Gilad Almogy

Second-harmonic generation in absorptive media

The solution of the coupled-wave equations for second-harmonic generation in a near-resonant three-level system is extended to include absorption. It is shown, within second-order perturbation theory, that double resonance is the optimal conversion condition, despite absorption enhancement. We extend the solution numerically, using nonperturbative susceptibilities derived within the rotating-wave approximation, to saturating intensities and discuss the modifications to the perturbative conclusions as well as the regimes of validity for the various approximations.

Intersubband-transition-induced phase matching

We suggest the use of the refractive-index changes associated with the intersubband transitions in quantum wells for phase matching in nonlinear materials. An improvement in the conversion efficiency of mid-IR second-harmonic generation by almost 2 orders of magnitude over non-phase-matched bulk GaAs is predicted. We also show that the linear phase contributions of intersubband transitions used for resonant enhancement of second-harmonic generation must be considered, as they could limit the conversion efficiency by increasing the phase mismatch on one hand or offset the bulks dispersion and lead to phase matching on the other.

Observation of birefringence induced by intersubband transitions in quantum wells

We present a direct measurement of the birefringence induced by the intersubband transitions in quantum wells. Phase delays of up to 40° were observed in our samples, corresponding to a 0.07 difference in the index of refraction between the polarizations parallel and perpendicular to the plane of the wells in a GaAs/AlGaAs silicon‐doped structure. The measurement was conducted at several wavelengths—enabling us to deduce the total linear birefringence near the absorption resonance. The observed birefringence is in close agreement with the Kramers–Kronig transform of the absorption spectra.

Monolithic integration of quantum well infrared photodetector and modulator

A modulation depth of 40% (0.7 dB/μm) was obtained with an infrared (10.6 μm) modulator consisting of a stack of 50 pairs of weakly coupled asymmetric quantum wells monolithically integrated with a quantum well infrared photodetector. The monolithic integration is shown to be a promising technique for the ‘‘ac’’ coupling of infrared focal‐plane arrays as well as for the direct study of the effects of electric fields and charge density variations on intersubband transitions.

Direct Measurement of Population-Induced Broadening of Quantum Well Intersubband Transitions

The dependence of the absorption spectral linewidth of quantum well intersubband transitions on the electron population in the well is experimentally demonstrated. We show that the dependence of the spectral linewidth on the population is substantial and accounts for some of the broadening previously attributed to donor scattering.