Population inversion in optically pumped asymmetric quantum well terahertz lasers
Intersubband carrier lifetimes and population ratios are calculated for three- and four-level optically pumped terahertz laser structures. Laser operation is based on intersubband transitions between the conduction band states of asymmetric GaAs-Ga1−xAlxAs quantum wells. It is shown that the carrier lifetimes in three-level systems fulfill the necessary conditions for stimulated emission only at temperatures below 200 K. The addition of a fourth level, however, enables fast depopulation of the lower laser level by resonant longitudinal optical phonon emission and thus offers potential for room temperature laser operation.
The relative importance of electron electron and electron phonon scattering in terahertz quantum cascade lasers
Abstract This theoretical work explores the carrier dynamical issues that must be confronted if the recent successes of the mid-infrared quantum cascade lasers [1] are to be translated into longer wavelength devices. Whilst the small subband separations lead to a partial suppression of longitudinal optic (LO) phonon emission, which is the dominant non-radiative loss mechanism in quantum cascade lasers, it also encourages intersubband electron–electron scattering, the rate of which can be comparable with LO phonon scattering. A triple quantum well active layer design is advanced which exhibits population inversion and operates at 7.7 THz (39 μm), it is shown that intersubband electron–electron scattering can dominate over phonon scattering for depopulating the lower laser level. The role of intrasubband electron–electron scattering on the non-equilibrium carrier distributions and its potential influence on the phonon scattering rate is also discussed. It is argued that only a full Monte Carlo simulation will describe completely the carrier dynamics of quantum cascade lasers.
An analytical approach to the design of electrodes in high-power, fast-transverse-flow CO2 lasers
Abstract Usually, the design of the electrodes of fast-transverse-flow CO 2 lasers relies more on experimental data than on theoretical analysis. Traditional systems sustain a stable, high-power discharge but the current distribution generates a peaked, non-uniform small-signal-gain pattern. We present a theoretical model suitable for the design of electrodes that achieves a quasi uniform gain distribution. The analysis, based on a two-temperature model and the gas-transport equations, computes the electron density that supports a flat gain pattern. Combining the desired electron density with the electron-balance equations, the model determines the required electric field. The results were used to design a new set of electrodes for a home-made fast transverse flow CO 2 laser. A stable, large volume discharge ( 4 × 8 × 90 cm 3 ) with very good uniformity was obtained. The resulting gain distribution was registered in a bi-dimensional map. The peak gain rose from 1.2 m - 1 , attained with the old electrode set, to 1.6 m - 1 with the new one and the gain showed a homogeneous profile.