of the refraction indices difference, Δn=n‖−n^⊥ are determined from the interference spectra of AgAsS₂ crystals. A transmission band with four maxima is observed in the transmission spectra of crystals placed between crossed polarizers. The optical parameters n, k, ε₁, and ε₂ for the E‖c and E⊥c polarizations are calculated from the reflection spectra by using the Kramers–Kronig relations.     

Electromodulation spectroscopy of excitons: High field regime

The effects of electric field on excitons have been investigated experimentally by means of electroabsorption (EA). The behavior of excitons with ionization fields F_I of the same order of magnitude as or smaller than typical applied fields is reported. EA spectra associated with excitons in TlBr, PbI₂, BiI₃, trigonal Se, and α-monoclinic Se will illustrate this case. In this situation the EA signal arises principally from the effect on the exciton ground state, and no finer structure could be detected. The exciton ground state undergoes a shift to higher energies with the applied field. Characteristic dependences with field of the sizes, energy position of the peaks, and zero crossing points of the EA signals have been found. Estimates of exciton parameters can be readily obtained from the EA spectra. Unusual features of the exciton EA spectra PbI₂ are discussed. Qualitatively, at least, theoretical predictions are obeyed.     

Collective state of interwell excitons in GaAs/AlGaAs double quantum wells under pulse resonance excitation

The time evolution and kinetics of photoluminescence (PL) spectra of interwell excitons in double GaAs/AlGaAs quantum wells (n-i-n structures) have been investigated under the pulse resonance excitation of intrawell 1sHH excitons using a pulsed tunable laser. It is found that the collective exciton phase arises with a time delay relative to the exciting pulse (several nanoseconds), which is due to density and temperature relaxation to the equilibrium values. The origination of the collective phase of interwell excitons is accompanied by a strong narrowing of the corresponding photoluminescence line (the line width is about 1.1 meV), a superlinear rise in its intensity, a long time in the change of the degree of circular polarization, a displacement of the PL spectrum toward lower energies (about 1.5 meV) in accordance with the filling of the lowest state with the exciton Bose condensate, and a significant increase in the radiative decay rate of the condensed phase. The collective exciton phase arises at temperatures T<6 K and interwell exciton densities n=3×10¹⁰ cm^?2. Coherent properties of the collective phase of interwell excitons and experimental manifestations of this coherence are discussed.     

Time-resolved spectra and kinetics of the exciton photoluminescence in different types of GaAs/AlAs superlattices

The temperature dependence of the recombination dynamics of excitons is investigated by time-resolved photoluminescence spectroscopy in(GaAs)_n / (AlAs)_n superlattices, where n denotes the layer thickness in monolayers, for different types of band structures. In direct-gap superlattices with a layer thickness of the order or larger than the exciton Bohr radius, the carrier dynamics is dominated by the transfer from light-hole to heavy-hole excitons. When the layer thickness becomes smaller than the exciton radius, the dynamics of free excitons is controlled by localization. vb In quasi-direct and indirect-gap superlattices, the influence of lateral potential fluctuations due to interface roughness completely governs exciton recombination.     

Spectroscopy of resonant excitation of exciton luminescence of GaSe–GaTe solid solutions

The luminescence excitation spectra of localized excitons in GaSe_0.85Te_0.15 solid solutions have been investigated at the temperature T = 2 K. It has been shown that the excitation spectra of excitons with the localization energy ε > 10 mV exhibit an additional maximum M _E located on the low-energy side of the maximum corresponding to the free exciton absorption band with n = 1. It has been found that the shift in the position of the maximum M _E in the excitation spectrum with respect to the energy of detected photons increases as the energy of detected photons decreases, i.e., with an increase in the localization energy of excitons. Under the resonant excitation of localized excitons by a monochromatic light from the region of the exciton emission band, in the exciton luminescence spectrum on the low-energy side from the excitation line, there is also a maximum of the luminescence (M _L ). The energy distance between the position of the excitation line and the position of the maximum in the luminescence spectrum increases with a decrease in the frequency of the excitation light. The possible mechanisms of the formation of the described structure of the luminescence excitation and exciton luminescence spectra of GaSe_0.85Te_0.15 have been considered. It has been concluded that the maximum M _E in the excitation spectrum and the maximum M _L in the luminescence spectrum are attributed to electronic–vibrational transitions with the creation and annihilation of localized excitons, respectively.     

Magneto-optical studies of excitons in AgGaS2

The reflection, absorption and luminescence spectra of the lowest excitons in AgGaS₂ are measured in magnetic fields. The results can be explained by the quasicubic model. The exchange splittings of the Γ₄, Γ₅ and Γ₃ exciton state are evaluated. The Zeeman splittings of the Γ₅ and Γ₅-Γ₃ states yield g-values of + 3.8 and 3.7, respectively. They are determined mainly by the contributions of the hole and conduction electron spins.     

Polarization dependence of the two quantum absorption spectrum of intrinsic excitons in ZnO

The intrinsic exciton spectrum of ZnO at 4.2°K has been investigated by two quantum absorption of light polarized parallel and perpendicular to hexagonal c-axis (light direction normal to c-axis). Three lines are observed with intensity and spectral position depending on polarization of the two interacting beams in respect to the c-axis. They are interpreted as n=2 states of the exciton series A, B and C with 2P₀ and 2P₊₁ envelope.     

Creation of spin-oriented 2P excitons in CdS by two-photon magneto-absorption

We report experimental results on the creation of 2P excitons in CdS by the two-photon magneto-absorption process involving a simultaneous absorption of visible and infrared photons. For a magnetic field parallel to the c axis and particularly for a visible beam propagating along the c axis, spin-oriented 2P (A) excitons are created from the A (Γ₉) valence band. We explain these results on the basis of the main contribution of the 1SΓ₅ (A) exciton intermediate state.     

Measurements of exciton absorption with pico-second light pulses produced by a self-phase modulation technique

Time-resolved absorption spectra of the A exciton in CdSe are measured by using pico-second white-light pulses produced by a self-phase modulation technique. The absorption peak of the n = 1 A exciton shifts to the high-energy side due to the high-density effect. The magnitude of the shift reaches a maximum at 20 psec after the pulse excitation, which is considered to indicate that the time constant for the formation of the n = 1 excitons by band-to-band excitation is about 20 psec.