Theoretical Studies Of Optical Properties And Applications For Asymmetric Coupled Quantum Well Structures

Recently, the fantastical optical characteristics of asymmetric coupled quantum well (ACQW)structures have intrigued scientists and material experts owe to the applicable aspiring in telecommunication systems, such as submillimeter wave emitter, optical modulator, optical switch, wavelength converter and so forth. In this paper, the linear and nonlinear optical properties of ACQW structure are analyzed for the applications. The main contents are presented as follows.(1) The behaviors of submillimeter wave radiation are theoretically studied. Inserting the decay term of carrier-carrier scattering and the carrier-LO (longitudinal optical) scattering in the density matrix equation, the results in accorting with the earlier experiments are obtained. Also, the generation and the damp processes of submillimeter wave are described, respectively; the submillimeter wave radiation due to the electronic wavepacket oscillation prepared by an ultrashort laser pulse can be viewed as a classical electromagnetism damping oscillator, whose lifetime depends strongly on carrier-LO scatting time and transfer time from one well to the other, and the intensity in frequency domain obeys the Lorentzian distribution. From an applicable point of view, the ACQW structure may be utilized as whether a submillimeter-wave-emitter or a receiver. In addition, the characteristics of the near-infrared emission are analyzed for this structure.(2) A double-coupled four-level model is proposed based on the electronic and the light-hole (lh) tunneling. Analyzed luminescence of the ACQW structure, there are two stronger peaks and two weaker ones, where a stronger peak relative to the central frequency of this structure is blue shift and the other is red shift. The quantum-confined Stark effect is stronger than that in a single quantum well. In particular, a photonic energy up-conversion feature of this structure is revealed and the corresponding applications are point out.(3) In terms of electronic tunneling and hh tunneling in the earler experiments a delocalized three-level configuration is put forward. Here the delocalization means that all of electronic levels and hh levels at resonance become delocalized, in which the electron in conduction-band and the hh in valence-band localize no longer in wide well. However, the large effective mass of hh gives only a much smaller energy splitting than the electronic so that the two unresolved levels in valence-band are viewed as one level. Utilized this configuration, the results in accord with in experiments are obtained. Compared with that in single quantum well, the relation between absorption and dispersion is remarkably changed due to the electronic wavepacket oscillation, resulting in that the group velocity of laser pulse is delayed, i.e., the group velocity is much less than that in vacuum, the propagating direction is whether along the straightness or inverse. Furthermore, a double modulator, an optical time delayer and a demultiplexier in telecommunation system, respectively, are designed.(4) The third-order nonlinear susceptibility associated with the Kerr effect and the saturation absorption is obtained from a perturbation theory. The radio of third-order susceptibility to linear absorptive coefficient, i.e., the efficiency of optical absorption transformed to the nonlinear effect, is studied. The results show that the electronic wavepacket oscillation leads to enhancement of the absorptive optical nonlinearity. By evaluation, the threshold value in the ACQW structures is less one thousand times than that in the single quantum wells. The nonlinearity leads to the absorptive profile suppressing and spectrum hole burning for a single quantum well and gives rise to that the lower energy peak is enhanced and the higher energy peak is suppressed for ACQW structures, as a result of LO-assisted phonon. Also, a giant Kerr coefficient is demonstrated. Based on the optical preoerties of ACQW structures, two kinds of all-optical switch schemes are put forward.(5) The third-order susceptibility of non-degenerate four-wave mixing is theoretically studied at bias. A frequency up-conversion peak in excess of 5 terahertz may be achieved, as a result of wavepacket oscillation; the third-order susceptibility, which is responsible for the wavelength–conversion efficiency, keeps approximately a constant in a small bias region. Furthermore, a possible application of the ACQW structure in the future telecommunication system, as an all-optical wavelength converter, may be theoretically expected. Analyzed by the dual-side Feymann diagram, the large Bragg reflection signal in four-wave mixing subjects to an assistant of LO phonon.