Abstract

A p-germanium laser in crossed electric and magnetic fields is usually operated in liquid helium.^{[1, 2]} The influence of heating of the laser is not clear. Andronov and cowokers^[1] estimated a critical temperature of 10-20 K, at which the simulated emission disappears. In this paper we report on a study of the temera- ture dependence of the emission characteristic of a p-germanium laser in the broad-band regime (80-120 cm⁻¹). To obtain different temperatures inside a superconducting magnet, we used a copper sample holder with a heater, that was surrounded by a low pressure helium atmosphere. The p-germanium crystal with a shallow acceptor concentration of 10¹⁴ cm⁻³ was inserted in the copper holder. The highly polished surfaces of the crystal served as laser mirrors for total internal reflection modes. Fig. 1 shows the result, when the laser was in an optimum operation point (H ≅ 1.8 T, E ≅ 3.25 kV/cm). Under these conditions laser oscillation was observed up to 20 K, while in other operation points the threshold occurred at lower temperatures. The signal curves for different temperatures show sharp increase of laser emission intensity and then a decrease. With increasing temperature the intensity decreases and, furthermore, the delay between electric pulse start (indicated by arrow in Fig. 1) and beginning of laser emission rises. The decrease of the emission intensity (Fig. 1) with time is explained by an additional heating of the crystal during the electric pump pulse. Figure 2 shows the temperature dependence of the unsaturated net laser gain (stars), which was obtained from the delay of the laser emission. For understanding of the experimental results we consider depopulation of the upper laser level (the light hole trap^[1])due to scattering of holes at acoustical phonons. Then the inverse lifetime of this level is l/t₂ = l/t_sp+AT, where t_sp ≈ 4.7 10⁻¹¹s^[3] is the spontaneous emission time, T the temperature of the lattice, A ≈ 8 10⁸ s⁻¹K⁻¹ a constant, which determines the strength of scattering of the light holes in the trap at acoustical phonons. From the last formula one can get the expression for the unsaturated net gain: g(T) = g_o/(l+ATt_sp) − L, where g_o is a gain at zero temperature and L the coefficient of radiation losses both due to intracavity absorption and coupling of the cavity with external space. The solid line (Fig. 2) shows the fit of the experimental data for go = 6.5 10⁻²cm⁻¹ and L = 3.5 10⁻² cm⁻¹. The deviation of the experimental data from the fit at low temperatures to smaller values can be explained by strong heating of the crystal at low temperatures where the specific heat is small.

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