Effects of optical activity on exciton luminescence of CdS

The luminescence spectra have been investigated in the vicinity of the B_n=1 exciton resonance in CdS at T = 2 K. The effect of “weak” optical activity on luminescence has been observed for the first time. The observed spectra are compared with the frequency dependences of the polariton transmission coefficients.     

Intrinsic luminescence of rare-earth oxyorthosilicates

The spectra and decay kinetics of luminescence and the excitation and reflection spectra of the luminescence of orthosilicates A ₂SiO₅ (A = Y, Lu, Sc, Gd), both nominally pure and doped with cerium, are measured using time-resolved VUV spectroscopy in the ranges of energies ?ν = 1.5–16 eV and temperatures T = 8–3000 K. The band structure of the crystals is calculated in the local electron density approximation (LDA). The origin of the intrinsic luminescence in the crystals studied is established, and the assumption regarding the existence of self-trapped excitons and their structure is made.     

Electronic excitations in BeAl<Subscript>2</Subscript>O<Subscript>4</Subscript>, Be<Subscript>2</Subscript>SiO<Subscript>4</Subscript>, and Be<Subscript>3</Subscript>Al<Subscript>2</Subscript>Si<Subscript>6</Subscript>O<Subscript>18</Subscript> crystals

Low-temperature (T = 7 K) time-resolved selectively photoexcited luminescence spectra (2–6 eV) and luminescence excitation spectra (8–35 eV) of wide-bandgap chrysoberyl BeAl₂O₄, phenacite Be₂SiO₄, and beryl Be₃Al₂Si₆O₁₈ crystals have been studied using time-resolved VUV spectroscopy. Both the intrinsic luminescence of the crystals and the luminescence associated with structural defects were assigned. Energy transfer to impurity luminescence centers in alexandrite and emerald was investigated. Luminescence characteristics of stable crystal lattice defects were probed by 3.6-MeV accelerated helium ion beams.     

Specific features of absorption and luminescence in CsBr: EuOBr crystals

The specific features of the absorption, photoluminescence, x-ray luminescence, thermally stimulated luminescence, and photostimulated luminescence spectra of CsBr: Eu²⁺ single crystals grown using the Bridgman method are investigated in the temperature range 80–500 K at the highest possible dopant content (0.1–0.4 mol % EuOBr in the batch) required for preparing perfect crystals. It is shown that an increase in the dopant content leads to a broadening of the absorption and photoluminescence excitation bands with maxima at wavelengths of 250 and 350 nm due to the interconfigurational transitions 4f⁷(⁸S_7/2) → 4f⁶5d(e _g , t_2g) in Eu²⁺ ions. The photoluminescence and photostimulated luminescence spectra of CsBr: EuOBr single crystals (0.1–0.4 mol % EuOBr) contain a band at a wavelength of λ_max=450 nm and bands at wavelengths of λ_max=508–523 and 436 nm. The last two bands are assigned to Eu²⁺-V_Cs isolated dipole centers and Eu²⁺-containing aggregate centers, respectively. It is revealed that the intensity of the luminescence associated with the aggregate centers (λ_max=508–523 nm) is maximum at an EuOBr content of less than or equal to 0.1 mol % and decreases with an increase in the dopant content. The possibility of forming CsEuBr₃-type nanocrystals that are responsible for the green luminescence observed at a wavelength λ_max=508–523 nm in CsBr: Eu crystals is discussed. The intensity of photostimulated luminescence in the CsBr: EuOBr crystals irradiated with x-ray photons is found to increase as the dopant content increases. It is demonstrated that CsBr: EuOBr crystals at a dopant content in the range 0.3–0.4 mol % can be used as x-ray storage phosphors for visualizing x-ray images with high spatial resolution.     

The interference of additional waves of forbidden polaritons excited by allowed polaritons in CuGaS2

Additional waves of exciton polaritons are studied in thin (1.5–1.8 μm) CuGaS₂ crystals at 9 K. The reflectivity spectra show a fine structure related to the interference of Fabry–Perot and additional waves which is a consequence of the polariton spatial dispersion. The main parameters of the exciton polaritons were determined from the spectra calculations. The Γ₄ excitons of big oscillator strength are shown to excite the additional polariton waves of the Γ₅ excitons of small oscillator strength, which interfere determining the fine structure in exciton resonance optical spectra.     

Photostimulated luminescence flash: from scientific photography to photonics of nanostructured materials

We have analyzed the possibilities of using the phenomenon of photostimulated luminescence flash for optical diagnosing of energy levels of structural and impurity defects of semiconductor crystals and nanostructures. New data on the spectra of deep localized states associated with adsorbed few-atom clusters Zn _n on the surface of ZnS; clusters Cd _n , Cu _n , and Ag _n on the surface of CdS; and clusters Ag _n on the surface of AgBr(I) have been presented, as well as results of investigation of photostimulated assembling processes of few-atom clusters on the surface of crystals using this phenomenon. We are the first to show the potential of the luminescence flash technique for studying the mutual arrangement of the levels of dye molecules and the bands of the crystal on the surface of which they are adsorbed, as well as of the spectra of localized states in colloidal CdS semiconductor quantum dots.     

Charge-transfer bands in crystals of alkaline earth fluorides with Eu<Superscript>3+</Superscript> and Yb<Superscript>3 + 1</Superscript>

The absorption, luminescence, and excitation spectra of CaF₂, SrF₂, and BaF₂ crystals with EuF₃ or YbF₃ impurity have been investigated in the range 1–12 eV. In all cases, strong wide absorption bands (denoted as CT₁) were observed at energies below the 4f ⁿ -4f ^{n ? 1}5d absorption threshold of impurity ions. Weaker absorption bands (denoted as CT₂) with energies 1.5–2 eV lower than those of the CT₁ bands have been found in the spectra of CaF₂ and SrF₂ crystals with EuF₃ or YbF₃ impurities. The fine structure of the luminescence spectra of CaF₂ crystals with EuF₃ impurities has been investigated under excitation in the CT bands. Under excitation in the CT₁ band, several Eu centers were observed in the following luminescence spectra: C _4v , O _h , and R aggregates. Excitation in the CT₂ bands revealed luminescence of only C _4v defects.     

Surface phonon–polaritons in rectangular quantum wires of polar ternary mixed crystals

In this paper, the dispersion relations of surface phonon–polaritons in freestanding rectangular quantum wire systems of polar ternary mixed crystals are derived. The numerical calculations for Al_xGa_1−xAs and Zn_xCd_1−xSe quantum wire systems are performed. The results reveal that the frequencies of surface phonon–polariton modes are sensitive to the geometric structures of the quantum wires, the wave-vectors in z-direction, and the compositions of the ternary mixed crystal materials. The effects of the “two-mode” and “one-mode” behaviors of the ternary mixed crystals on the surface phonon–polariton modes are also discussed.     

Light scattering by electrons in the excitonic absorption region of GaAs

A study has been made of the effect of the additional generation of photoexcited electrons on the excitonic absorption and luminescence spectra of ultrapure GaAs samples at T=2 K. The observed increase in the absorption coefficient for the ground (n=1) excitonic state is shown to originate from the polariton character of the energy spectrum of this state and to be due to an increase of polariton damping. The increased damping observed under electron generation is caused by polariton scattering from hot electrons as the latter undergo thermalization. As a result, the polaritons are heated. The changes observed in the luminescence spectra are produced by the reverse effect of electron heating and polariton cooling. Fiz. Tverd. Tela (St. Petersburg) 39, 1011–1016 (1997)     

Spectroscopic studies of erbium-doped potassium-lead double chloride crystals KPb<Subscript>2</Subscript>Cl<Subscript>5</Subscript>:Er<Superscript>3+</Superscript>: 1. Optical spectra and relaxation of excited states of the erbium ion in potassium-lead double chloride crystals

Optical spectra, intensities of radiative and nonradiative transitions, and luminescence kinetics in erbium-doped potassium-lead double chloride crystals KPb₂Cl₅:Er³⁺s(KPC:Er³⁺) were investigated. The crystals were grown by the Bridgman-Stockbarger method. Their absorption and luminescence spectra were studied experimentally. The crystal-matrix absorption edge was determined at 80 and 300 K. Intensity parameters, radiative transition probabilities, branching ratios, and nonradiative relaxation rates were estimated by the Judd-Ofelt method. The luminescence kinetics from the emitting levels ⁴ G _11/2, ² G _9/2, ⁴ S _3/2, and ⁴ F _9/2 upon selective excitation was studied.     

Luminescence of yttrium aluminum borate single crystals doped with manganese

The optical absorption and luminescence spectra of single crystals of yttrium aluminum borate YAl₃(BO₃)₄ doped with manganese ions are measured. It is established that the optical absorption spectra of yttrium aluminum borate single crystals doped with manganese ions are determined primarily by the contribution from the Mn⁴⁺ ions. The luminescence spectra of the YAl₃(BO₃)₄ single crystals doped with manganese exhibit narrow lines attributed to the Mn⁴⁺ ions and an extended multiband structure which is associated primarily with the contribution from the Mn²⁺ ions.     

Exciton–phonon luminescence and Raman scattering in CuGaS2 crystals

Photoluminescence and resonance Raman scattering spectra of CuGaS₂ crystals are investigated at low temperature (10 K) under the excitation with the radiation from a spectral interval obtained by passing the radiation of an incandescent or xenon lamp through a monochromator. The thermalized luminescence of the Γ₄ and Γ₅ excitons was revealed under the excitation by an interval of photon energies higher than the energy of the ground state of long-wavelength Γ₄ excitons (ω_Т(Γ₄) + ). The energy conversion between the polariton modes in luminescence and resonance Raman scattering spectra is considered.     

Growth and spectroscopic studies of NaLa(MoO4)2:Tm3+ crystals: A new promising laser material

Single crystals of scheelite-like disordered double molybdate NaLa(MoO₄)₂ doped with thulium ions with dopant concentrations of 3.0 × 10¹⁹ and 1.2 × 10²⁰ cm^?3 are grown by the Czochralski method. Spectral and luminescent studies of crystals grown are performed in wavelength ranges of 800 nm (the transitions ³ H ₆?³ H ₄) and 2 μm (the transitions ³ H ₆?³ F ₄), including the measurements of the polarized absorption and luminescence spectra of these crystals and the decay kinetics of their excited states. A model that satisfactorily describes the decay kinetics of the luminescence from the ³ H ₄ level taking into account cross-relaxation interactions is proposed. The prospects for the use of these crystals as active media in two-micrometer solid-state lasers are discussed.     

“Thermal barrier” effect in energy relaxation of polaritons in HgI2

The paper presents the results of measurements of polariton radiation spectra, polariton band excitation spectra and spectra of resonance Raman scattering HgI₂. These are registered at different temperatures and units of energetic derangement between the exciting radiation quantum energy and that of transverse exciton. The threshold value of the temperature (T=12 K) starting from which there is attained the equilibrium between the polariton and phonon subsystems is found. The scattering mechanism that guides the formation of “thermal barrier” favouring the polaritons thermalization, is revealed.     

Growth and Study of Scintillation Properties of Orthovanadate Crystals Ca:GdVO<Subscript>4</Subscript> and Ca:YVO<Subscript>4</Subscript>

New scintillation orthovanadate crystals Ca:GdVO₄ and Ca:YVO₄ featuring intrinsic luminescence are grown. The spectra of pulsed cathodoluminescence of new scintillators and their luminescence decay time are studied. Using γ-rays with an energy of 662 keV from the ¹³⁷Cs source, total absorption spectra (so-called photopeaks) for new orthovanadate crystals are measured. It is shown that the light yield of Ca:YVO₄ and Ca:GdVO₄ crystals is 28100 photon/MeV and 14000 photon/MeV, respectively.     

Effects of exciton-photon interaction and spatial dispersion in xenon crystals

New data on luminescence from xenon crystals are discussed in terms of the polariton theory, and the treatment of a fine spectrum structure is proposed based on the polariton scattering with participation of some acoustic phonons.The effect of spatial dispersion on transmission and reflection spectra of solid Xe is considered. Verification of the basic parameters of polariton spectrum in xenon is made by comparing calculated and experimental data.     

VUV spectroscopy of Ce<Superscript>3+</Superscript>-doped Na<Subscript>0.4</Subscript>Lu<Subscript>0.6</Subscript>F<Subscript>2.2</Subscript> single crystals

The short-wave transmission spectrum of Na_0.4Lu_0.6F_2.2 with the visible/ultraviolet transmission edge of 8 eV was studied. Absorption spectra of the 4f—5d transitions of the Ce³⁺ ion in the region of 4–8 eV were studied in Ce³⁺-doped Na_0.4Lu_0.6F_2.2 single crystals. Luminescence spectra in the ultraviolet and visible spectral regions, luminescence decay kinetics and reflection and luminescence excitation spectra in the visible/ultraviolet and ultraviolet regions (4–20 eV) were investigated at helium and room temperatures.     

Upconverted 5d-4f luminescence from Er and Nd ions doped into fluoride hosts excited by ArF and KrF excimer lasers

We report on observation of upconverted VUV luminescence due to 5d-4f radiative transitions in Er³⁺ and Nd³⁺ ions doped into some fluoride crystals, under excitation by ArF and KrF excimer lasers, respectively. Only spin-forbidden 5d-4f luminescence of Er³⁺ (at 165 nm) was detected from the LiYF₄:Er³⁺ crystal whereas both spin-forbidden (at 169 nm) and spin-allowed (at 160.5 nm) components are observed from the BaY₂F₈:Er³⁺ crystal, the latter being much weaker than in the case of one-photon excitation. Nd³⁺ 5d-4f luminescence at 180 and 173 nm has been detected from the LiYF₄:Nd³⁺ and LaF₃:Nd³⁺ crystals, respectively. The shift of short-wavelength edge of 5d-4f emission spectra towards longer wavelengths is observed under temperature increase from 15 to 293 K. The observed effects in the spectra of Er³⁺ and Nd³⁺ doped crystals were interpreted as a result of reabsorption of 5d-4f luminescence escaping from the bulk of the crystals.     

Electron spin resonance and luminescence in Ga2−xEuxS3 single crystals

In present work electron spin resonance (ESR) and luminescence have been studied in Ga_2−xEu_xS₃ single crystals. The ESR and photoluminescence (PL) spectra of Ga_2−xEu_xS₃ were observed and the intensity of these spectra has increased linearly with the Eu concentrations in the samples. At the low temperature of 3.8 K in Ga_2−xEu_xS₃ single crystals, when the magnetic field has been applied as the perpendicular to the direction of [110], the hyperfine structure of ESR spectra has been observed. The exchange interaction of Eu²⁺ atoms in Ga_2−xEu_xS₃ single crystals have been determined as, g=4.2. The PL spectrum of pure Ga₂S₃ single crystals was recorded at 4.2 K and consisted of a narrow high-energy band at 2.53–2.38 eV and a low-energy band at 2.14–1.59 eV. Ga_2−xEu_xS₃ resulted in complete quenching of the impurity luminescence bands gave rise to a green europium band (2.38–2.14 eV) in the luminescence spectrum. Crystals of Ga_2−xEu_xS₃ emitted bright green electroluminescence (EL) spectrum when they excited with static and alternating electric fields at 77 K. We have also determined the field and frequency dependences of the EL.     

Bound and many-particle states in polariton Raman spectra of NH4Cl crystals

The properties of polariton Raman spectra for the internal vibration region of NH₄Cl crystal are discussed. These spectra have been recorded by means of a copper laser ( $\text{λ = 5105 and 5782}\text{A}\text{?}$) for sample temperature T=80 and 10°K.According to the theory a polariton singularities in the region of a polar bound state has been obtained. The two-particle bands have been studied and a very complicate structure inside of these bands has been revealed. Moreover, multiphonon singularities at polariton curves have been observed. The experimental polariton law has been compared to the theory and a conclusion has been made as to the necessity of a further development of the theory.