B. Gu

Theory of Luminescence and Optical Refrigeration in p-doped Semiconductors

We present a microscopic many-body theory of optical refrigeration of p-doped semiconductors. Conceptually, the refrigeration mechanism is the upconversion of pump photons through absorption and subsequent luminescence by electron-hole pairs. The electron-hole pair can be an unbound pair, a pair bound by the attractive Coulomb interation (exciton), or a pair in which the hole is located at an acceptor site. Assuming the electron-hole pairs to be in quasi-thermal equilibrium, our theory calculates its absorption and luminescence spectra within a diagrammatic (real-time) Greens function approach at the self-consistent T-matrix level. The strong on-site Coulomb repulsion of holes at acceptor sites is taken into account via a truncation of the acceptor Fock space, which excludes states with higher than single-hole occupation. The resulting absorption and luminescence spectra are used in a cooling threshold analysis for GaAs that also takes into account other losses into heat. We compare the present results for p-doped GaAs with previous ones obtained for undoped GaAs.

The relation between light absorption and luminescence in laser cooling of two-dimensional semiconductor systems

In efforts underway to achieve laser cooling of semiconductors, an electron-hole population is generated in the sample and maintained in a steady state. The analysis of light absorption by and luminescence from this population is basic to the understanding of feasibility and efficiency issues of the cooling process. It is commonly understood that, when this electron-hole plasma is in quasi-thermal equilibrium (equilibrium at a fixed density), the KMS (Kubo-Martin-Schwinger) relation holds between its luminescence and absorption spectra: their ratio is proportional to the Bose distribution function characterized by the temperature and chemical potential of the plasma. The proportionality factor, which affects the total luminescence rate, may generally depend on the dimensionality and geometry of the system. In this Contribution, as a preliminary step to extend our theoretical analysis of semiconductor cooling to quantum well systems, we discuss the application of the KMS relation to their spectra. In particular, we derive and discuss the geometrical proportionality factor in the KMS relation for quantum wells and compare it to its counterpart for bulk semiconductors.

Relation Between Interband Dipole and Momentum Matrix Elements in Semiconductors

The relation between dipole and momentum matrix elements in crystals, treated with periodic boundary conditions, is revisited. A correction term to standard expressions is found to be large for bulk GaAs, small for THz transitions.

Theoretical approach to the excitonic response of GaAs nanomembranes in the averaged-strain approximation

GaAs nanomembranes are thin crystalline GaAs semiconductor structures that can be bent or otherwise elastically deformed from their natural shape. We present a microscopic theory of the linear optical response of such deformed structures. Our approach combines conventional structural analysis (based on the theory of elasticity), the valence band Hamiltonians (Luttinger and Pikus–Bir) for III–V semiconductors, and the semiconductor Hamiltonian including Coulomb interaction. We formulate the general equation of motion for the interband polarization for thin elastically deformed nanomembranes. A simple limiting case results from the single-subband approximation and the averaged-strain approximation. Within this approximation scheme, we present numerical results for excitonic spectra for a cylindrically deformed membrane.