S.G. Pavlov

Active mode locking of a p-Ge hot hole laser

The generation of 200 picosecond pulses of far-infrared radiation from a p-Ge hot hole laser (50–140 cm−1) was achieved due to active mode locking by electrical intracavity modulation of the gain.

Population Inversion and Far-Infrared Emission from Optically Pumped Silicon

A mechanism for the generation of population inversion and far-infrared emission in Si with shallow donors is investigated. By optically pumping a phosphorus doped Si crystal with a CO2 laser it was possible to achieve population inversion between the metastable 2p0 state and the 1s(E) and 1s(T) states. Spontaneous emission from these transitions was detected. Frequency measurements of this emission are in agreement with the expected transition frequency. Absorption measurements with a far-infrared probe laser confirm the existence of population inversion between the 2p0 state and the 1s(E) and 1s(T) states.

Mode fine structure of the p-Ge Intervalenceband laser measured by heterodyne mixing spectroscopy with an optically pumped ring gas laser

Abstract Heterodyne mixing of the far infrared p -Ge intervalenceband pulse laser and the optically pumped continuous wave ring gas laser at 117.7 μm (CH 2 F 2 ), 118.8 μm (CH 3 OH) and at 184.3 μm (CH 2 F 2 ) revealed the p -Ge laser mode fine structure with a resolution below 1 MHz, the absolute mode linewidth and a mode tunability of 25 MHz during the p -Ge laser pulse due to sample heating. For the first time an acousto-optical spectrometer has been used in such experiments which analysed the mode spectrum during a single pulse in a 1 GHz band. We have combined the methods of grating, homodyne mixing and heterodyne mixing spectroscopy for the investigation of the p -Ge laser mode fine structure in the broad band emission regime with different resonators including a high-Q tunable resonator for single mode operation.

Single axial-mode selection in a far-infrared p-Ge laser

Single axial-mode operation of the pulsed far-infrared p-Ge laser with an intracavity Fabry–Perot type frequency selector has been observed by means of Fourier-transform spectroscopy. A spectral resolution better than 1 GHz has been achieved on an ordinary continuous-scan spectrometer using the event-locked technique for pulsed emission sources. A laser active-cavity finesse of at least unity has been directly confirmed from the measured emission spectral width. Analysis of the envelope of the corresponding interferogram suggests that the finesse exceeds 10.

FIR lasing based on group V donor transitions in silicon

The observation of population inversion and far-infrared emission from shallow donor transitions in silicon is reported. Silicon crystals doped with phosphor and bismuth have been optically excited by radiation from a CO2 laser. For Si : P, the population inversion is due to the suppression of acoustical phonon assisted intracenter relaxation between the 2p0 and 1s(E) states and stimulated emission from the transition between these states has been observed. For Si : Bi, the population inversion is due to a resonance interaction with optical phonons which deplete the 2p0 state. In this case only spontaneous emission was detected. The experimental results are compared with theoretical calculations and found to be in good agreement. # 2001 Elsevier Science B.V. All rights reserved.

Terahertz Luminescence of GaAs-Based Heterostructures with Quantum Wells under the Optical Excitation of Donors

Spontaneous emission from selectively doped GaAs/InGaAs:Si and GaAs/InGaAsP:Si heterostructures is studied in the frequency range of ∼3–3.5 THz for transitions between the states of the two-dimensional subband and donor center (Si) under the condition of excitation with a CO2 laser at liquid-helium temperature. It is shown that the population inversion and amplification in an active layer of 100–300 cm−1 in multilayered structures with quantum wells (50 periods) and a concentration of doping centers ND ≈ 1011 cm−2 can be attained under the excitation-flux density 1023 photons/(cm2 s).

Piezo-controlled intracavity wavelength selector for the far-infrared p-Ge laser

An electrically-controlled tunable intracavity frequency selector is demonstrated for the far-infrared p-Ge laser. The tunable laser mirror was driven by a piezo-element inside a liquid helium bath. This design allows very small controllable displacements of the tunable mirror that were impossible in previous mechanical systems. High-resolution spectroscopy of the laser output reveals the nature of mode-hop tuning characteristic for the tuning-element construction used.

Active Mode Locking of a P-GE Light-Heavy Hole Band Laser by Electrically Modulating its Gain: Theory and Experiment

The p-Ge intervalenceband (IVB) far-infrared laser (50cm−1 < v < 140cm−1), operating in crossed electric and magnetic (E ⊥ B) fields, is based on a population inversion between accumulated light holes and streaming heavy holes in the k-space region below the optical phonon energy ɛop.1 Its broad amplification band (Δv ≈ 100cm−1) allows amplification of far-infrared pulses at a picosecond timescale.2 We have investigated the feasibility of achieving active mode locking in a p-Ge IVB laser by locally modulating the gain in a small part of the p-Ge crystal. By applying a radiofrequency (RF) electric field to small additional contacts along the magnetic field direction, the population inversion in the active part between these contacts is modulated strongly: at peak amplitudes of the RF cycle the light holes are accelerated out of the so-called passive region below Ɛop, while at the RF nodes the light hole population is restored due to the fast pumping by optical phonon scattering from streaming heavy holes. Fig. la shows the expected response of the small-signal gain due to the direct IVB transition during one period of a 500MHz RF electric field, as it has been calculated using a Monte Carlo simulation program.3 From Fig. lb it is clear that a peak-to-peak modulation depth of 30–40 % can be obtained in the modulated part of the p-Ge crystal with only a small RF field amplitude E RF 0 of approximately 20Vcm−1.

Characterization of a quantum cascade laser as local oscillator in a heterodyne receiver at 2.5 THz

A quantum cascade laser operating at 2.5 THz has been investigated with respect to performance parameters which are relevant for a local oscillator in a heterodyne receiver. The beam profile has been measured and transformed in to a close to Gaussian profile resulting in a good matching between the field patterns of the quantum cascade laser and the antenna of a superconducting hot electron bolometric mixer. Noise temperature measurements with the hot electron bolometer and a 2.5 THz quantum cascade laser yielded the same result as with a gas laser as local oscillator.

THz silicon lasers based on donor center transitions

Summary form only given. Results of experimental and theoretical study of terahertz frequency (1.3-7 THz, wavelength range 230-42 /spl mu/m) stimulated emission based on the intracenter optical transitions of group-V shallow donor centers (phosphor P, antimony Sb, arsenic As, bismuth Bi) in silicon crystal are reviewed and discussed. The population inversion of bound Coulomb center states and appropriate light amplification is formed due to low-temperature (T<30K) phonon-assisted intracenter relaxation under optical excitation e.g. using CO/sub 2/ laser radiation. The lifetimes of principle impurity states, small signal gain and laser threshold intensity are considered in the framework of simplified multi-valley theoretical model of donor center states. Laser performances differed for different donor centers are summarized and compared with theoretical calculations. Special attention is paid to the effect of Si crystal lattice deformation under applied uniaxial stress (P<2kbar) on the phonon-assisted scattering which results in the essential enhancement of laser gain and the appropriate decrease of the laser threshold intensity (Si:P,Si:Sb) and can generate the shift of impurity transitions responsible for stimulated emission (Si:As).