Transient-type stimulated emission in the terahertz (THz) frequency range has been achieved from phosphorus doped silicon crystals under optical excitation by a few-picosecond-long pulses generated by the infrared free electron lasers FELIX and CLIO. The analysis of the lasing threshold and emission spectra indicates that the stimulated emission occurs due to combined population inversion based lasing and stimulated Raman scattering. Giant gain has been obtained in the optically pumped silicon due to large THz cross sections of intracenter impurity transitions and resonant intracenter electronic scattering. The transient-type emission is formed under conditions when the pump pulse intervals exceed significantly the photon lifetime in the laser resonator.

We demonstrate several mechanisms for switching QCL frequency comb on and off in double-resonance mid-infrared InGaAs/InAlAs Quantum Cascade Lasers using nonstationary pump current and unstable part of the I-V curve. All of them are based on Risken-Nummedal-Graham-Haken instability.

We analyze the cascade capture of charge carriers due to the interaction with acoustic phonons in highly doped semiconductors using a model that describes the recombination of photo-ionized carriers as a continuous relaxation of carriers in the energy space at both positive and negative energies in the field of a set of impurity ions. Such description enables simultaneous calculation of non-equilibrium carrier distribution formed by interaction with acoustic phonons in the presence of impurity traps, and the time of recombination in a wide range of concentrations of capture centers and phonon temperatures. Additionally, we calculated the time of cascade recombination in the presence of fast scattering processes forming a Maxwellian distribution of free carriers. We show that experimentally observed concentration and temperature dependence of carrier life times in highly doped semiconductors can be described within the model of the cascade capture to uniformly spaced capture centers, and the main factor that determines the regime of cascade capture is the ratio of the thermal energy and the energy of the overlap of impurity potentials.

We report on experimental observation of low-threshold Risken-Nummedal-Graham-Haken (RNGH) instability in mid-infrared InGaAs/InAlAs Quantum Cascade Lasers (QCLs) operating at 8 μm wavelength. The devices employ sequential resonant tunneling and can provide optical gain on the diagonal or vertical transition. Depending on the tunneling resonance detuning, the lasers can operate either on a stable part of I-V curve or with unstable electric field domain formation. We report how all these features impact the occurrence of the RNGH instability with broadband multimode emission, and show how they can be tailored with an applied bias or via optical excitation of free carriers. We present the results of 2nd order interferometric autocorrelation measurements and time-resolved spectral measurements in free-running QCLs and QCLs subjected to optical pumping. We explain the diversity of dynamic regimes in QCLs using a simple analytical expression for the second threshold.

In this letter we propose an approach to obtain directive radiation from wire lasers with subwavelength transverse dimensions and length much larger than the radiation wavelength (wire lasers) based on spatial filtering of their radiation using a combination of a spherical lens and a diaphragm. Theoretical modeling based on the antenna model for wire lasers shows that a directive beam with the uniform phase front can be formed when the diaphragm separates the maximum of the image field of the laser created by the lens. We demonstrate spatial filtering of wire laser radiation experimentally using a terahertz quantum cascade laser.

The multimode Risken-Nummedal-Graham-Haken (RNGH) instability in a CW laser arises due to Rabi splitting of the lasing transition induced by the lasing mode. Observation of large spectral broadening on the order of 2Ω Rabi provides an indication of multimode RNGH instability. Results of the experimental studies on RNGH instability in Mid-IR InGaAs/AlGaAs quantum cascade lasers (QCLs) operating at 8 μm wavelength are presented. This paper reports on steady-state and time-resolved spectral measurements and 2D order interferometric autocorrelation (IAC) measurements in free running QCLs and QCLs subjected to optical pumping and characterize it via spectral measurements and via 2D order interferometric autocorrelation (IAC) measurements. Results show that effect of applied bias field on the lasing transition, which can be controlled either with applied bias or optical excitation of free carriers on interband transition, can switch on and off broad multimode emission associated with RNGH instability.

We report on broad-band two-photon absorption (TPA) in several commercially available MIR inter-band bulk semiconductor photodetectors with the spectral cutoff in the range of 4.5–6 μm. The highest TPA responsivity of 2 × 10−5 A·mm2/W2 is measured for a nitrogen-cooled InSb photovoltaic detector. Its performance as a two-photon detector is validated by measuring the second-order interferometric autocorrelation function of a multimode quantum cascade laser emitting at the wavelength of 8 μm.

The low‐temperature capture processes of non‐equilibrium holes into gallium acceptors in moderately doped p‐germanium (NA ≈ 2 × 1015 cm−3) has been investigated by a single‐color pump–probe experiment using the free electron laser FELBE. The capture time decreases with increasing average photon flux density of the excitation pulse from about 10.9 ns (at ∼1.2 × 1024 cm−2 s−1) to ∼1.2 ns (∼2 × 1026 cm−2 s−1). Relaxation inside the valence band is almost independent on pump light intensity and its characteristic time is about 200 ps. In Addition, the intracenter relaxation times of the lowest excited Ga states were measured. The lifetimes scale with the phonon density of states controlling the bound hole − acoustic phonon interaction. The lifetime of the lowest excited state, , was measured to be ∼275 ps; while the lifetimes of the higher excited states, and , were found to be ∼160 ps.

We demonstrate the formation of a narrow beam from a long (L >> lambda) laser with subwavelength transverse dimensions (wire laser) as an image of the subwavelength laser waveguide formed by a spherical lens. The beam is linearly diverging with the angle determined by the ratio of the wavelength to the lens radius, while the minimum beam spot size is the same as that of the image of a point source. We realize such a beam experimentally using a terahertz quantum cascade wire laser.

In this paper, we calculate the structure of radiation from slow modes of wire lasers (lasers with the length much larger than the wavelength and subwavelength transverse dimensions) after transformation by spherical lenses and specially designed phase plates, and analyze the ways to optimize the coupling of wire laser radiation to phase-sensitive detectors.

Specific features of radiation structure of terahertz quantum cascade lasers are determined by wire geometry of their waveguides with small and sub-wavelength transverse dimensions and the length much larger than the wavelength. Here we present an overview of the results of beam profile investigations and of the methods proposed for the control of radiation structure of such lasers.

We show that the discrete spectra of shallow impurity centers in delta-doped semiconductor structures can be tailored using a system of wells and barriers with the width smaller than localization radii of impurity states (typically few nanometers). Optimization of the relaxation processes in such structures opens the way to a significant reduction of generation threshold of silicon impurity lasers.

In this paper we report a study on relaxation of the impurity photoresponse in strained p-Ge/GeSi heterostructures excited by pulsed THz radiation. The relaxation time is found to increase with the applied dc electric field, which is interpreted within the model of cascade carrier capture by the impurity centers. A second time scale of the impurity photoconductivity relaxation is observed near the impurity breakdown field, and in post-breakdown electric fields the relaxation time (decreasing with the field) is shown to be governed by the impact ionization rather than by the recombination.

Analysis of the main factors of the temperature dependence of inverse population on shallow-impurity transitions in semiconductors is carried out in the context of the four-level inversion scheme: the population of the lower state of the laser transition by heating from the ground state and a decrease in the population of the long-lived impurity state due to thermal ionization and an increase in the rate if direct recombination to the ground state with optical-phonon emission occurs. The temperatures at which these factors become substantial are determined. It is shown that thermal ionization from the long-lived state is the major factor determining the temperature quenching of stimulated emission upon shallow-donor transitions in silicon.

In n-GaAs/InGaAsP and p-Ge/GeSi quantum well heterostructures excited by both the picosecond broad band and the nanosecond narrow band pulses of THz radiation the relaxation times of the impurity photoconductivity were measured. In one of the sample Ge/GeSi this more narrow quantum well we observed the first time increases maximum relaxation time from 9 to 14 ns at the narrow band THZ radiation frequency decrease from 60 cm−1 down to 25 cm−1 that can be explained by a dispersion of shallow acceptor binding energies. The frequency of 60 cm−1 nearly corresponds to the binding energy of acceptors situated nearby Ge QW center while that of 25 cm−1 corresponds to ionization energy for the hole (in Ge QW) bound with a remote acceptor ion in the center of GeSi barrier. We explain the observed larger relaxation time for the acceptor with lower binding energy by an enhanced probability for the hole captured by an excited impurity state to be thermally activated again into the valence band.

Highly divergent radiation is expected from lasers with sub-wavelength apertures due to diffraction. However the short range variations of intensity have been observed in the far field of THz quantum cascade lasers with sub-wavelength apertures. We developed an antenna model for long lasers with subwavelength transverse dimensions (wire lasers). Based on this model we show that the observed far field variations are formed due to the interference of the radiation from the longitudinal mode distribution. We predict that radiation from subwavelength wire laser can be concentrated in a narrow beam with a divergence determined by the ratio of the wavelength to the length of the laser.

The need to reach single-mode lasing and minimize at the same time the electrical dissipation of cryogenically operated terahertz quantum cascade lasers may result in small and subwavelength cavity dimensions. To assess the influence of such dimensions on the shape of the laser emission, we have measured the beam pattern of two metal-metal cavity quantum cascade lasers. The patterns show regular angular intensity variations which depend on the length of the laser cavity. The physical origin of these features is discussed in terms of interference of the coherent radiation emitted by end and side facets of the laser bar.