Second-Harmonic Generation in Asymmetric AlGaAs Quantum Wells

Abstract

Optical transitions between conduction subbands of GaAs-AIGaAs quantum wells are of considerable interest for mid-infrared (MIR, applications due to their very large oscillator strengths 1. Indeed, the dipole moments associated with intersubband transitions only involve matrix elements between envelope wave functions which translate into effective dipole lengths scaling like the quantum well thickness 2. For comparison, the dipole length associated with an interband transition would be much shorter, of the order of the lattice constant because the matrix elements involve the periodic part of the Bloch wavefunctions. This large value of the intersubband dipole moments offers the basis for efficient linear and non-linear optical interaction schemes as long as the ground conduction subband of the quantum well is populated. This is usually achieved by means of epitaxial n-doping of the quantum well heterostructure. Photodetectors 3 and optical Stark-shifted modulators 4 based on a linear intersubband absorption process in n-doped quantum wells have been investigated first. The work on photodetection has considerably evolved since then with the demonstration of high quantum efficiencies under polarized irradiation for photodetectors operating in the photoconductive, 5-7 photovoltaic 8,9 or photon-drag 10,11 regime. Recently, photoinduced intersubband absorption in undoped GaAs-AIGaAs mUlti-quantum wells (MQW) was reported by several authors 12-14 with special emphasis on intersubband-interband spectroscopy of the quantum wells 14 and on all-optical modulation of MIR radiations 15. Intersubband emissions from GaAs/AIGaAs superlattices using either parallel or perpendicular electronic transport have been demonstrated 16,17.The intersubband resonance enhancement of optical non-linearities have also been investigated. Efficient second-order non-linearities based on intersubband resonance enhancement were first predicted by Gurnick and De Temple in 1983 18. Because in a centro-symmetric system the second-order susceptibilities vanish 19, the authors have considered quantum well heterostructures whose conduction band profile reproduces a Morse well potential. Huge increases in the second-order susceptibilities associated with second-harmonic generation (SHG) and optical rectification were predicted. The first experimental evidence of an intersubband-resonance enhancement of the second-harmonic generation was obtained by Fejer et a1 20 using an applied electric field to break the symmetry of the envelope wavefunctions. Previously, this dc-biased situation had been theoretically studied by Ikonic et al 21. However, larger second-order susceptibilities Intersubband Transitions in Quantum WellsEdited by E. Rosencher et al., Plenum Press, New York, 1992