Abstract
We present femtosecond pump-probe measurements of the nondegenerate (1960$\,$nm excitation and 1176--1326$\,$nm probe) two-photon absorption spectra of 8 $\,$nm GaAs/12$\,$nm Al$_{0.32}$Ga$_{0.68}$As quantum well waveguides. Experiments were performed with light pulses co-polarized normal and tangential to the quantum well plane. The results are compared to perturbative calculations of transition rates between states determined by the $\mathbf{k}\cdot\mathbf{p}$ method with an 8 or 14 band basis. We find excellent agreement between theory and experiment for normal polarization, then use the model to support predictions of orders-of-magnitude enhancement of nondegenerate two-photon absorption as one constituent photon energy nears an intersubband resonance.
Highlights
Nondegenerate two-photon absorption (ND-2PA) is a process whereby absorption of an optical field is induced by a second, high irradiance field at a different wavelength
III, and coupled nonlinear Schrödinger equations were used to fit the curves to ND-2PA coefficients
The TM-TM theory matches the data without any nonphysical scaling parameter; this excellent agreement is likely due to the dominance of allowed transitions
Summary
Nondegenerate two-photon absorption (ND-2PA) is a process whereby absorption of an optical field is induced by a second, high irradiance field at a different wavelength. Pasquarello and Quattropani [17] relaxed some of Shimizu’s approximations and extended the analysis to nondegenerate photon pairs, predicting large enhancements as. We present pump-probe measurements of ND2PA coefficients in GaAs quantum wells. The TM-TM model shows that predictions of intersubband resonance enhancements of ND2PA apply to finite wells, suggesting the possibility of extremely sensitive gated detection of subband-gap pulses [1]. Appendix B contains nonlinear wave propagation analysis, as well as the techniques used to match the calculation results to experimental curves. The rest of the Appendixes contain derivations and background for important equations: Appendix C gives a detailed derivation of ND-2PA coefficients in semiconductor quantum wells. An analytical equation for nonlinear propagation is derived in Appendix G
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