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Journal of Nanchang University

Light Localization in Random Laser with Coherent Feedback and Explain on the Sushil Mujumdar's Experiment

Zhong Feng

Published Jan 1, 2007• 0 citations
OpenAlex

Abstract

The paper puts forward that there are two kinds of light localizations at same time in random laser with coherent feedback: the one is that the recurrent light scattering loop restricts photons in the loop,the another is that the scattering particles restrict photons in the entire gain medium,and on the base,it expatiates the work principle of the random laser with coherent feedback.For the first localization,it puts forward that it is easier to lasing in the loop with less particles,for the second localization,it gets the average life of the laser photon when gain region is cube.On the above base,it explains the new phenomenon which was reported by Sushil Mujumdar et al.

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The paper puts forward that there are two kinds of light localizations at same time in random laser with coherent feedback: the one is that the recurrent light scattering loop restricts photons in the loop,the another is that the scattering particles restrict photons in the entire gain medium,and on the base,it expatiates the work principle of the random laser with coherent feedback.For the first localization,it puts forward that it is easier to lasing in the loop with less particles,for the second localization,it gets the average life of the laser photon when gain region is cube.On the above base,it explains the new phenomenon which was reported by Sushil Mujumdar et al.
In any quantum or wave system dissipation leads to decoherence. Therefore, it was surprising in first instance when experiments on strongly lossy random lasers showed unambiguously by measurements of the photon statistics and of the lasing mode volume that coherent feedback is possible in such systems3-5,8. In coherent-feedback lasers the photons form a far-from-equilibrium condensate in the sense that a single quantum state is occupied by a macroscopic number of photons15. We demonstrate that the lossy dynamics of random lasers alone imply a three dimensional finite lasing mode extent, thus resolving the puzzle about coherent feedback without resonator. Our theory of random lasing including nonlinear gain and gain saturation predicts a characteristic dependence of this lasing mode volume on the pump intensity, which can be tested experimentally.
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.
We derive the photon-number probability distribution and the resulting degree of second-order optical coherence for light emission from a uniformly distributed active species within a multiple-light-scattering medium. This is obtained from a master equation describing the probability distribution for photons in the vicinity of position r, traveling with a wave vector k, related, in turn, to a coarse-grained average of the optical Wigner coherence function. Using a simple model for isotropic, spatially uncorrelated scatterers, this reduces to a generalization of the master equation of a conventional laser in which the medium behaves like a random collection of low-quality factor cavities that are coupled by photon diffusion between a given cavity and its neighbors. Laserlike coherence, on average, is obtained in the random laser above a specific pumping threshold. Photon-number statistics above and below the lasing threshold are computed by first assuming that the atomic response to the local electromagnetic fields is nearly instantaneous. Corrections to this simple model, arising from nonadiabatic atomic dynamics, are then estimated. The dependence of the photon statistics on scatterer density, gain concentration, and position within a sample reveal that, on average, increase of the scattering strength (decrease of the photon transport mean free path) in the medium leads to a sharper peak in the local photon-number distribution, characteristic of increased local coherence in the optical field. We also evaluate the coherence of the output field at points outside the random-laser medium. This is a weighted average of radiation emitted at different positions in the sample, exhibiting varying degrees of coherence due to variations in the local pumping intensity.
We derive the photon number probability distribution of light emitted from a random multiple-light-scattering medium with gain, using a generalized master equation. Our model treats the random laser medium as a collection of low quality-factor cavities, coupled by random photon diffusion. We demonstrate that, with stronger scattering, the pumping threshold for the transition from chaotic to isotropic coherent light emission decreases and the local second order coherence, above threshold, increases.
Cold atoms have emerged as interesting quantum system to study coherent transport properties of light. First experiments have established that dilute samples with large optical thickness allow studying weak localization of light. The present goal of this research is to study coherent transport of photons in dense samples. The quest for Anderson localization of light is among the most interesting questions currently studied. One important aspect in the quest of Anderson localization of light with cold atoms is the relation of Anderson localization and Dicke super- or subradiance. With large and dense samples of cold atoms the interplay between these different phenomena can be studied. During this research one also has to study anomalous diffusion (Lévy flights of photons) and new situations such as a random lasing can arise if one combines gain with multiple scattering. Cold atoms thus continue to allow for new exciting physics to be investigated.
Random lasing action and weak localization of light were observed in multiple visible wavelengths with highly transparent, Nd+3 doped phosphate glass gain media, with strong energy redistribution between different frequencies under different pumping powers. A tentative model based on photoinduced scattering was proposed and verified with a He-Ne laser beam in probing the lasing zone. The remarkable coherency and intensity changes of the probing laser beam confirmed the local, random scattering behind the photoinduced random lasing and the localization of light.