Excitonic versus plasma screening in highly excited gallium arsenide

We report band edge absorption spectra of GaAs under conditions of high electron-hole pair excitation in the range of 10¹³-10¹⁷ cm^-3 achieved with picosecond light pulses tuned (i) to the 1s exciton energy and (ii) to the continuum (ω#62;E_g), where critical densities for bleaching of the exciton absorption lines are much lower. A substantial blue shift of the 2s absorption line relative to the 1s exciton was found, the latter remained constant in energy up to complete bleaching.     

Coherent Pump-Probe Response of Quantum Wells: Polarization Dependence

We consider the coherent pump–probe response from quantum wells using the exciton–boson formalism approach. We obtain the mean-field contribution and that of the biexciton as well as of a molecular scattering state in analytic form and show that each of these contributions may completely disappear in one or another polarization configuration of the pump and probe pulses, which makes the response strongly dependent on polarization. We apply the derived formulas for calculating the differential absorption spectrum of quantum-well samples under excitation conditions supporting the slow exciton–exciton scattering. The obtained results are in good qualitative agreement with available experimental observations.     

On the nonlinear dielectric function of CuCl in pump and probe experiments

We study the dispersion and the absorption changes of CuCl under high excitation conditions in two-beam experiments with parallel and crossed linear polarizations of both beams. The induced optical non linearities are shown to depend sensitively on the presence of interactions between the degenerate transverse exciton states.     

Stability of high-density one-dimensional excitons in carbon nanotubes under high laser excitation

Through ultrafast pump-probe spectroscopy with intense pump pulses and a wide continuum probe, we show that interband exciton peaks in single-walled carbon nanotubes (SWNTs) are extremely stable under high laser excitations. Estimates of the initial densities of excitons from the excitation conditions, combined with recent theoretical calculations of exciton Bohr radii for SWNTs, suggest that their positions do not change at all even near the Mott density. In addition, we found that the presence of lowest-subband excitons broadens all absorption peaks, including those in the second-subband range, which provides a consistent explanation for the complex spectral dependence of pump-probe signals reported for SWNTs.     

Optical nonlinearities in GaSe and InSe crystals upon laser excitation

The nonlinear absorption of light and its temporal evolution in the vicinity of exciton resonance in layered GaSe and InSe crystals under high optical excitation have been experimentally investigated. The decisive factor for the observed temporal dependence of the absorption coefficient and its dependence on the excitation intensity is screening excitons by nonequilibrium-carrier plasma. It is shown that the increase in the transmittance in the absorption-band edge in GaSe with a simultaneous blue shift of the band edge is caused by filling the energy bands under high optical excitation.     

High-excitation photoluminescence studies of CdS1−x Se x quantum dots

Linear absorption and excitation dependent luminescence measurements on CdSSe quantum dots in glass are reported. The new aspects of the spectra are: (i) Stokes shift of linear emission versus absorption and (ii) appearance of new structures in the high energy wing of the luminescence peak under high excitation, which are interpreted as consequences of Coloumb effects, important for dot radii around the exciton Bohr radius.     

Exciton-polariton state in nanocrystalline SiC films

We studied the features of optical absorption in the films of nanocrystalline SiC (nc-SiC) obtained on the sapphire substrates by the method of direct ion deposition. The optical absorption spectra of the films with a thickness less than ~500 nm contain a maximum which position and intensity depend on the structure and thickness of the nc-SiC films. The most intense peak at 2.36 eV is observed in the nc-SiC film with predominant 3C-SiC polytype structure and a thickness of 392 nm. Proposed is a resonance absorption model based on excitation of exciton polaritons in a microcavity. In the latter, under the conditions of resonance, there occurs strong interaction between photon modes of light with λ_ph=521 nm and exciton of the 3С polytype with an excitation energy of 2.36 eV that results in the formation of polariton. A mismatch of the frequencies of photon modes of the cavity and exciton explains the dependence of the maximum of the optical absorption on the film thickness.     

Induced absorption and gain from high density excitons in CdS

Induced absorption and gain in CdS at 1.8 K has been investigated under excitation densities up to 10 MW cm^?2. The absorption and gain below the free exciton energy is governed by exciton interactions and optical conversion of excitons into excitonic molecules. At the highest density, induced transparency due to excitonic molecule recombinations is observed. E_M=5.1002 eV is determined.     

Peculiarities of the nonlinear absorption of colloidal solutions of CdSe/ZnS quantum dots under stationary single-photon excitation of excitons

Peculiarities of the nonlinear absorption of a colloidal solution of CdSe/ZnS quantum dots with various sizes under resonant stationary excitation of the ground electron–hole (exciton) transition have been revealed by the pump and probe technique. The detected peculiarities of the nonlinear change in absorption are explained by the coexistence and competition of the effects of state filling and charge-induced Stark and temperature long-wavelength shift of the absorption spectra.     

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.     

Bandshapes in polymer spectra

The shape of the vibrational envelope of an absorption band in the solution spectrum of a helical polymer is related to the shape of the corresponding band in the monomer. The approximation used is expected to be valid in both the strong and weak coupling limits. In the strong coupling limit it is shown that under certain conditions the polymer band consists of an extremely narrow peak shifted from the monomer line, as has been observed in the spectra of some cyanine dyes. The identification of this peak with absorption by a propagating exciton mode is supported by a calculation of the effective distance of excitation transfer as a function of coupling strength.     

Photoluminescence properties of exciton–exciton scattering in a GaAs/AlAs multiple quantum well

We report on the photoluminescence (PL) properties of a GaAs (20 nm)/AlAs (20 nm) multiple quantum well under high-density-excitation conditions at excitation energies near the fundamental exciton energies. The biexciton-PL band is dominant in a relatively low-excitation-power region. The PL originating from exciton–exciton scattering, the so-called P emission, suddenly appears with an increase in excitation power. The excitation-energy dependence of the intensity of the P-PL band indicates that the excitation energy higher than the fundamental heavy-hole exciton by the energy of the longitudinal optical (LO) phonon is the most efficient for the P PL. This suggests that the LO-phonon scattering plays an important role in the relaxation process of excitons leading to the P PL. The appearance of the P-PL band remarkably suppresses the intensity of the biexciton-PL band; namely, the exciton–exciton scattering process prevents the formation of biexcitons. Furthermore, we have confirmed the existence of optical gain due to the exciton–exciton scattering process with use of a variable-stripe-length method.     

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.     

Indirect optical cooperative excitation of two-particle electronic excitations in doped organic molecular crystals

A theory on the indirect optical cooperative excitation of a two-particle electronic excitation composed of one host exciton and one guest-molecule one-particle electronic excitation through one electric dipole-moment active singlet guest molecule one-particle electronic excitation is developed by considering the nonradiative coupling bstween one- and two-particle electronic excitations.The complex dielectric tensor and the corresponding absorption coefficient are found out, which are expressed in terms of the Fourier-transform of the retarded Green function: The derived formulas are discussed in an explicit form for the one-dimensional model of the doped organic molecular crystal. Within the framework of ths one-dimensional model also the conditions for the existence of the host exciton-guest molecule one-particle electronic excitation bound states are established and discussed.     

Photoluminescence due to exciton–exciton scattering in a GaAs/AlAs multiple quantum well

We have investigated photoluminescence (PL) properties of a GaAs (20 nm)/AlAs (20 nm) multiple quantum well at 10 K under intense excitation conditions. It has been found that a PL band due to exciton–exciton scattering, the so-called P emission, is observed in addition to the biexciton PL under an excitation energy higher than the fundamental heavy-hole exciton by the energy of the longitudinal optical phonon. On the other hand, the P band could never be observed at an excitation energy much higher than the exciton energy, where a band-filling phenomenon appears in the PL spectrum. Furthermore, we confirmed the existence of optical gain leading to stimulated emission in the energy region of the P band using a variable-stripe-length method.     

Resonant Raman scattering at the indirect exciton in silver bromide

By excitation in the indirect exciton absorption of AgBr at low temperatures selectively enhanced two-phonon Raman scattering is observed. Using different excitation wavelengths the resonance enhancement is found to be associated with the Г-L exciton as intermediate state for the resonant scattering process. The resonant phonons involved are pairs of LA and TA phonons with opposite wave vectors near L. Measurements in the temperature range 2 K ? T ? 40 K show a decrease of the scattering intensity with increasing temperature. The origin of this temperature dependence is due to lifetime broadening of the scattering state. Several features of the indirect exciton absorption of AgBr are discussed.     

神经网络法研究多溴代二苯胺热力学性质

通过时间分辨光谱技术详细地研究了ZnCuInS/ZnSe/ZnS量子点的激子弛豫动力学. 基于速率分布模型,波长依赖的发射动力学表明本征激子、界面缺陷态中的激子、给-受体对态中的激子都会参与量子点的发光过程,整个发光过程主要依赖于给-受体对态发射. 瞬态吸收数据表明激发后本征激子和界面缺陷物种可能会同时出现,在高激发光强下,光强依赖的俄歇复合过程也存在于量子点中     

Electronic excitations and luminescence of SrMgF4 single crystals

The electronic and crystal structures of SrMgF₄ single crystals grown by the Bridgman method have been investigated. The undoped SrMgF₄ single crystals have been studied using low-temperature (T = 10 K) time-resolved fluorescence optical and vacuum ultraviolet spectroscopy under selective excitation by synchrotron radiation (3.7–36.0 eV). Based on the measured reflectivity spectra and calculated spectra of the optical constants, the following parameters of the electronic structure have been determined for the first time: the minimum energy of interband transitions E _g = 12.55 eV, the position of the first exciton peak E _n = 1 = 11.37 eV, the position of the maximum of the “exciton” luminescence excitation band at 10.7 eV, and the position of the fundamental absorption edge at 10.3 eV. It has been found that photoluminescence excitation occurs predominantly in the region of the low-energy fundamental absorption edge of the crystal and that, at energies above E _g , the energy transfer from the matrix to luminescence centers is inefficient. The exciton migration is the main excitation channel of photoluminescence bands at 2.6–3.3 and 3.3–4.2 eV. The direct photoexcitation is characteristic of photoluminescence from defects at 1.8–2.6 and 4.2–5.5 eV.     

Combined Effect of Uniaxial Strain and Magnetic Field on the Exciton States in Semiconducting Single-Walled Carbon Nanotubes

The exciton states of semiconducting carbon nanotubes are calculated by a tight-binding model supplemented by Coulomb interactions under the combined effect of uniaxial strain and magnetic field. It is found that the excitation energies and absorption spectra of zigzag tubes(11,0) and(10,0) show opposite trends with the strain under the action of the magnetic field. For the(11,0) tube, the excitation energy decreases with the increasing uniaxial strain, with a splitting appearing in the absorption spectra. For the(10,0) tube, the variation trend firstly increases and then decreases, with a reversal point appearing in the absorption spectra. More interesting,at the reversal point the intensity of optical absorption is the largest because of the degeneracy of the two bands nearest to the Fermi Level, which is expected to be observed in the future experiment. The similar variation trend is also exhibited in the binding energy for the two kinds of semiconducting tubes.