Spatially selective laser cooling of carriers in semiconductor quantum wells

Abstract

A successive four-step model is proposed for spatially selective laser cooling of carriers in undoped semiconductor quantum wells. The four physical steps include the following processes: (1) cold electrons with nearly zero kinetic energy are initially excited across a bandgap in a coherent and resonant way by using a weak laser field; (2) the induced cold carriers in two different bands are heated via inelastic phonon scattering to higher-energy states above their chemical potentials; (3) the resulting hot electrons and holes radiatively recombine to release photons, thus extracting more power from the quantum well than that acquired during the weak pump process; and (4) hot phonons in two surrounding hot barrier regions thermally diffuse into the central cool quantum well, thereby cooling the entire lattice with time. Based on this model, a thermal-diffusion equation for phonons including source terms from the carrier-phonon inelastic scattering and the thermal radiation received by the lattice from the surrounding environment is derived to study the evolution of the lattice temperature. At the same time, an energy-balance equation is applied to adiabatically find the spatial dependence of the carrier temperature for a given lattice temperature at each moment. There are two interesting findings in thismore » paper. First, a V-shape feature in the carrier temperature is predicted by numerical calculations, which becomes apparent only for initial lattice temperature above 150 K. Second, a thermal-drag of the carrier temperature is found as a result of the strong carrier-phonon scattering. The difference between the lattice and carrier temperatures resulting from the thermal-drag effect is larger in the barrier regions than in the well region.« less