The Research Of THz-wave Difference Frequency Generation And Optical Properties Based On Asymmetric Quantum Well

Recently, THz-wave (λ=30-300μm,1-10THz), for its great interest andapplication from both a fundamental and an applied point of view，has been developedvery rapidly. However, the lack of efficient source is the key hindrance in thedevelopment of THz applications. Asymmetric multiple quantum wells are suited tothe design of THz devices because their energies and value of these transitions can betailored by changing the geometry of the well in the THz field. Besides, large χ(2)inquantum well structures also can be achieved by tailoring so that all interacting fieldsare in resonance with intersubband transitions.The main research contents and innovation points can be summarized as follows:1. An asymmetric quantum well is designed to emit terahertz wave by usingdifference frequency generation with the structure of GaAs/Al0.2Ga0.8As/Al0.5Ga0.5As.We find that the second-order nonlinear susceptibility χ(2)and Optical rectificationcoefficient vary with the two pump wavelengths, and they can reach the peak value of2.5019μm/V and13.208μm/V respectively, almost104-105times bigger thancorresponding bulk material.2. The numerical results show that the refractive index of one pump wave in theAQW is concerned with not only its own wavelength and light intensity alsoconcerned with the other. The reason comes from the third order items of refractiveindex. The extreme values of refractive index can be acquired on some wavelengthand light intensity. Through the tuning of the two pump wavelengths and lightintensity, the wave vector phase-matching inside the asymmetric quantum well can besuccessfully obtained.3. The characteristics of absorption coefficients are analyzed on parabolicenergy-band conditions in detail. We find that the absorption coefficients of pumpwaves and THz wave vary with the two pump optical intensity, and the αTHzreachesthe maximum of1.553107m-1when the pump wavelengths are given as λp1=9.69μmand λp2=10.64μm respectively. When the light intensity increases to a certain value,the absorption of asymmetric quantum well will be appeared saturated absorptionphenomenon at fixed wavelength.4. The output of THz wave, absorption coefficients and refractive index of asymmetric quantum well have been discussed on non-parabolic condition ofconduction band and different well width. The second-order nonlinear susceptibilityχ(2)reaches the peak value of1.539×10-4m/V on the non-parabolic condition.Compared with parabolic condition, the total absorption coefficient and refractiveindex show a red shift under non-parabolic condition, which is due to the decrease inthe energy difference between the ground and excited states. A red or blue shift ofabsorption coefficients and refractive index can be occurred by changes in well width.5. The single split-ring resonator and asymmetric planar dual-split-ring resonatorare designed and their characters are simulated in the THz region. Gallium arsenide isinserted between the gap of two samples, through modulation of its conductivity, wehave achieved the switching of three different resonance modes and researched theinfluence of conductivity on transmission based on current and electric fielddistributions.