Dynamics And Spectral Properties In Quantum Well Under THz Waves And Optical Fields

When external electric and/or magnetic fields are applied to semiconductorsystems, the optical and electrical properties of the semiconductor systems willbe modified significantly and many interesting physical efects and phenomenawill show up. Considerable work has been devoted to investigating the efects ofapplied low frequency electric fields, e.g., dc electric field, on the optical and elec-trical properties of semiconductors, however, the cases for high frequency electricfields, e.g., terahertz field, attract attention until comparatively recent times. Inthis thesis, the intersubband polarization dynamics and spectral properties of anasymmetric semiconductor quantum well under a terahertz field and an opticalfield have been investigated theoretically. We show that the application of aterahertz field can give rise to many intriguing phenomena. The main work andresults are listed as follows:(1)The master equations of an asymmetric semiconductor quantum well withthree subbands under a terahertz field and an infrared field are established byusing the density matrix theory. With the help of the quantum regression theo-rem, we calculate the probe absorption spectra of the asymmetric quantum wellin the steady state limit. The efects of the Rabi frequencies of the two pumpfields on the absorption spectra are analyzed in detail. The quantum coherentefects observed in the absorption spectra are explained clearly by the dress-statepicture for the interaction system of electrons and external fields.(2)Based on the density matrix theory, we establish the equations of motionfor electron distribution and intersubband polarization in an asymmetric semi-conductor quantum well driven by external fields, i.e., the intersubband Semicon-ductor Bloch Equations. By employing the obtained intersubband SemiconductorBloch Equations, we calculate the optical absorption spectra of the semiconductorquantum well driven by a growth-direction-polarized and an in-plane-polarizedterahertz field, respectively. The efects of various characteristic parameters ofthe terahertz field on the absorption spectra are analyzed. The results show that the terahertz field can strongly change the optical properties of the con-cerned semiconductor system. Many fascinating phenomena, e.g., formation ofthe terahertz-sidebands for infrared absorption, ac Stark efect, terahertz dy-namic Franz-Keldysh efect, Autler-Townes splitting, and the steep switch fromabsorption to gain are explored. These phenomena not only have abundantphysical meaning, but also promise potential applications in the development ofterahertz frequency optic-electric devices.(3) We investigate the terahertz intersubband gain due to the particle inver-sion between the first and third subband arisen by an infrared pump field in athree-subband asymmetric quantum well. By employing the intersubband Semi-conductor Bloch Equations, we calculate the terahertz gain at diferent pumpintensities. The results predict a saturation phenomenon of terahertz gain whenthe pump intensity is enough large, which must be considered in designing opti-cally pumped terahertz emission devices based on intersubband transitions.