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Indian journal of physics A and Proceedings of the Indian Association for the Cultivation of science A

TRANSPORT AND WIGNER DELAY TIME DISTRIBUTION ACROSS A RANDOM ACTIVE MEDIUM

S. K. Joshi, Abhijit Kar Gupta, A. M. Jayannavar

Published Jan 1, 1998• 0 citations
OpenAlex

Abstract

We siudy the wave propagation through a single-channel (single-mode) coherently amplifying disoulered medium A new crossovei length scale is introduced in the icgime ol stiong disoidei and weak amplification We show that in an active medium reflectance anscs due to .synergeticeffect of localization and coherent amplification Oui study reveals that ihe tail ot the Wigner delay lime distribution from a disordered passive medium exhibits a universality in the sense that H is independent ol the nature ol disorder

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We siudy the wave propagation through a single-channel (single-mode) coherently amplifying disoulered medium A new crossovei length scale is introduced in the icgime ol stiong disoidei and weak amplification We show that in an active medium reflectance anscs due to .synergeticeffect of localization and coherent amplification Oui study reveals that ihe tail ot the Wigner delay lime distribution from a disordered passive medium exhibits a universality in the sense that H is independent ol the nature ol disorder
We theoretically study reflection and transmission of light in a one-dimensional disordered phase-conjugating medium. Using an invariant imbedding approach a Fokker-Planck equation for the distribution of the probe light reflectance and expressions for the average probabilities of reflection and transmission are derived. A new crossover length scale for localization of light is found, which depends on the competition between phase conjugation and disorder. For weak disorder, our analytical results are in good agreement with numerical simulations.
We study local and nonlocal correlations of light transmitted through active random media. The conventional approach results in divergence of ensemble-averaged correlation functions due to the existence of lasing realizations. We introduce a conditional average for correlation functions by omitting the divergent realizations. Our numerical simulation reveals that amplification does not affect local spatial correlation. The nonlocal intensity correlations are strongly magnified due to selective enhancement of the contributions from long propagation paths. We also show that by increasing gain, the average mode linewidth can be made comparable to the average mode spacing. This implies that light transport through a diffusive random system with gain may exhibit some similarities with that through a localized passive system.
A coherently amplifying medium containing dense random weak scatterers can exhibit mirror-less lasing beyond a threshold of optical pumping even when the active medium has linear dimensions much smaller than the estimated transport mean free path. The threshold pump power decreases with decreasing mean free path. This mirror-less lasing can be understood in terms of the sub-mean free path scatterings which are normally statistically rare, but are now made effective by the coherent amplification that more than offsets their otherwise low probability of occurrence.
In the last decade Anderson Localization of Light and Random Lasing has attracted a variety of interest in the community of condensed matter theory. The fact that a system so inartificial as a thin layer consisting of dust particles, which have amplifying properties, can produce coherent laser emission, is of a simple elegance, which promises new insights in fundamental physics. There is no need of an external feedback mechanism, like in the system of a conventional laser. Whereas several attempts try to enter the subject numerically by considering cavity approximations, this thesis is concerned with building a microscopically self-consistent theory of random lasing. Designing this method, we had to study light localization effects in random media including absorption and gain. Incorporating interference effects by means of Cooperon contributions we derived Anderson localized states for passive media. Mapping this on a system which consists of laser active Mie-scatterers in a passive medium, we found that by incorporation of the Cooperon, we loose the Anderson localization, but the system is weakly localized, which is sufficient for population inversion and stimulated emission.
The model based on the method of finite-difference time-domain is applied to calculate the general feature of localized modes in active random media. The evolution of the localized modes from passive random media to active ones is investigated. By varying the gain lineshape for an active random medium, we have observed a selective excitation of localized modes, including both single-mode and multi-mode excitations. When the eigenfrequencies for some localized modes are near the centre of the gain spectrum, the optical gain offers coherent amplification to them so that they can obtain more amplification and have stronger field amplitude than other modes. The pumping level dependence of the field amplitude of the localized modes exhibits a threshold behaviour.