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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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

Terahertz absorption and emission upon the photoionization of acceptors in uniaxially stressed silicon

Experimental data on the spontaneous emission and absorption modulation in boron-doped silicon under CO2 laser excitation depending on the uniaxial stress applied along the [001] and [011] crystallographic directions are presented. Room-temperature radiation is used as the probe radiation. Low stress (less than 0.5 kbar) is shown to reduce losses in the terahertz region by 20%. The main contribution to absorption modulation at zero and low stress is made by A+ centers. Intersubband free hole transitions additionally contribute to terahertz absorption at higher stress. These contributions can be minimized by compensation

Time-resolved electronic capture in n-type germanium doped with antimony

The low temperature (T=5–40 K) capture of free electrons into hydrogenlike antimony centers in germanium has been studied by a time-resolving experiment using the free electron laser FELBE. The analysis of the pump-probe signal reveals a typical capture time of about 1.7 ns that decreases with pump energy to less than 1 ns while the number of ionized donors increases. The dependence on the pump-pulse energy is well described by an acoustic phonon-assisted capture process. In the cases when (i) a significant number of the electrons is in the conduction band (flux densities larger than 5× 1E25 photons/(cm2 s), (ii) the lattice temperature is above ~20 K, or (iii) a static electric field above ~2V/cm is applied to the crystal, the pump-probe technique reveals an additional intraband relaxation process with a characteristic time of ~100 ps, which is much shorter than that of the capture of free electrons into the antimony ground state

Towards a life-time-limited 8-octave-infrared photoconductive germanium detector

Ultrafast, ultra-broad-band photoconductive detector based on heavily doped and highly compensated germanium has been demonstrated. Such a material demonstrates optical sensitivity in the more than 8 octaves, in the infrared, from about 2 mm to about 8 μm. The spectral sensitivity peaks up between 2 THz and 2.5 THz and is slowly reduced towards lower and higher frequencies. The life times of free electrons/holes measured by a pump-probe technique approach a few tenths of picoseconds and remain almost independent on the optical input intensity and on the temperature of a detector in the operation range. During operation, a detector is cooled down to liquid helium temperature but has been approved to detect, with a reduced sensitivity, up to liquid nitrogen temperature. The response time is shorter than 200 ps that is significantly faster than previously reported times

Extremely fast electron capture in moderately doped germanium

We report the observation of ultrafast capture of free electrons (n-Ge:Sb:Ga) and holes (p-Ge:Ga:Sb) in moderately doped (net concentration > 2×10^15/cm3) Ge with high compensation, up to 50%. The typical range of the relaxation times for photoinduced (far-infrared photo- ionization) transmittance is in the range of 20-300 ps dependent on the sample characteristics and the far-infrared light intensity. Measurements of the photoinduced decay have been carried out by a contactless pump-probe technique at the infrared free electron laser facility at HZDR, Dresden. The observed relaxation times of free charge carriers are up to two orders of magnitude shorter than the ~ 2 ns response time observed in the photoconductive response of neutron transmutation doped isotope-fixed compensated p-Ge:Ga:Se:As. This indicates the potential for very fast broad band detection of infrared pulses by extrinsic photoconductive germanium detectors with optimized doping, compensation, geometry and an appropriate electrical readout circuit

Optimizing the operation of terahertz silicon lasers

A thorough characterization of terahertz silicon lasers with respect to doping concentration and operation temperature has been carried out. Several factors limiting the laser operation, such as heating of the laser crystal and absorption by photoinduced free carriers are discussed. The optimal doping concentration has been determined. The influence of the pump geometry on the laser efficiency has been investigated. It was found that an external uniaxial force applied to the laser crystal lowers the pump threshold and increases the output power

Polarization of the induced THz emission of donors in silicon

The polarization of the terahertz (4.9–6.4 THz) stimulated emission of Group-V (Sb, P, As, Bi) donors in single-crystal silicon under pumping (photoionization) by a CO2 laser (photon energy 117 meV), depending on the uniaxial compressive deformation of the crystal along the [100] axis, is experimentally investigated. The influence of the field direction of the pump wave on its efficiency is discussed