Intersubband terahertz lasers using four-level asymmetric quantum wells

We demonstrate the potential for laser operation at far-infrared wavelengths (30–300 µm, 1–10 THz) by using intersubband emission in four-level GaAs/AlGaAs asymmetric (stepped) quantum wells. Achieving population inversion in these devices depends critically on the lifetimes of the nonradiative intersubband transitions, and so we have performed detailed calculations of electron–electron and electron–phonon scattering rates. Our four-subband structures show potential for the realization of room temperature lasing, unlike previously considered three-subband structures which did not give population inversions except at impractically low electron densities and temperatures. Auger-type electron–electron interactions involving the highly populated ground subband effectively destroyed the population inversion in three level systems, but in these four subband structures the inversion is maintained by strong phonon-mediated depopulation of the lower laser level. The largest population inversions are calculated at low temperatures (< 30 K), but for the structures with higher emission energies, room temperature (300 K) operation is also predicted. © 1999 American Institute of Physics

Self-consistent solutions to the intersubband rate equations in quantum cascade lasers: Analysis of a GaAs/AlxGa1-xAs device

The carrier transition rates and subband populations for a GaAs/AlGaAs quantum cascade laser operating in the mid-infrared frequency range are calculated by solving the rate equations describing the electron densities in each subband self-consistently. These calculations are repeated for a range of temperatures from 20 to 300 K. The lifetime of the upper laser level found by this self-consistent method is then used to calculate the gain for this range of temperatures. At a temperature of 77 K, the gain of the laser is found to be 34 cm(-1)/(kA/cm(-2)), when only electron–longitudinal-optical phonon transitions are considered in the calculation. The calculated gain decreases to 19.6 cm(-1)/(kA/cm(-2)) when electron–electron transition rates are included, thus showing their importance in physical models of these devices. Further analysis shows that thermionic emission could be occurring in real devices. © 2001 American Institute of Physics

Nonequilibrium electron heating in inter-subband terahertz lasers

Inter-subband laser performance can be critically dependent on the nature of the electron distributions in each subband. In these first Monte Carlo device simulations of optically pumped inter-subband THz lasers, we can see that there are two main causes of electron heating: intersubband decay processes, and inter-subband energy transfer from the "hot" nonequilibrium tails of lower subbands. These processes mean that devices relying on low electron temperatures are disrupted by electron heating, to the extent that slightly populated subbands can have average energies far in excess of the that of either the lattice or other subbands. However, although these heating effects invalidate designs relying on low temperature electron distributions, we see that population inversion is still possible in the high-THz range at 77 K in both stepped and triple-well structures, and that our 11.7 THz triple-well structure even promises inversion at 300 K. © 2002 American Institute of Physics

Electric field domains in p-Si/SiGe quantum cascade structures

The formation of domains in quantum cascade structures is one of the mechanisms strongly affecting the operation of quantum cascade lasers, quantum-well infrared detectors, and other devices. In this paper, we consider the problem of domain formation in p-doped Si/SiGe quantum cascades, using a carrier scattering transport framework. In effect, the hole flow along the cascade is described via scattering between quantized states belonging to neighboring periods, caused by phonons, alloy disorder, and carrier-carrier interactions. The generation of either periodic or of nonperiodic domains is studied in uniformly doped cascades, as well as the influence of modulation doping of cascades on the domain formation

Electron temperature and mechanisms of hot carrier generation in quantum cascade lasers

A technique for calculating the temperature of the nonequilibrium electron distribution functions in general quantum well intersubband devices is presented. Two recent GaAs/Ga(1–x)Al(x)As quantum cascade laser designs are considered as illustrative examples of the kinetic energy balance method. It is shown that at low current densities the electron temperature recovers the expected physical limit of the lattice temperature, and that it is also a function of current density and the quantised energy level structure of the device. The results of the calculations show that the electron temperature T(e) can be approximated as a linear function of the lattice temperature T(l) and current density J, of the form T(e) = T(l) + a(e–l)J, where a(e–l) is a coupling constant (~6–7 K/kA cm(–2) for the devices studied here) which is fixed for a particular device. © 2002 American Institute of Physics

Self-consistent energy balance simulations of hole dynamics in SiGe/Si THz quantum cascade structures

Analysis of hole transport in cascaded p-Si/SiGe quantum well structures is performed using self-consistent rate equations simulations. The hole subband structure is calculated using the 6×6 k·p model, and then used to find carrier relaxation rates due to the alloy disorder, acoustic, and optical phonon scattering, as well as hole-hole scattering. The simulation accounts for the in-plane k-space anisotropy of both the hole subband structure and the scattering rates. Results are presented for prototype THz Si/SiGe quantum cascade structures. © 2004 American Institute of Physic

Self-consistent scattering model of carrier dynamics in GaAs-AlGaAs terahertz quantum-cascade lasers

Intersubband electron scattering transport in terahertz GaAs–AlGaAs quantum cascade lasers is analyzed, using a full 13-level self-consistent rate equation model. The approach includes all relevant scattering mechanisms between injector–collector and active region states in the cascade structures. Employing an energy balance equation which includes the influence of both electron longitudinal optical phonon and electron–electron scattering, the method also enables evaluation of the average electron temperature of the nonequilibrium carrier distributions in the device. The electron temperature is found to give a strong influence on the output characteristics, particularly at very low temperatures. The threshold currents and electric field-current density characteristics are in very good agreement with experiment, implying that the model has a strong predictive capability

Observations of Birds and Mammals at Bluenose Lake

Reports 4 Aug - 3 Sept 1953 observations at this lake at 1800 ft elevation in a large shallow basin in the Melville Hills of Mackenzie District. This belated note is prompted by the lack of other investigations there and by the fact that most of the breeding-bird records constitute range extensions. Only 35 bird species were recorded. The absence of some species could be correlated to an absence of varied habitat: no extensive willow thickets and no rock outcrops. Eleven mammalian species were seen. A barren-ground grizzly was observed to approach a muskox and turn away. There is no known record of a grizzly killing a muskox.Observations sur les oiseaux et les mammif&egrave;res du lac Bluenose. On d&eacute;crit les oiseaux et mammif&egrave;res observ&eacute;s au cours d'une exploration biologique du lac Bluenose, par 68&ordm;28'N et 119&ordm;45'W. On a vu trente-cinq esp&egrave;ces d'oiseaux, dont beaucoup &eacute;taient en train de couver et dont nombre constituent des extensions de leur domaine connu. On a identifi&eacute; onze esp&egrave;ces de mammif&egrave;res. Un manque de variabilit&eacute; dans l'habitat est probablement le principal facteur qui limite le nombre des esp&egrave;ces, surtout d'oiseaux