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Физика и техника полупроводников

Photon echo in germanium with shallow donors

В.В. Цыпленков, V. N. Shastin

Published Jan 1, 2022• 1 citation
Publisher

Abstract

A theoretical study was made of the conditions for observing the photon echo effect in a germanium crystal doped with shallow donors. A numerical calculation of the medium polarization excited by a sequence of two optical pulses at a frequency close to the impurity transition frequency has been made. The effect of excitation pulse parameters, such as the pulse duration, the inhomogeneous broadening of impurity transitions and the relaxation rate of population and coherence in the system on the echo is considered. The key aspect in the experimental implementation of the effect is the control of the crystal temperature under photoexcitation, since the rate of coherence relaxation in the system strongly depends on the temperature of the crystal lattice. Keywords: germanium, shallow donors, coherent effects, photon echo.

DOI
10.21883/sc.2022.08.54454.22

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A theoretical study was made of the conditions for observing the photon echo effect in a germanium crystal doped with shallow donors. A numerical calculation of the medium polarization excited by a sequence of two optical pulses at a frequency close to the impurity transition frequency has been made. The effect of excitation pulse parameters, such as the pulse duration, the inhomogeneous broadening of impurity transitions and the relaxation rate of population and coherence in the system on the echo is considered. The key aspect in the experimental implementation of the effect is the control of the crystal temperature under photoexcitation, since the rate of coherence relaxation in the system strongly depends on the temperature of the crystal lattice. Keywords: germanium, shallow donors, coherent effects, photon echo.
A theoretical study was made of the conditions for observing the photon echo effect in a germanium crystal doped with shallow donors. A numerical calculation of the medium polarization excited by a sequence of two optical pulses at a frequency close to the impurity transition frequency has been made. The effect of excitation pulse parameters, such as the pulse duration, the inhomogeneous broadening of impurity transitions and the relaxation rate of population and coherence in the system on the echo is considered. The key aspect in the experimental implementation of the effect is the control of the crystal temperature under photoexcitation, since the rate of coherence relaxation in the system strongly depends on the temperature of the crystal lattice.
Abstract The relaxation rates of lower excited states 1s(T), 2p0, 2s, 3p0, and 2p± of arsenic donors in a germanium crystal are calculated upon the interaction with long-wavelength acoustic phonons depending on the uniaxial stress in the crystallographic direction [111]. The populations of states under optical excitation are estimated for calculated times. It is shown theoretically that optical excitation of the medium forms an inverse population of arsenic donor levels and leads to the possibility of the implementation of a four-level laser scheme with the radiative transition between 2p states and the 1s triplet state at zero strain. The estimated value of the expected gain in the medium under optical excitation conditions by CO2 laser radiation in the medium at a donor concentration of 2 × 1015 cm–3 is ~0.35 cm–1 at a frequency of 1.98 THz if the laser transition is 2p± → 1s(T) and 1.25 THz if the laser transition is 2p0 → 1s(T).
We present a theory of photon echoes from impurity molecules in mixed crystals that takes into account electronic interactions among impurity molecules and inhomogeneous broadening of the electronic absorption spectrum. The system is modeled as a collection of randomly placed interacting two level systems with a distribution of transition energies. The observable of the echo experiment is expanded in powers of the density of two level systems, and the first two terms of this expansion are calculated. A scaling argument suggests that the density expansion is rapidly convergent for concentrations of experimental interest, and the density expansion is used to construct a Padé approximant. At times comparable to or less than the inverse of the width of the inhomogeneous distribution of resonance frequencies, the time dependence of the signal is determined primarily by the interactions between the impurities and is independent of the inhomogeneous width. At longer times the inhomogeneity becomes significant and the echo signal displays a much weaker time dependence. We examine the effect of different microscopic models of inhomogeneous broadening on the echo observable. We demonstrate that the photon echo experiment can provide a sensitive probe of interactions between impurities in mixed crystals.