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Physical review. B, Condensed matter

Phonon-mediated excitonic optical bistability in polymers

X. S. Li, D. L. Lin, Thomas F. George, X. Sun

Published Aug 15, 1990• 11 citations
Publisher

Abstract

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.

DOI
10.1103/physrevb.42.2977

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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.
Effects of external optical feedback on the behavior of input-cavity-field relations are investigated for an electron–phonon system inside an optical cavity exposed to an incident laser field. The external feedback by conventional mirrors and by phase-conjugate mirrors is considered separately. Steady-state results are obtained numerically for a typical phonon–exciton material of polydiacetylene. In the case of conventional external feedback, the threshold, the contrast, and the switching behavior of optical bistability can be changed by adjustment of the feedback parameters without causing any instability, whereas in the case of phase-conjugate feedback the cavity field intensity shows five states corresponding to a single input field intensity as long as the feedback is strong enough. However, all these states are unstable because of the amplified phase-conjugate reflection, which limits the application of phase-conjugate mirrors in optical multistabilities.
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.
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.
An exciton-phonon mechanism for producing cavityless optical bistability is suggested. We consider the influence of the oscillating subsystem of a crystal on the absorption process of a laser wave in the region of exciton frequencies. It is shown that exciton-phonon interaction leads to nonlinear dependence of the absorption coefficient on the density of the exciton gas. The increase of absorption with increasing exciton density, however, is possible only in the frequency region on the long-wavelength tail of the exiton band. As a result, the exciton density of the crystal becomes a hysteresis function of external parameters (intensity of the incident wave, temperature, etc.). We calculate the absorption coefficient of excitons and define the frequency and temperature regions where optical bistability may exist. We analyze different relaxation mechanisms of excitons and explain the experimental results of observing optical bistability in different semiconductor crystals.