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 bulk's dispersion and lead to phase matching on the other

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

Quantum Well Intersubband Transitions: Nonlinear Optics, Refractive Index and Infrared Modulation

The nonperturbative theory for the nonlinear optical interaction of quantum well intersubband transitions is developed. The nonlinear optical response of intersubband transitions in quantum wells is rigorously derived and the implications of their resonantly-enhanced nature are examined. Limitations on the use of the standard expansion of the induced polarization in terms of perturbative nonlinear coefficients are presented and it is shown that an alternative nonperturbative formalism is necessary for analyzing intersubband device applications. Upper limits are derived on the magnitudes of several key intersubband transition-induced nonlinear processes. It is shown that for both electrooptic and all-optic modulation, resonantly-enhanced absorption modulation is inherently preferable to phase modulation. A limit on the second-harmonic intensity that may be generated in a given propagation length and modified design criteria for optimizing second-harmonic generation in quantum wells are also obtained from the nonperturbative formalism. The large and highly dispersive refractive index contribution of intersubband transitions was observed for the first time through the birefringence induced in a GaAs/AlGaAs multi-quantum well stack. It is shown that this index, rather than the absorption induced by intersubband transitions, may become the dominant limitation on frequency conversion efficiencies. Potential applications of this controllable refractive index for a novel phase-matching technique of second-harmonic generation and for improved waveguiding in semiconductors is suggested and analyzed. Removal of charge integration limitations upon the performance of thermal imagers through the 'ac'-coupling of infrared focal-plane arrays is suggested. This is achieved by the monolithic integration of an intersubband infrared absorption modulator and detector leading to a modulation depth of 45% at a wavelength of 10.6µm. The uniquely accurate design of the coupled quantum well infrared modulator was based on a self-consistent computer model of the Schrodinger and Poisson equations in quantum wells, taking into account many body effects, band nonparabolicity and flat band boundary conditions. Monolithic integration of the modulator and detector also turns out to be a simple and accurate method of studying the optical properties of quantum wells under bias. This technique led to the first observation of the exchange-interaction's contribution to the charge transfer between coupled quantum wells.</p