Influence of infrared radiation on the excitonic absorption edge of CuCl

We report changes in the excitonic absorption edge of CuCl caused by intense CO₂ laser radiation at 10.6 m, a wavelength which lies in the infrared (ir) transparency region of CuCl. With an ir intensity of 0.4 GW/cm² we observe a 100% absorption increase for the Kr⁺ laser probe wavelength of 406.7nm. The effect scales linearly with ir intensity but does not depend on relative polarization. We explain the effect by laser field induced electroabsorption of the exciton. The magnitude of the effect is closely related to electroabsorption induced by static external fields and by internal electric fields from optical phonons.     

Fluorescence properties of Bi4Ge3O12(BGO) single crystals under laser excitation: Excited state dynamics and saturation effects

Laser-excited techniques were used to investigate the optical properties of bismuth germanate crystals. Absorption, reflectivity, excitation, emission, lifetime, time-resolved fluorescence, photoconductivity, thermally stimulated conductivity measurements were performed at various temperatures on single crystals of different origins.The absorption is shown to occur in bismuth and germanate centers while both intrinsic and perturbed Bi³⁺ ions together with impurities contribute to the total fluorescence.The emission mechanism at room temperature involves a thermally activated energy migration, and at low temperature localized emitting centers. Formation of deep holes in the wide emission band at room temperature reveals saturation effects on various luminescent centers, promoted by energy migration. Trapped exciton models are proposed to explain the excited state dynamics occurring at low and room temperature.     

Scattering of an exciton polariton by impurity centers in GaAs

Scattering of an exciton polariton by impurity centers at low temperatures has not been investigated comprehensively in spite of its significant role in processes accompanying Bose–Einstein condensation of an exciton polariton. For studying the peculiarities of the interaction of an exciton polariton with impurity centers, we have studied the integrated absorption of the ground state (n ₀ = 1) of the exciton in GaAs in thin (micrometer-thick) wafers with an appreciable optical transmission. Comparative analysis of the transmission in the vicinity of the exciton resonance performed on 15 samples of crystalline GaAs wafers with different concentrations N of impurity has revealed an unexpected regularity. The value of N increases by almost five decimal orders of magnitude, while the normalized spectrally integrated absorption of light exhibits a slight increase, following the power dependence N ^m on the concentration, where m = 1/6. It has been shown that this dependence indicates the diffusion mechanism of propagation of the exciton polaritons through the bulk of the semiconductor, which is present along with the ballistic propagation of light through the sample.     

ZnSe/ZnS抛物量子阱中激子的极化子效应(英文)

采用推广的LLP方法研究了ZnSe/ZnS抛物量子阱中激子的极化子效应。考虑电子和空穴与LO声子的相互作用,得到了激子基态能量和结合能随阱宽的变化关系。结果表明,阱宽较小时,能量随着阱宽的增大而急剧减小;阱宽较大时,能量减小的比较缓慢。和我们以前的工作对比,我们发现ZnSe/ZnS抛物量子阱对激子的束缚强于GaAs/Ga1-xAlxAs抛物量子阱。     

Absorption and photoluminescence studies of the temperature dependence of exciton life time in lattice-matched and strained quantum well systems

We present systematic studies of the temperature dependence of linewidths and lifetimes of excitonic transitions in quantum wells grown by molecular beam epitaxy using both photoluminescence(PL) and optical absorption. The temperature ranged from 6K to room temperature. Samples under investigation were lattice-matched GaAs/AlGaAs and InGaAs/InAlAs, and strained InGaAs/GaAs and InGaAs/AlGaAs quantum wellssystems. In addition, the effects of well-size variations in GaAs/AlGaAs quantum wells were measured and analyzed. In all cases we were able to observe the excitonic transitions up to room temperature. By a careful fitting of the experimental data we separated the exciton transitions from band-to-band transitions. By deconvoluting the excitonic transitions we obtained the homogeneous and inhomogeneous linewidths. The homogeneous linewidths were used to calculate the exciton lifetimes as a function of temperature using the Heisenberg uncertainty principle. We found the lifetime decreases significantly with temperature and increases with increasing well size. These results are interpreted in terms of the exciton-phonon interaction and are expected to be very useful for the design of semiconductor optical devices operating at different temperatures.     

Modeling and Simulation of Saturation Gain of IR Integrated Photonic Amplifiers

IR integrated photonic amplifiers at 1.55m operation will have good foreground in optical phasedarray radars for splitters and signal processing. The saturation gain characteristics of IR integrated photonic waveguide amplifiers (taken Er³⁺Yb³⁺ co-doped phosphate glass waveguide amplifiers as an example) are studied theoretically. For the homemade laser glass materials the calculated saturation intensities are 2.22kw /cm² for signal and 10.15kw/cm² for pump. The effects of absorption saturation of signal and pump lights on the gain of amplifiers are discussed.     

Novel glass-forming ferroelectric liquid-crystal material of high second-order nonlinearity

We report on a novel ferroelectric liquid-crystal material with an exceptionally large and stable nonlinear optical response. The material shows a phase transition from theSmC* mesophase to the glassy state near room temperature. This allowed us to state-freeze a stable nonlinear optically active configuration close to room temperature at 10° C for one day. The stored orientations can easily be forced to relax back by heating slightly above the glass-transition temperature. The material and the freezing process have been characterized in detail by analyzing the frequency-doubling process of the fundamental radiation of a Nd: YAG laser in the liquid crystal. Under favourable conditions, we obtained second-order susceptibilities as large as _zzz = 0.5 pm/V.     

Superhigh resolution spectroscopy in methane with cold molecules

Use of optical selection of cold particles in saturated absorption spectroscopy is discussed. Owing to this we observed directly the splitting of a methane absorption line (=3.39 m) due to recoil effect in a light beam of 0.5 cm in diameter. The obtained width of nonlinear resonance of about 1 kHz corresponds to an effective temperature of particles of about 10^–1 K. The new direct absolute frequency measurement of the central hyperfine component of theF ₂ ⁽²⁾ line in methane (7–6 transition) made under these conditions gave the following value 88376181600.7±0.5 kHz. The behaviour of the resonance intensity of saturated absorption in the transit-time conditions is analysed. It has been shown that with an absorption cell inside a cavity the saturation resonance intensity in the transit-time region increases sharply.     

Physical properties of GaAs on glass

Physical properties of GaAs layers grown by molecular beam epitaxy on glass substrates are investigated by Raman light scattering, Photothermal Deflection Spectroscopy (PDS), optical absorption and Scanning Electron Microscopy (SEM) measurements. The results indicate that the layers are polycrystalline and strain-free. Raman spectra exhibit GaAs-TO and LO modes at 260 and 283 cm^–1, respectively. The peaks are shifted by about 10 cm^–1 with respect to bulk GaAs which we attribute to local heating effects induced by laser excitation. The phonon lines are strong and have a bandwidth of about 5 to 8 cm^-1 indicating a good crystalline quality. However, neither photoluminescence nor the Hall effect could be observed that is suggestive of the presence of a large number of electronic defects. SEM micrographs taken from the surface and the interface exhibit a granular structure with the polycrystal sizes of well under 1 m. PDS results show about 10¹⁸ cm^–3 defects and some disorder at the band gap.     

Contrast Saturation Resonances in the Absorption Band of Rubidium Molecules

We study Doppler-free saturation resonances in the absorption band $ {{\mathrm{B}}^1}{\Pi_{\mathrm{u}}}-{{\mathrm{X}}^1}\Sigma_{{^{\mathrm{g}}}}^{+} $ of rubidium diatomic molecules in the frequency range near the D₂ line of lithium atoms (671 nm). We observe contrast saturation resonances and record a variation in the laser light transmission of 4% due to optical saturation. The large optical nonlinearities in the molecular diatomic gas can be used for investigating the four-wave mixing and other nonlinear effects.     

Photoluminescence and Optical Absorption of Cs2NaScF6:Cr3+

The main objective of this paper is the characterization of the spectroscopic properties of new materials that are prospective laser media. This approach allows for the comparison of the properties of the Cr³⁺ in different environments. Here, we have studied the photoluminescence and optical absorption of Cs₂NaScF₆:Cr³⁺ single crystals. On the basis of near-infrared luminescence measurements at 2, 77, and 300 K the observed lines originated from the Cr³⁺-centres were associated with the transition and the lifetimes were obtained. In spite of the quenching observed as a function of temperature at least 10% of the 2 K emission intensity for Cs₂NaScF₆ doped with 1% of Cr³⁺ remains at room temperature. Besides, the 2 K emission broad band could be well described in terms of normal modes of the octahedral complex [CrF₆]³⁻, and the Racah and crystal-field parameters calculated.     

Anomalous linewidths and peak-height ratios in137Ba hyperfine lines

We have investigated several effects of optical pumping in the hyperfine spectrum of the6s6p ¹ P ₁6s ^{2 1} S ₀ transition in¹³⁷Ba. Most of these effects are explained by absorption strength changes which occur because of redistribution of population among magnetic substates. At very low laser intensities, no redistribution effects are observed. At higher intensities, it is possible to either empty the magnetic substates that are accessible to optical excitation, or to redistribute the population among these states until a steady-state condition is achieved. The first case results in the familiar disappearance of a hyperfine line. The less familiar second case can result in peak-height ratios in the Ba¹ P ₁¹ S ₀ hyperfine spectrum that differ by almost a factor of three from the low-intensity case. In this second case, the observed linewidth can either broaden or narrow, depending on whether redistribution decreases or increases absorption strength. At high intensities, saturation effects dominate and branching to intermediateD states becomes apparent. We report here the result of a numerically integrated rate equation model which shows good agreement with our experiments.     

Simple room temperature synthesis and optical studies on Mg doped ZnO nanostructures

We report simple room temperature synthesis of Mg doped ZnO nanostructures through the sol–gel method. X-ray diffraction shows the prepared ZnO particles are in wurtzite structure and replacement of Zn²⁺ by Mg²⁺ alters the position of the X-ray diffraction peak slightly towards higher angle. Measured optical absorption spectra show the exciton peaks of ZnO present around 366, 296 and 235 nm. Room temperature photoluminescence measurements show strong peaks around 385, 394 nm are attributed to band edge exciton emission; other peaks found at 469 and 558 are attributed to oxygen ion vacancy and formation of Vo⁺ and Vo⁺⁺ centers in nanostructures.     

Light-induced charge transport in BaTiO3 via three charge states of rhodium

By combined studies of electron spin resonance and optical absorption at low temperatures, the charge-transfer bands of Rh⁵⁺ and Rh⁴⁺ are identified to be peaked near 1.6 and 1.9 eV, respectively. On this basis, the light-induced charge-transfer processes in BaTiO₃:Rh are unraveled at room temperature. It is shown that three charge states of Rh are involved, leading to two levels: the shallow Rh^4+/5+ and the deeper Rh^3+/4+ level. The optical behaviour of these two levels corresponds to those expected from a two-center model. The present paper represents the first atomic-scale identification of three charge states of one element leading to optical two-level response.     

Ultrafast nonlinear optical response and high density excitation effects of the stacking fault exciton in BiI3

On the exciton states localized at a two-dimensional stacking fault interface in a layered crystal BiI₃, some nonlinear optical phenomena clearly appear reflecting large transition probability. The optical Stark shift and other high density exciton effect on the energy-shift and the spectral broadening are observed with clear separation under intense laser pumping by time-resolved measurements. The optical Stark shift is analyzed based on the dressed exciton model. The ultrafast optical response faster than 3 ps for the pump-laser field is confirmed on the Stark shift. Degenerate four-wave-mixing signals show fairly long dephasing time of 40 ps in this system. The dephasing probability depends linearly on the pump-laser intensity in the same manner as that of the spectral line-broadening reflecting the relaxation process. The dephasing mechanisms are understood by the exciton scattering at high density in parallel with the spectral changes. The blue-shift due to the high density excitons are discussed on the basis of exciton-exciton interaction in connection with a phase-space filling theory in two-dimensional systems.     

Photochromic behaviour of doped Bi4Ge3O12 single crystal scintillators

The changes induced by ultraviolet (UV) illumination on the optical absorption and electron paramagnetic resonance (EPR) spectra of Bi₄Ge₃O₁₂ single crystals, doped either with Fe (and Gd) or Mn, have been followed at room temperature (RT). In both crystals several overlapping optical absorption bands develop under UV illumination, covering from 0.7 eV up to the band edge of the matrix. The optical damage can be bleached by heating the samples above RT or by illumination with visible light. Although these optical changes temporarily correlate with the variation of the Gd³⁺ and Mn²⁺ concentrations, it has been concluded that other defects are present and partially responsible for the optical damage.     

The luminescence of unactivated alkali iodides

Two shorter wave-length emission bands have been observed in activated alkali iodides at — and light excitation apart from activator emission. The shortest wave-length band, observed only at low temperatures, is excited in exciton absorption bands and it may be due to exciton emission.This emission has greater intensity in unactivated crystals grown from the solution. We have observed this emission on CsI, KI and RbI crystals.The intermediate emission band is excited in a narrow excitation band, situated on the sharp slope of the fundamental absorption in crystals and it can apparently be ascribed to structural defects.We wish to thank M. D. Galanin for interest in this work and L. M. amovski for providing us with crystals.     

The coupled coherent and incoherent motion of excitons and its influence on the line shape of optical absorption

We treat the coupled coherent and incoherent motion of Frenkel excitons by a model calculation. The model contains the four parametersa (distance of neighbouring atoms),J (exchange interaction integral), γ_o (describing the strength of the local energy fluctuations) and γ₁ (a measure of the fluctuations of the exchange interaction integral, i.e. nonlocal fluctuations). Calculation of the optical absorption of systems with two differently oriented molecules/unit cells results in the Davydov-splitting given by Δ=8J and the linewidth given by Γ=γ_o+γ₁. From the equation of motion of the density matrix we derive a diffusion equation. The diffusion constant is given by \(D = \frac{{a^2 }}{\hbar }\left( {2\gamma _1 + \frac{{J^2 }}{{\gamma _1 + \Gamma }}} \right)\) . Comparison with experiment yields γ_o=70cm^?1, γ₁=0.1 cm^?1 at room temperature and Γ<1 cm^?1 at 4.2 °K. Using these numerical values and the criterium of Haken and Strobl we derive that at room temperature the exciton motion is incoherent and may be described by a hopping process whereas at low temperature it is coherent.     

Critical Dependence of the Excitonic Absorption in Cuprous Oxide on Experimental Parameters

We study the modification of the exciton absorption in cuprous oxide by the presence of free carriers excited through above band gap excitation. Without this pumping, the absorption spectrum below the band gap consists of the yellow exciton series with principal quantum numbers up to more than n = 20, depending on the temperature, changing over to an about constant, only slowly varying absorption above the gap. Careful injection of free carriers, starting from densities well below 1 μm^–3, leads to a reduction of the band gap through correlation effects. The excitons in the Rydberg regime above n = 10 remain unaffected until the band gap approaches them. Then they lose oscillator strength and ultimately vanish upon crossing with the band gap.