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physica status solidi (b)

On the Multispike Structure of a Transmitted Pulse in Monocrystalline Films with Absorption Bistability

Lev T. Perelman, Edward I. Vitkin

Published Nov 1, 1991• 0 citations
Publisher

Abstract

Abstract The dynamics of motion of boundaries between strongly and weakly absorbing regions in monocrystalline films with increasing absorption bistability is considered. It is shown that the multispike structure of a transmitted pulse is attributed to the simultaneous influence of the film boundary effects and bleaching of the sample at large exciton concentrations.

DOI
10.1002/pssb.2221680128

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Abstract The dynamics of motion of boundaries between strongly and weakly absorbing regions in monocrystalline films with increasing absorption bistability is considered. It is shown that the multispike structure of a transmitted pulse is attributed to the simultaneous influence of the film boundary effects and bleaching of the sample at large exciton concentrations.
We compute the peak velocity of a broad Gaussian pulse propagating in an absorbing thin film modeled with a one-oscillator dielectric function. The exact numerical calculation is done for a GaP:N film in the frequency region of the bound A exciton. For absorption lengths greater than the film thickness, the pulse velocity essentially coincides with the group velocity, and with the experimental results of S. Chu and S. Wong [Phys. Rev. Lett. 48, 738 (1982)]. However, outside the region of strong absorption the pulse velocity depends sensitively upon the thickness of the slab. Our results give a better account of the Chu and Wong experiment than the calculation based on the group velocity. For film thickness with smaller than one absorption length the pulse velocity disagrees with the group velocity even in the strong absorption region, as a consequence of Fabry-P\'erot reflections of the pulse inside the film.
The peculiarities of nonlinear transmission of ultrashort pulses of resonance laser radiation by a semiconductor thin film in the excitonic spectral region have been studied, taking into account the exciton-phonon interaction and concentration gain of the dipole moment of the excitonic transition. It has been shown that under exact resonance conditions the film transmits only the leading edge of the incident rectangular pulse and completely reflects the remaining part. For a nonzero off-resonance, a residual transmission occurs. A series of possibilities for transforming Gaussian pulses is predicted. The state equations for steady-state bistable transmission (reflection) have been derived, and the stability of the solutions has been studied. A theorem of areas, which predicts area restriction of the transmitted pulses, has been stated; namely, the transmitted pulse area cannot exceed π/2.
Abstract We have studied the nonstationary transmission of two ultrashort laser pulses incident on a thin film. The radiation frequency of one of these pulses is in resonance with the two-photon transition from the ground state of the crystal to the biexciton state, while the other pulse coherently mixes the exciton and biexciton states, leading to strong renormalization of the energy spectrum of the crystal. We have obtained a system of nonlinear equations describing the time evolution of the exciton and biexciton amplitudes and the fields of three pulses transmitted through the film. We have analyzed the effect of amplitudes and widths of incident pulsed and of the time delay between them on the peculiarities of film transmission of these pulses. We have predicted the effect of a substantial time delay in the generation of the pulse transmitted through the film relative to the incident pulse, which is in resonance with the exciton–biexciton transition frequency. The possibility of generation of a precursor, viz., a pulse transmitted through the film sooner than the peak of the incident pulse reaches the film, as well as possibility of generation of a reflected pulse in the absence of the incident pulse, has been proven.
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.