Energy Characteristics of Non-Resonator Generation in Nanodispersed Active Media Irradiated with Femtosecond Laser Pulse

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ENERGY CHARACTERISTICS OF NON-RESONATOR GENERATION IN NANODISPERSED ACTIVE MEDIA IRRADIATED WITH FEMTOSECOND LASER PULSES P. A. Babushkin,1,3 V. A. Donchenko,2 Al. A. Zemlyanov,1,3 V. K. Oshlakov,1 R. V. Ryambov,3 and A. V. Trifonova3

UDC 537.523

Results of experimental measurements of the energy characteristics of stimulated emission in coumarin-30 and rhodamine 6G dyes with nanoparticles under irradiation with femtosecond laser pulses are presented. It is shown that the stimulated emission thresholds of the active medium with one-photon absorption containing nanoparticles are by a factor of 1.4 lower than of the pure active medium. Keywords: nanoparticles, femtosecond pulse, stimulated emission thresholds.

INTRODUCTION Laser radiation generation by randomly inhomogeneous active media was theoretically substantiated by V. S. Letokhov in 1967 [1]. In his work, he proposed a possible mechanism of forming the optical feedback due to strong scattering in an amplifying medium. This effect was experimentally demonstrated by V. M. Markushev [2]. The experiment was carried out using dielectric powder samples activated by neodymium ions. Later on, laser generation was excited and investigated in various randomly inhomogeneous active media. At present, the effect of laser radiation generation in such media was called random lasing by a random laser [3]. For a classical laser, lasing is determined, as a rule, by two factors – the optical amplification due to the population density inversion in the excited active substance and the feedback formed by a laser resonator. In this situation, the light scattering phenomenon plays a negative role and is always reduced to a minimum. However, in the stochastic laser, the chaotic strongly scattering medium creates conditions for the lasing effect. H. Cao in [4] defined the stochastic laser as a non-resonator (without external mirrors) laser in which the mechanism of positive feedback is realized due to light scattering in the active medium with optical inhomogeneities. According to the method of obtaining stimulated emission, the stochastic lasers can be subdivided into two classes [3]. The first class contains the stochastic lasers based on solutions of laser dyes with incorporated nanoparticles of various substances. In these structures, the effect of multiple scattering on small inhomogeneities increases the time the photons emitting stimulated radiation dwell inside the active medium. The stochastic lasers the principle of lasing for which is based on stimulated emission in the so-called microresonators whose mirrors are well reflecting microcrystal facets belong to the second class. The closed resonators are randomly formed under the action of a pumping pulse. The composite laser active media draw attention of researchers because the lasing thresholds of these active media are considerably lower than of the active media without nanoparticles [5].

1

V. E. Zuev Institute of Atmospheric Optics of the Siberian Branch of the Russian Academy of Sciences, Tomsk, Russia, e-mail: bpa@