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physica status solidi (b)

Transient Light‐Induced Gratings Due to Excitons in CdSe

Ilja Rückmann, K. Jarašiūnas, Eugenijus Gaubas

Published Apr 1, 1985• 13 citations
Publisher

Abstract

Abstract Transient light‐induced gratings due to free carriers and excitons are created in CdSe in a medium density range of 1015 to 1017 cm−3 by two coherent intense (ns) IR‐pump beams. Detailed investigations of probe‐beam diffraction due to thin excitonic gratings in the spectral region just below the An = 1 ‐exciton line at 85 K and its dependence on sample temperature and on the excitation level are presented. First‐order diffraction efficiences up to 5% and a nonlinear dependence of diffraction on the excitation level can be observed. For the first time numerical calculations of probe‐beam diffraction due to excitonic phase and amplitude gratings are presented. The comparison with the experiment shows, that the damping of a dense exciton gas increases proportional to the excitation intensity in the case of two‐photon interband excitation, and that the light‐induced absorptive changes Δ due to dense excitons dominate over the dispersive changes Δn in the spectral region under consideration.

DOI
10.1002/pssb.2221280229

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Physics of the Solid State•Sep 1, 2010•14 citations
Abstract Transient light‐induced gratings due to free carriers and excitons are created in CdSe in a medium density range of 1015 to 1017 cm−3 by two coherent intense (ns) IR‐pump beams. Detailed investigations of probe‐beam diffraction due to thin excitonic gratings in the spectral region just below the An = 1 ‐exciton line at 85 K and its dependence on sample temperature and on the excitation level are presented. First‐order diffraction efficiences up to 5% and a nonlinear dependence of diffraction on the excitation level can be observed. For the first time numerical calculations of probe‐beam diffraction due to excitonic phase and amplitude gratings are presented. The comparison with the experiment shows, that the damping of a dense exciton gas increases proportional to the excitation intensity in the case of two‐photon interband excitation, and that the light‐induced absorptive changes Δ due to dense excitons dominate over the dispersive changes Δn in the spectral region under consideration.
A picosecond time-resolved four-wave mixing technique is used to study the dephasing of nonlinear optical resonances in CdSe. At low temperature strong coherent, nonlinear signals at the free exciton resonance, at the biexciton resonance, and at the exciton-biexciton transition are observed. The observed correlation traces, i.e. the diffracted intensity versus the delay between the interfering light pulses, yield information on the dephasing process. Dephasing times for excitons as long as 40 ps, decreasing with increasing excitation density and temperature, are observed. A two-level model is applied to describe theoretically the dynamical behavior of the resonances, and to extract their homogeneous broadening, as well as their inhomogeneous broadening.
Self-diffraction at a one-dimensional dynamic photonic crystal formed in the colloidal solution of CdSe/ZnS quantum dots has been discovered. This self-diffraction appears simultaneously with self-diffraction at induced transparency channels at the resonant excitation of the main electron–hole (excitonic) transition of quantum dots by two laser beams with a Gaussian intensity distribution over the cross section. It is shown that a nonlinear change in the absorption of colloidal quantum dots results in the formation of a transparency channel and an induced amplitude diffraction grating, and a significant nonlinear change in the refractive index (Δ n ≈ 10 −3 ) in the absorbing medium is responsible for the formation of the dynamic photonic crystal. Self-diffracted laser beams are revealed propagating not only in directions corresponding to self-diffraction at the induced diffraction grating but also in directions satisfying the Laue condition.
We present a thorough investigation of the different relaxation channels opened in a strongly interacting system of excitonic quasiparticles in CuCl for density regimes ranging up to 50 MW/cm2{\mathrm{cm}}^{2}cm2. A spectrally narrow pump beam consisting of 30-ps pulses is tuned around one-half the biexciton energy. It creates an initial distribution of biexcitons through resonant two-photon absorption. A spectrally broad probe beam made of pulses that also have 30-ps duration monitors the excitation-induced variation of the transmission spectrum of CuCl samples near the Z3{\mathit{Z}}_{3}Z3​ exciton resonance. Coherent processes corresponding to parametric emissions of various orders are observed. Absorption and gain bands that reflect the evolution of biexciton and exciton populations on their respective dispersion curves are followed as functions of the time delay between pump and probe pulses.
The possibility and the conditions of the formation of correlated “gratings” of electrons and their spins in a semiconductor film containing CdSe/CdS nanoparticles (which are the quantum dots) have been studied. Notably, the induction of electron spin “gratings” on quantum dots has been considered for the first time. We have suggested to perform an excitation of the film in the two photon absorption mode using two crossed (at an angle of 60?) femtosecond laser beams. As a result of the excitation, two nonequilibrium “gratings” are induced simultaneously in the excitation area of the film. If the exciting pulses have the same polarizations, then induced “gratings” of electrons are formed. However, in the case when the exciting pulses have mutually orthogonal polarizations, the formation of induced “gratings” of electron spins takes place. The same exciting pulses excite the electrons of CdSe/CdS nanoparticles into a superposition state. Then the free induction decay signal is generated. This signal being diffracted by induced “gratings” plays the role of a probe pulse in the suggested experimental scheme. The diffracted signals propagating in two mutually opposite directions are correlated (i.e., they are identical). Their wavefronts can carry the correlated transient holograms. Other possible applications of correlated “gratings” (in spintronics and in coherent femtosecond laser spectroscopy) are also discussed.
Electromagnetically induced phase grating is theoretically investigated in the driven two-level quantum dot exciton system at the presence of the exciton-phonon interactions. Due to the phonon-induced coherent population oscillation, the dispersion and absorption spectra are sharply changed and the phase modulation is enhanced via the high refractive index with nearly-vanishing absorption, which could effectively diffract a weak probe light into the first-order direction with the help of a standing-wave control field. Moreover, the diffraction efficiency of the grating can be easily manipulated by controlling the Huang-Rhys factor representing the exciton-phonon coupling, the intensity and detuning of the control field, and the detuning of the probe field. The scheme we present has potential applications in the photon devices for optical-switching and optical-imaging in the micro-nano solid-state system.