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Journal of Applied Physics

Biexcitonic effect on optical nutation in semiconductor quantum wires

Pranay K. Sen, Manish K. Bafna, Saral Kumar Gupta

Published Nov 5, 2004• 6 citations
Publisher

Abstract

We have proposed a simplified theoretical model to examine the optical nutation in one-dimensional semiconductor quantum wires. We explored the role of excitons, biexcitons, and the effect of confinement in enhancing transient optical processes in such semiconductor nanostructures. We apply the time dependent perturbation technique to the coherent radiation-semiconductor interaction for a three-level system in the near band gap resonant excitation regime. We have incorporated the relaxation and dephasing mechanisms phenomenologically. The induced polarization is calculated theoretically and the role of biexciton density on the polarization is analyzed. Numerical estimations of refractive and absorptive optical nutation have been made for a GaAs∕AlxGa1−xAs quantum wire duly shined by femtosecond pulse Ti: sapphire laser. Our results agree qualitatively with available experimental observations.

DOI
10.1063/1.1794895

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We have proposed a simplified theoretical model to examine the optical nutation in one-dimensional semiconductor quantum wires. We explored the role of excitons, biexcitons, and the effect of confinement in enhancing transient optical processes in such semiconductor nanostructures. We apply the time dependent perturbation technique to the coherent radiation-semiconductor interaction for a three-level system in the near band gap resonant excitation regime. We have incorporated the relaxation and dephasing mechanisms phenomenologically. The induced polarization is calculated theoretically and the role of biexciton density on the polarization is analyzed. Numerical estimations of refractive and absorptive optical nutation have been made for a GaAs∕AlxGa1−xAs quantum wire duly shined by femtosecond pulse Ti: sapphire laser. Our results agree qualitatively with available experimental observations.
Transient optical nonlinearities of semiconductor quantum dots in the strongly confined regime are studied, in which the exciton, biexciton and magnetic field effects are taken into account. The photo induced band-to-band electronic transitions are considered from both the heavy-hole (hh) and light-hole (lh) valence bands to the lowest (1s) exciton state below the fundamental absorption edge. The time dependent perturbation technique for a three-level system, under the near band gap resonant excitation regime, has been employed; with the incorporation of the relaxation and dephasing mechanisms phenomenologically. The induced polarization is calculated from where the real and imaginary parts of the third-order nonlinear optical susceptibility, responsible for the occurrence of nonlinear refraction and nonlinear absorption respectively, have been derived. The temporal and detuning spectra of nonlinear refraction and absorption; and their dependence on density of biexcitons and a moderately strong magnetic field, have been analyzed. It is observed that the nonlinear refraction and nonlinear absorption enhance with increasing magnetic field in small CdS quantum dots.
Using a coherent radiation-semiconductor interaction model based upon the time dependent perturbation technique, the occurrence of optical nutation has been analytically investigated in direct-gap semiconductors such as GaAs, GaSb, InAs, and Hg1-xCdxTe, duly irradiated by near resonant laser beams. The present approach is much more simplified and straightforward than the conventional Maxwell-Bloch approach used so far for the study of coherent optical transient effects in molecular and atomic systems. The dispersion and absorption expressed in terms of the real and imaginary parts of the crystal optical susceptibility\chi_{r}and\chi_{i}exhibit temporally damped ringing behavior of Neumann and Bessel types, respectively. These processes have been identified as dispersive and absorptive optical nutation. The theory is developed for arbitrary laser intensity in presence of damping due to nonoptical processes. To understand the mechanisms of dispersive and absorptive optical nutation, we have restricted only to low-power resonant band-to-band transitions neglecting excitonic and damping effects. While studying analytically the transient behavior of\chi_{r}and\chi_{i}for a given pump field strength, it is noticed that the absorptive component exhibit remarkable qualitative agreement with the experimental observations of optical nutation in13CH3F.
The possibility of occurrence of the coherent optical transient effect known as optical nutation has been analytically established in the semiconductor quantum well (QW) structure, namely GaAs/GaxA11−xAs most extensively used in optical electronics. Ultra-short-pulse low-intensity band-to-band excitation of electrons to the 1s Wannier-Mott exciton state of the crystal has been considered to play an important role in the coherent radiation—QW interaction. Numerical estimations of the complex optical susceptibility and the transmitted intensity under the transient regime reveal ringing behaviour confirming the occurrence of optical nutation in III-V semiconducting QW structures.
We present a theoretical analysis to examine the strain effect on light and heavy hole energy levels and role of exciton and bi-excitons on the absorptive optical mutation in InxGa1-xAs/GaAs quantum dots (QDs) near the band edge. The a nalysis is based upon the strain induced Luttinger Hamiltonian and time dependent perturbation technique of the coherent radiation-semiconductor interaction model for a three-level system under near band gap resonant excitation regime. The relaxation and dephasing mechanisms phenomenological have been incorporated. Numerical estimations of refractive and absorptive optical mutation have been made for a InGaAs/GaAs quantum dot duly shined by femtosecond pulse Ti: sapphire laser. Our results are found to agree qualitatively with available experimental observations.
The paper discusses the analytical investigation of the occurrence of coherent optical transient processes such as dynamic Stark effect and free induction decay in direct-gap semiconductor quantum wires with thickness and width smaller than the bulk Wannier-Mott exciton Bohr radius. The role of a moderately strong magnetic field has been critically evaluated in the occurrence of the transient processes. The analysis is based upon the time dependent perturbation treatment of the coherent radiation-semiconductor interaction model under near band gap resonant excitation regime. The excitation intensity is kept below the Mott transition regime. Numerical estimates made for a GaAs/AlGaAs quantum wire irradiated by femtosecond pulse Ti:Sapphire laser establishes that the Stark shift, Stark broadening as well as free induction decay are all considerably influenced by the magnetic field. Moreover, the wire width also affects Stark splittings.
The coherent response of excitons in semiconductor nanostructures measured in four-wave mixing (FWM) depends strongly on the inhomogeneous broadening of the exciton transition. We investigate GaAs/Al0.3Ga0.7As single quantum wells (SQW) and AlxGa1-xAs mixed crystals. Additional to the usual phase-space filling nonlinearity, excitation-induced dephasing (EID) and biexciton formation (BIF) are important. EID leads to a strong dependence of the signal on the angle between the linear input polarizations. We find that EID persists in inhomogeneous systems, showing that the mutual density-dependent dephasing rate difference between two subsystems within the inhomogeneous distribution is strongly dependent on their energy difference. BIF is strongly affecting the cross-linear polarized FWM response. The signal for positive delay is dominated by the transitions from the one-exciton state X to the two-exciton states. Here, the third-order polarization at X-XX is not at the same resonance as the first-order polarization at 0-X. Consequently, the rephasing of the microscopic third-order polarization to the macroscopic FWM photon echo is blurred by the non-perfect correlation of X and XX energies, leading to a fast and non-exponential signal decay in delay time. For inhomogeneous broadenings larger than the biexciton binding energy, we find an enhanced biexciton binding energy, and a quenching of the oscillator strength of the unbound biexciton.