Abstract

Recently, there have been extensive studies of intersubband transitions. The large oscillator strengths of transitions between subbands of quantum wells¹ have led to investigations of their application to lasers, detectors and nonlinear optical devices. In this paper, we discuss the extremely large second order optical susceptibility [χ⁽²⁾] predicted and observed² in structures with inversion symmetry broken through external electric fields or compositional asymmetry. Second order susceptibility of the quantum well system is obtained by solving effective mass Hamiltonian in the conduction band. The large oscillator strengths and relatively narrow linewidths lead one to expect large . Experimental measurements of χ(²) have been obtained for second harmonic generation over the tuning range of CO₂ laser in both electric field biased and compositionally asymmetric modulation doped structures, resulting in 28 nm/V and 100 nm/V near resonance, respectively. These are extremely large compared to χ⁽²⁾of bulk GaAs, for example, which is 0.38 nm/V at similar wavelengths. In actual frequency conversion devices, the strong absorption at resonance adversely affects efficiency, which ultimately depends on the ratio of χ⁽²⁾ to absorption. We discuss the design of quantum well devices for large conversion efficiency.

© 1990 Optical Society of America