Sergei G. Pavlov

Far‐Infrared Active Media Based on Shallow Impurity State Transitions in Silicon

Two mechanisms of the inverse population of shallow impurity states in silicon under optical pumping have been proposed and analyzed, using a procedure allowing to reduce the number of required matrix elements of transitions. The first mechanism is based on the resonance interaction of the 2p0 state in Si : Bi with optical phonons. The other one is based on the suppression of acous- tic-phonon-assisted relaxation from the 2p0 state in Si : P due to the momentum conservation law. Spontaneous emission was registered from shallow donors in Si : P under photoionization by a CO2 laser. The dependence of the spontaneous emission intensity on the intensity of pumping radiation confirms the possibility of amplification on impurity transitions. Introduction. The interest in far-infrared (FIR) active media based on shallow impu- rity states in silicon is caused by two reasons. The first one is the low level of lattice absorption of FIR radiation in silicon. The second is the cascade character of the main relaxation processes along the excited coulombic impurity states (1), allowing to expect high efficiency of pumping of impurity excited states population, which is important for reaching continuous lasing. Cascade relaxation means that transitions with a small re- duction of carrier energy are predominating, and the probability that a heated carrier takes part in the amplification is rather high when one of the first excited states or the group of excited states within the step of phonon relaxation are inversely populated. On the other hand, the fast acoustical-phonon-assisted relaxation causes the main complication in obtaining the inverse population of impurity states, since it tends to form the equilibrium distribution at lattice temperature. Thus, the inverse population of impurity states implies conditions in which the distribution is formed by processes with threshold character (interaction with optical phonons, optical pumping) and being faster than acoustical-phonon-assisted and Auger processes, or it implies conditions, where the latter are suppressed for particular states. The other complication originates from the fact that absorption for impurity transitions lies in the same frequency region as possi- ble amplification and thus can prevent it. Hence the possibility of amplification depends on the details of nonequilibrium distribution of charge carriers over excited impurity states.

Gain of the mode locked p-Ge laser in the low field region

Following the earlier observation of active mode locking in the high field region of the Voigt configured p-Ge intervalence band laser, presently mode locking in the low field region is also reported. The experimental results on the effective small signal gain for active- as well as for self-mode locked operation are given.

High-resolution study of composite cavity effects for p-Ge lasers

The temporal dynamics, spectrum, and gain of the far-infrared p-Ge laser for composite cavities consisting of an active crystal and passive transparent elements have been studied with high temporal and spectral resolution. Results are relevant to improving the performance of mode-locked or tunable p-Ge lasers using intracavity modulators or wavelength selectors, respectively. It is shown that an interface between the active p-Ge crystal and a passive intracavity spacer causes partial frequency selection of the laser modes, characterized by a modulation of their relative intensities. Nevertheless, the longitudinal mode frequencies are determined by the entire optical length of the cavity and not by resonance frequencies of intracavity sub-components. Operation of the p-Ge laser with multiple interfaces between Ge, Si, and semi-insulating GaAs elements, or a gap, is demonstrated as a first step toward a p-Ge laser with an external quasioptical cavity and distributed active media.

Actively mode-locked p-Ge laser in Faraday configuration

Active mode locking of the far-infrared p-Ge laser has been achieved in the Faraday configuration of electric and magnetic fields applied to the laser crystal. The laser generates 200 ps pulses of 80–110 cm−1 radiation with a laser-cavity roundtrip frequency of 454 MHz. The mechanism of gain modulation by the external rf electric field is based on induced electric-field gradients inside the active crystal and requires less rf power than was found previously for Voigt geometry.

BROAD BAND P-GE OPTICAL AMPLIFIER OF TERAHERTZ RADIATION

A solid state broad band amplifier of terahertz radiation (1.5–4 THz), based on intersubband transitions of hot holes in p-Ge is demonstrated. The gain is investigated as a function of applied magnetic and electric fields by transmission measurements using a laser system with two p-Ge active crystals, when one operates as an oscillator and one as an amplifier. A peak gain higher than usually reported for p-Ge lasers has been achieved using time separated excitation of the oscillator and amplifier. Distinct differences in gain dependence on applied fields are noted between low- and high-frequency modes of p-Ge laser operation.

Pulse separation control for mode-locked far-infrared p-Ge lasers

Active mode locking of the far-infrared p-Ge laser giving a train of 200 ps pulses is achieved via gain modulation by applying an rf electric field together with an additional bias at one end of the crystal parallel to the Voigt-configured magnetic field. Harmonic mode locking yields a train of pulse pairs with variable time separation from zero to half the roundtrip period, where pulse separation is electrically controlled by the external bias to the rf field.

Pulsed and mode-locked p-Ge THz laser: wavelength-dependent properties

Wavelength dependent properties of the p-Ge THz laser are reported for pulsed as well as for mode locked operation. The original small mirror laser outcoupler has been replaced by a mesh outcoupler, resulting in clear improvements of laser action. The optical output has been analyzed using a grating spectrometer and fast Schottky diode detectors. FOr 0.25 <EQ B <EQ 0.6T, 170-185 micrometers emission occurs. Laser action starts at short wavelength; during the pulse, longer wavelength components gain intensity, until simultaneous emission across the whole band occurs. With the mesh outcoupler instead of a small mirror, the small signal gain is found to increase, for instance from 0.015 cm-1 to 0.04 cm-1 at 172 micrometers . With the rf field modulation applied, 770 MHz mode locking of the laser is achieved at 172 micrometers , yielding a train of 100 ps FWHM pulses. For 0.5 <EQ B <EQ 1.4T, 75-120 micrometers emission is observed, dependent on both B and E field. Time-and wavelength dependence is complicated; often an oscillatory behavior of spectral components is seen. Although this effect complicates the formation of stable pulse trains under mode locked conditions, 140 ps pulses have been produced.

Influence of group II and III shallow acceptors on the gain of p-Ge lasers

The influence of intracenter absorption on the gain and the emission spectrum of p-Ge lasers was investigated. Self-absorption of p-Ge laser emission by group III acceptors in active media leads to a decrease of the gain as well as to a spectral shift in the amplification and emission spectra. Experimental spectra of p-Ge laser emission show evidence of transitions between shallow impurity states, even in the presence of high applied electric and magnetic fields.

Mode locking of far-infrared p-Ge lasers

Active mode locking of the THz p-Ge light-to-heavy-hole-band laser is reported yielding 200 ps FIR pulses with a few Watts peak power. It is achieved via radio-frequency (RF) gain modulation at one end of the laser crystal in Voigt configuration. Applying an additional bias voltage at the RF contacts allows one to optimize the gain and improve mode locking characteristics. This compensates an intrinsic bias onset caused by charging of the laser crystal due to the asymmetry of (warped-band) hole transport in crossed electric and magnetic fields for the orientation B/spl par/[112], E/spl par/[11~0] used in our experiments. A transition from single-pulse mode locking to second harmonic mode locking is observed for a crystal with roundtrip frequency equal to the RF frequency, and separation between the two pulses is tuned from zero to half the cavity roundtrip period by changing the external bias to the modulating RF field.

The far-infrared p-Ge laser: cavity and modulation advances

Summary from only given. We have made advances in laser cavity design, wavelength selection, cooling system, and excitation electronics that enhance p-Ge semiconductor laser viability for commercial applications.