Terahertz lasers based on intracentre transitions of group V donors in uniaxially deformed silicon

This paper presents a brief overview of available experimental data on the characteristics of stimulated terahertz emission (4.9 – 6.4 THz) from optically excited neutral group V donors (phosphorus, antimony, arsenic and bismuth) in crystalline silicon subjected to uniaxial compressive strain along the [100] axis. Strain is shown to have a significant effect on the characteristics in question. Optimal strain depends on the dopant and may reduce the threshold pump intensity and improve lasing efficiency. We discuss possible mechanisms behind this effect and estimate the limiting output emission parameters

Relaxation of Coulomb States in semiconductors probed by FEL radiation

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The physical principles of terahertz silicon lasers based on intracenter transitions

The first silicon laser has been reported in the year 2000. It is based on impurity transitions of the hydrogen-like phosphorus donor in monocrystalline silicon. Several lasers based on other group-V donors in silicon have been demonstrated since then. These lasers operate at low lattice temperatures under optical pumping by a mid-infrared laser and emit light at discrete wavelengths in the range from 50 µm to 230 µm (between 1.2 THz and 6.9 THz). Dipole-allowed optical transitions between particular excited states of group-V substitutional donors are utilized for donor-type terahertz (THz) silicon lasers. Population inversion is achieved due to specific electron-phonon interactions inside the impurity atom. This results in long-living and short-living excited states of the donor centers. Another type of the THz laser utilizes stimulated resonant Raman-type scattering of photons by a Raman-active intracenter electronic transition. By varying the pump laser frequency, the frequency of the Raman intracenter silicon laser can be continuously changed between at least 4.5 THz and 6.4 THz. The gain of the donor and Raman-type THz silicon lasers is in the order of 0.5 cm-1 to 10 cm-1 which is similar to the net gain realized in THz quantum cascade lasers and infrared Raman silicon lasers. In addition, fundamental aspects of the laser process provide new information about the peculiarities of electronic capture by shallow impurity centers in silicon, lifetimes of nonequilibrium carriers in excited impurity states, and electron-phonon interaction

Stimulated terahertz emission due to electronic Raman scattering in silicon

Silicon-based semiconductors are intensively investigated over the past years as promising candidates for optoelectronic devices at terahertz (THz) frequencies [1]. Optically pumped intracenter silicon lasers, realized in the past decade in the THz range, are based on direct optical transitions between shallow levels of different shallow donors [2]. Recently, terahertz Raman laser emission has been demonstrated in silicon doped by antimony [3] and phosphorus [4]. We report on realization of terahertz lasers based on intracenter electronic Raman scattering in silicon doped by arsenic (Si:As, frequency range 4.8 – 5.1 THz and 5.9 – 6.5 THz) and silicon doped by bismuth (Si:Bi, 4.6 – 5.9 THz) under optical excitation by infrared frequency-tunable free electron laser at low lattice temperatures. The Stokes shift of the observed laser emission is equal to the Raman-active donor electronic transition between the ground 1s(A1) and the excited 1s(E) donor states. Raman terahertz gain of the lasers is similar to those observed for the donor-type terahertz silicon donor lasers

Fast relaxation of free carriers in compensated n- and p-type germanium

The relaxation of free holes and electrons in highly compensated germanium doped by gallium (p-Ge:Ga:Sb) and antimony (n-Ge:Sb:Ga) has been studied by a pump-probe experiment with the free-electron laser FELBE at the Helmholtz-Zentrum Dresden-Rossendorf. The relaxation times vary between 20 ps and 300 ps and depend on the incident THz intensity and compensation level. The relaxation times are about five times shorter than previously obtained for uncompensated n-Ge:Sb and p-Ge:Ga. The results support the development of fast photoconductive detectors in the THz frequency range

Thz Silicon Lasers Based On Impurity State Transitions

The operating principles and recent results on THz lasers based on intracenter optical transitions of shallow donors in silicon under optical excitation are reviewed. The aspects of THz lasing from double donor and acceptor centers in bulk silicon as well as electrical current excitation by resonant tunneling in Si/SiGe heterostructures are discussed

Nonequilibrium electron distribution in terahertz intracentre silicon lasers

Optical excitation of shallow donor centres in silicon creates an inverted electron distribution on the Coulomb centres at low lattice temperature. The population inversion is formed due to the peculiarities of the electron-phonon interaction of free and bound charge carriers. At certain conditions stimulated emission in the THz frequency range from group-V donors in silicon can be obtained

Stimulated terahertz emission from arsenic donors in silicon

Stimulated emission has been obtained from intra-center donor transitions in silicon monocrystals doped by arsenic. The Si:As laser was optically excited by radiation from a CO2 laser. The emission spectrum consists of two lines corresponding to the 2p±-->1s(E) and 2p±-->1s(T2) intra-center arcenic transitions. The population inversion is formed due to fast 2s-->1s(A1) electron relaxation assisted by intervalley longitudinal acoustic f-phonon emission. This keeps the excited donor states below the 2p± state unpopulated. Thus population inversion occurs between the 2p± state and the 1s(E), 1s(T2) states

Possibility of THz donor lasing in electrically pumped silicon

A possible way to create silicon terahertz laser under electric field excitation is presented. Electrical pulses with both period and duration in nanosecond range should be applied to moderately doped stressed bulk silicon. The purpose of short pulse excitation is impurity breakdown followed by capture and population of upper lasing state. The mechanisms responsible for population inversion and losses are described