Research On The Nonlinear Optical Properties And Applications Of Asymmetric Quantum Wells

THz-wave has good perspective, security and high spectral resolution. Over the last decade, THz-wave has been widely used in biology, medicine and communication fields, which has great scientific value and broad application prospects. Now many methods have been employed to obtain the THz radiation including electronics and photonics methods, among which the difference-frequency generation (DFG) of THz-wave based on the nonlinear optical frequency mixing has attracted great interest for its numerous advantages, such as operating at room temperature, widely tunable range, high peak power, low cost and compactness. Crystals commonly used have small second-order nonlinear difference-frequency coefficients, which induce very low conversion efficiency. But asymmetric quantum wells have very large second-order nonlinear coefficients and their magnitude are generally microns per volt. Using asymmetric quantum wells as the difference-frequency generation crystal will greatly improve the poor conversion efficiency. Therefore, the application of asymmetric quantum well in the difference-frequency generation has a great potential.In this paper, linear, second-order and third-order nonlinear effects in a special GaAs/AlGaAs quantum well were investigated. Furthermore, the use of quantum well to generate coherent THz-wave via difference-frequency generation was mainly examined, and the THz-wave modulation devices for practical applications were studied. The main contents and the creative points are as follows:1. Based on the ladder-type asymmetric quantum well (AQW), energy levels, wave functions and optical nonlinear effects of the asymmetric quantum well taking into account the effect of conduction-band nonparabolicity were discussed. The second-order nonlinear coefficient of the asymmetric quantum well taking into account the effect of conduction-band nonparabolicity was calculated and the numerical results were presented for typical asymmetric quantum well. When the quantum well width L1 equals 70 Egypt and L2 equals 300 Egypt, the optical difference-frequency coefficient is up to GaAs/Al0.2Ga0.8As/Al0.5Ga0.5As X_DFG,max²≈2.5019μm/V(higher than GaAs crystal's optical difference-frequency coefficient by about 5 orders of magnitude), and the optical rectification coefficient is up to 13. 208?m /V (higher than GaAs crystal's optical rectification coefficient by about 4 orders of magnitude). 2. The linear and nonlinear intersubband refractive index and absorption of THz-wave in a special GaAs/AlGaAs quantum well were investigated. The numerical results reveal that the THz-wave modulator controlled by pump optical intensity or electric field can not only be a THz-wave switch but also modulate THz-wave intensity and phase conveniently.3. By calculating the relative change of the first-order and third-order nonlinear refractive index coefficient in the quantum well, we find that by adjusting the incident light intensity the phase matching in the collinear case of difference-frequency generation can be achieved. At last, the experimental setup of difference-frequency generation of THz-wave in asymmetric quantum well was designed.