E.E. Orlova

Beam patterns of terahertz quantum cascade lasers with subwavelength cavity dimensions

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.

Far-infrared stimulated emission from optically excited bismuth donors in silicon

Far-infrared stimulated emission from optically pumped neutral Bi donors in silicon has been obtained. Lasing with wavelengths of 52.2 and 48.6 μm from the intra-center 2p±→1s(E:Γ8),1s(T2:Γ8) transitions has been realized under CO2 laser pumping. The population inversion mechanism is based on fast optical-phonon-assisted relaxation from the 2p0 and 2s excited states directly to the ground 1s(A) state leading to relatively small population in the intermediate 1s(E), 1s(T2) excited states.

Stimulated terahertz emission from group-V donors in silicon under intracenter photoexcitation

Frequency-tunable radiation from the free electron laser FELIX was used to excite neutral phosphorus and bismuth donors embedded in bulk monocrystalline silicon. Lasing at terahertz frequencies has been observed at liquid helium temperature while resonant pumping of odd parity impurity states. The threshold was about two orders of magnitude below the value for photoionization pumping. The influence of nonequilibrium intervalley TO phonons on the population of excited Bi impurity states is discussed.

Terahertz optically pumped Si:Sb laser

Far-infrared stimulated emission from optically pumped neutral Sb donors in silicon has been obtained. Lasing with a wavelength of 58.2 μm from the intracenter 2p0→1s(T2:Γ8) transition has been realized under CO2 laser pumping at liquid helium temperature. The population inversion mechanism is based on the relatively long-living excited state, 2p0, leading to an accumulation of the photoexcited electrons.

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.

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.

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.

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.

Relaxation of the impurity photoconductivity in p-Ge/Ge1−xSix quantum well heterostructures

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.

The emission spectra of optically pumped Si-based THz lasers

THz stimulated emission of phosphorus and bismuth donors in silicon under resonant intra-center optical pumping by the tunable free electron laser FELIX has been observed at low temperatures. The dependence of the emission on the excitation wavelength and power is reported.