Nai H. Kwong

Distortionless light pulse delay in quantum-well Bragg structures

We describe a reflection scheme that allows Bragg-spaced semiconductor quantum wells to be used to trap, store, and release light. We study the temporal and spectral distortion of delayed light pulses and show that this geometry allows multibit delays and offers a high degree of distortion compensation.

Optical characterization of electron-phonon interactions at the saddle point in graphene.

The role of many-body interactions is experimentally and theoretically investigated near the saddle point absorption peak of graphene. The time and energy-resolved differential optical transmission measurements reveal the dominant role played by electron-acoustic phonon coupling in band structure renormalization. Using a Born approximation for electron-phonon coupling and experimental estimates of the dynamic lattice temperature, we compute the differential transmission line shape. Comparing the numerical and experimental line shapes, we deduce the effective acoustic deformation potential to be Deff(ac)≃5  eV. This value is in accord with recent theoretical predictions but differs from those extracted using electrical transport measurements.

Mechanism of all-optical spin-dependent polarization switching in Bragg-spaced quantum wells

The authors outline a microscopic theory of pump-induced anisotropy in the optical response of Bragg-spaced quantum wells (BSQWs). Their theory explains the manipulation of the band structure of the BSQWs by the pump through the microscopic interactions between excitons in the quantum wells. They apply their theory to understand the mechanism of an all-optical polarization switch implemented on a BSQW structure. They trace the relation between the strongly spin-dependent exciton-exciton interactions and the switching signal. Reasonably good agreement is found between their theoretical results and experimental data.

Theory of optical refrigeration in p-doped semiconductors

We present a microscopic theory for luminescence of doped GaAs and its application to a study of optical refrigeration. We find that p-doping affects the temperature dependence of the cooling threshold in a complex way.

Theory of ultrafast optical gain in the nonlinear reflectivity of semiconductor Bragg structures

We present a microscopic theory for the nonlinear reflection of semiconductor Bragg structures. Our theoretical results showing ultrafast optical gain complement recent experimental observations and allow for the identification of the underlying many-particle processes.

Stopping, storing and releasing light in quantum well Bragg structures

Stopping, storing and releasing of light in resonant photonic bandgap structures made from Bragg-spaced quantum wells is studied. The expected performance of almost ideal infinite systems is contrasted with that of existing structures.

Probing Electron-Phonon Interactions at the Saddle Point in Graphene

High frequency differential transmission spectroscopy of graphene, probing near the M-point, is performed and analyzed theoretically. Electron-phonon coupling is identified as the chief mechanism for renormalization with an effective acoustic deformation potential of approximately 5eV.

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.

Instability induced all-optical switching in planar semiconductor microcavities

Using a microscopic theory, we predict all-optical switching in planar semiconductor micro-cavities where a weak beam switches a stronger signal. The scheme is similar to that recently demonstrated in atomic vapors.

Polarization- and spin-dependent ultrafast optical nonlinearities of Bragg-spaced quantum wells

Spin- and polarization-dependent ultrafast blue shifts, transient gain and self-wave-mixing are observed in Bragg-spaced InGaAs/GaAs quantum wells. The data are in agreement with a microscopic theory.