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Physical review. B, Condensed matter

Transient nonlinear optical phenomena in exciton-phonon systems

Xiao-Lin Li, D. L. Lin, Thomas F. George, Xin Sun

Published Nov 15, 1992• 2 citations
Publisher

Abstract

Propagation of a light pulse in the bulk and hole bleaching in the absorption spectrum from a film of polydiacetylene-toluene sulfonate are investigated in the transient regime. This polymer is treated as an example of an exciton-phonon coupling system with only two phonon modes dominating the coupling. For the propagation, we use the Maxwell wave equation to express the field in terms of the polarization density, which is found microscopically from a set of optical Bloch-like equations in the semiclassical approximation. Phonon-mediated phenomena predicted in this work include the photon echo in a solid and bistable behavior of both the absorption coefficient and refraction index. The absorption spectrum is calculated for a thin film of an exciton-phonon system under a strong pump field and a weak probe field from the optical Bloch-like equations only. Phonon-mediated bleaching of holes is obtained in good agreement with experiments.

DOI
10.1103/physrevb.46.13035

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Propagation of a light pulse in the bulk and hole bleaching in the absorption spectrum from a film of polydiacetylene-toluene sulfonate are investigated in the transient regime. This polymer is treated as an example of an exciton-phonon coupling system with only two phonon modes dominating the coupling. For the propagation, we use the Maxwell wave equation to express the field in terms of the polarization density, which is found microscopically from a set of optical Bloch-like equations in the semiclassical approximation. Phonon-mediated phenomena predicted in this work include the photon echo in a solid and bistable behavior of both the absorption coefficient and refraction index. The absorption spectrum is calculated for a thin film of an exciton-phonon system under a strong pump field and a weak probe field from the optical Bloch-like equations only. Phonon-mediated bleaching of holes is obtained in good agreement with experiments.
For a polymer inside an optical cavity, optical multistability mediated by phonon modes and virtual exciton coupling is investigated. The cavity field, excitons, and phonon modes are treated as damped oscillators, and the equations of motion for the dynamical variables are obtained by applying Dekker's theory for dissipative systems. Steady-state results are obtained numerically for polydiacetylene [bis(p-toluene sulfonate)] with the input field frequency on or off resonance with the cavity field. The variation of the cavity-field intensity follows a reversed hysteresis loop on resonance, while it may follow either a normal or a reversed hysteresis loop off resonance depending on the course of change of the driving-field intensity at the unstable turning point. The variation is inverted when the detuning changes its sign. It is also found that the virtual excitons, phonon modes, and feedback of the cavity are all indispensable to such tristability.
Using laser pulses of duration comparable to semiconductor relaxation and dephasing times, the coherent phenomena common to two-level systems have been demonstrated in II-VI and III-V semiconductors. These light-matter interactions are characterized by electrons and holes in their initially excited states and/or a macroscopic polarization that persists after the excitation. A nonthermal carrier distribution was observed in bulk CdSe, demonstrating the extremely fast energy relaxation possible when carrier-LO-phonon scattering is included. Quantum-confined CdSe microcrystallites were then employed to limit the available decay avenues. The one and two-pair transitions were characterized, and using spectral hole-burning, energy relaxation was shown to be substantially slower than in bulk. Dipole dephasing remained very rapid. Persistent macroscopic polarization was demonstrated in semiconductors through pump and probe experiments conducted on time scales faster than the inverse linewidth of the exciton. In the first set of experiments, it was found that the exciton optical Stark shift deviates from that predicted by a steady-state theory. The shift is smaller than expected and lasts longer than the pump pulse. Both effects are explained by free evolution of the exciton's polarization. Transient exciton bleaching is observed and shown to be a result of adiabatic following. In a second study of coherent polarization, spectral oscillations at negative delay times were studied. Oscillations were measured in a wide variety of semiconductors, temporally preceding several different pump-probe phenomena. By comparison with a semiclassical theory, it was shown that spectral oscillations in the spectral region of the exciton are evidence of persistent exciton polarization. Oscillations in the band are due to four wave mixing of the pump and probe beams.
Surface effects on optical bistability and pump-probe processes for coupled exciton-phonon systems are studied theoretically. When a laser beam is directed on an excitron-phonon coupling system inside an optical cavity, the cavity field intensity is investigated as a function of the driving field intensity for various distances between the sample and a metallic surface representing one of the mirrors of the cavity. It is found that the direction of switch of the optical bistability can be controlled by adjusting the distance from the surface. For a strong laser pump field and a weak probe field directed on this system, the pump-field-induced absorption spectrum is investigated for different distances between this sample and the metal surface. It is found that the position and line shape of the spectrum are significantly changed due to the presence of the surface.