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physica status solidi (b)

Excitation spectroscopy of the I2 bound exciton and the excitonic molecule in CdS II. Relation between I2‐line and EM‐lines

Joachim Puls, Ilja Rückmann, J. Voigt

Published Dec 1, 1979• 20 citations
Publisher

Abstract

Abstract Using a narrow band dye laser the creation of EM's in CdS via TPA is demonstrated in the transmission spectra. Knowing the excited states of the I2 bound exciton from Part I the connection between TPA and EM emission lines and the relation between the excitation spectra of the I2 bound exciton and the EM are investigated. The creation of a cold EM gas under resonant two‐photon excitation is firstly found in CdS and the intrinsic nature of the M‐lines is shown.

DOI
10.1002/pssb.2220960218

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Excitonic molecules in semiconductors

V. D. Kulakovskiĭ, Valery G. Lysenko, Vladislav B. Timofeev

Great progress has been made in recent years in experimental studies of excitonic molecules, both in direct-band and indirect-band semiconductors. The exciton systems in these semiconductors differ substantially. In direct-band semiconductors with dipole-allowed optical transitions, the direct-recombination times are extremely small. Consequently the processes of binding of excitons into excitonic molecules and their decay occur under strongly nonequilibrium conditions. Hence one can obtain the fundamental information on biexcitons in these semiconductors by nonlinear optical methods. This review discusses the processes on which the various methods are based: two-photon resonance excitation of biexcitons, induced one-photon conversion of an exciton into a biexciton, two-photon resonance Raman scattering involving biexciton states, etc., and their application to studying biexcitons with the example of CdS. Problems of bistability involving biexcitons are also briefly discussed. In indirect semiconductors, where recombination occurs with the emission of a phonon, the lifetimes of excitons are large, and quasiequilibrium can be established in the exciton system. Under these conditions the information on the properties of the molecules is obtained by analyzing their radiative-decay spectra. The problem of detecting biexcitons in indirect semiconductors, radiative decay of molecules, and the effect on their stability of external electric, strain, and magnetic fields are discussed in detail with the examples of Si and Ge. Also the results of investigating the quantum statistical behavior of a dense gas of spin-oriented excitons (in a magnetic field under conditions in which the biexcitons are destabilized) are briefly presented (using the example of uniaxially compressed germanium), and the question of the possibility of their Bose condensation is discussed.

Soviet Physics Uspekhi•Sep 30, 1985•34 citations
Abstract Using a narrow band dye laser the creation of EM's in CdS via TPA is demonstrated in the transmission spectra. Knowing the excited states of the I2 bound exciton from Part I the connection between TPA and EM emission lines and the relation between the excitation spectra of the I2 bound exciton and the EM are investigated. The creation of a cold EM gas under resonant two‐photon excitation is firstly found in CdS and the intrinsic nature of the M‐lines is shown.
Abstract Under high excitation conditions thick CdS samples show pronounced broad‐band nonlinear transmission in the bound exciton region and up to a wavelength of about 515 nm at cryo‐temperatures. This behavior is only explainable in a model based on impurity neutralization and bound exciton creation. The suitability of these nonlinearities to yield optical bistability will be shown. Bistable operation is investigated in dependence of crystal thickness, impurity concentration, excitation density, wavelength, and temperature. A strong correlation to acceptor‐bound exciton generation is obtained, and the explanation of this bistable operation fits well with that of the above mentioned transmission behavior.
Various optical spectra such as time-resolved luminescence spectra are studied for excitonic molecules in CuCl under two-photon resonance excitation by using picosecond and nanosecond frequency-tunable laser pulses. Two sharp emission lines observed under resonance excitation are ascribed to the radiative decay of “cold” excitonic molecules created directly at k ∼2 k 0 , while the broad emission band on the low energy side of the sharp line to the luminescence of “hot” molecules created through single excitons. The stimulated emission process gives a considerable influence on both the sharp luminescence lines and the hot molecule band, and further on two-photon resonant excitonic Raman lines as well. Discussions are made on the Bose condensation of the excitonic molecules and also on whether the emission under just-resonance excitation is due to Raman scattering or luminescence.
Pulse propagation experiments are performed with coherent picosecond laser pulses in resonance with the donor bound A-exciton (I2) state in CdS. Using bandwidth-limited time-resolved spectroscopy, we investigate the intensity of the transmitted laser pulse as a function of monochromator energy position and time. This method permits the determination of both the amplitude and the phase of the complex electric-field amplitude. Studying the dependence of pulse propagation on the input intensity, for small-area pulses we found at the rear of the sample the spectral hole formation and beating characteristic of a 0π pulse. At large pulse areas close to 2π the pulse propagates nearly unperturbed, showing no oscillatory reshaping or significant absorption losses. This provides clear evidence for self-induced transparency at an excitonic state in solids. Our interpretation is substantiated by an analysis within the framework of the combined Maxwell and optical Bloch equations.
Giant two-photon absorption spectrum for the direct generation of excitonic molecules is measured with using a frequency tunable dye laser, and absorption peak is found at 3.1870 eV. Emission spectrum with excitation at the giant two-photon absorption band depends on the spectral width, ΔΩ1\varDelta\varOmega_{1}ΔΩ1​, of excitation light. In the case of ΔΩ1=0.25\varDelta\varOmega_{1}{=}0.25ΔΩ1​=0.25 meV, the two-photon resonance Raman scattering is found to be predominant. The Raman process involves an excitonic molecule and a longitudinal exciton as the intermediate and final states, respectively. On the other hand, for the case of ΔΩ1=2.3\varDelta\varOmega_{1}{=}2.3ΔΩ1​=2.3 meV, the emission spectrum shows a very sharp line at 3.1649 eV, which has been previously ascribed to be due to the Bose-Einstein condensation of excitonic molecules. Discussions are made on the dependence of emission spectrum upon the energy band-width of excitation light.
Abstract Measuring the transform‐limited time‐resolved spectrum of a coherent laser pulse allows one to determine the amplitude and phase of the pulse electric field. From the reshaping of pulses transmitted through a thin semiconductor platelet in resonance with an exciton transition in case of the donor bound exciton in CdS the effects of coherent exciton–photon interaction for different laser intensities can be studied. While at low intensities the expected polariton beating connected with the formation of a 0π‐pulse is observed, at sufficiently high intensities changes in the pulse shape occur that are ascribed to self‐induced transparency of the bound exciton states.
Great progress has been made in recent years in experimental studies of excitonic molecules, both in direct-band and indirect-band semiconductors. The exciton systems in these semiconductors differ substantially. In direct-band semiconductors with dipole-allowed optical transitions, the direct-recombination times are extremely small. Consequently the processes of binding of excitons into excitonic molecules and their decay occur under strongly nonequilibrium conditions. Hence one can obtain the fundamental information on biexcitons in these semiconductors by nonlinear optical methods. This review discusses the processes on which the various methods are based: two-photon resonance excitation of biexcitons, induced one-photon conversion of an exciton into a biexciton, two-photon resonance Raman scattering involving biexciton states, etc., and their application to studying biexcitons with the example of CdS. Problems of bistability involving biexcitons are also briefly discussed. In indirect semiconductors, where recombination occurs with the emission of a phonon, the lifetimes of excitons are large, and quasiequilibrium can be established in the exciton system. Under these conditions the information on the properties of the molecules is obtained by analyzing their radiative-decay spectra. The problem of detecting biexcitons in indirect semiconductors, radiative decay of molecules, and the effect on their stability of external electric, strain, and magnetic fields are discussed in detail with the examples of Si and Ge. Also the results of investigating the quantum statistical behavior of a dense gas of spin-oriented excitons (in a magnetic field under conditions in which the biexcitons are destabilized) are briefly presented (using the example of uniaxially compressed germanium), and the question of the possibility of their Bose condensation is discussed.