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.
Carrier dynamical issues for extending the operating wavelength of quantum cascade lasers
Electron-longitudinal optic phonon and electron-electron intersubband scattering rates are calculated for a variety of quantum well systems. It is demonstrated that the internal quantum efficiency of a Terahertz radiative intersubband transition can be greater than in the mid-IR at 4K, however by room temperature has fallen to around 20 percent. A study of the internal quantum efficiency of a Terahertz energy intersubband transition in double and single quantum wells, has shown that the vertical intrawell transition is more efficient than the diagonal interwell transition.
Optical rotatory dispersion and absolute configuration—III
Abstract Optical rotatory dispersion measurements on pyrrolidine, piperidine, and tetrahydro-isoquinoline alkaloids indicate that the absolute configuration of the α-asymmetric center may be assigned directly from the dispersion curve, provided either that the nitrogen atom is not protonated, or that the compound contains a chromophore absorbing above 200 mμ.