Transport And Optical Properties Of Semiconductor Systems Under Intense Terahertz Fields

The receatly development of the free-electron laser, which provides continuously tunablesource of linearly polarized terahertz (THz) electromagnetic radiation of high intensity, hasmade it possible to irradiate a semiconductor with an intense far-infrared field under variousconditions. Nonlinear dynamics of the electron gas driven by THz radiation fields in semi-conductors has become a central focus of many experimental and theoretical studies in theliterature. To deal with the transport and optical properties of the intense THz-driven semi-conductor systems the perturbative treatment of electron-photon interaction will be no longervalid. Recently, several nonperturbative approaches have been developed. The balance-equationapproach as one of them provides convenieat tools to investigate the nonlinear transport andoptical behavior of semiconductor systems due to large amplitude of the field and resulting formultiphoton emission and absorption channels. In this thesis we use it to investigate the trans-port and optical properties of three-dimensional (3D) and quasi-two-dimensional (quasi-2D)semiconductor systems under the influence of THz fields.In THz driven 3D electron system, we find, when the dc bias field is weak, the electrondc mobility peaks around a certain value of the amplitude of the terahertz field at low latticetemperature. At relatively high lattice temperature or in the case of strong dc field strength thedc mobility is suppressed with increasing the amplitude of the terahertz fields. With ascendingthe frequecy of THz fields or lattice temperature or dc biae field strength, the effect of theTHz irradiation decreases. The electro heating effect is observed. The electron temperatureincreases with the THz field intensity. In GaAs/AlGaAs heterojunction the dc mobility descendswhen the THz field ascends.'We also studied the magnetotransport properties of typical semiconductor systems, such as3D polar and nonpolar semiconductor systems, GaAs/AlGaAs heterojunction, infiuenced by anintense terahertz radiation at reltively high temperature. We find that the magnetoresistivityas a function of the magntic fields exhibits, in addition to the conventional magnetophononresonant peaks (zero-photon peaks), many new peaks which correspond to one-, two-, and three-photon emission and absorption processes. With an increase of the strength of the terahertzfield zero-photon peds descend, while the multiphoton (nonzero) peaks ascend.The noIilinar free-carrier absorption coefficient of an intense terahertz electromagneticwave propagating in a bulk semiconductor and the perceatage when an iatense radiation passedthrough a quasi2D sheet are calculated. We find that at frequency 1THz, the absorptioncoefficient in a bulk GaAs system increases with increasing aznplitude of the radiation fieldfrom zero and rW a maxhnUtn at around 8 kV/cm before decreasing quicldy with furtherincrease of the field strength. The absorption percentage of a quasi2D system exhibits an evenstronger nonlinearity than that of a 3D bulk. It is shown that the high order multiphotonprocesses pl8y a maor role in d6teredng the absorption of an intense THz field.Using the first harmonic approach to dealing with the ceatermass velocity we extend thebalance opatho aPproach to study the strength dependence cyclotron resonance in quasi-2Dsendconductor sy8tem. We find tliat with ascending the THz radiation intensity the resonancepeak value of the absOrption perc6ntage emerge a peak. The electron temperature exhibitscyclotron resonance and increase8 with the intensity of the THz fields.