Photoinduced Absorption and Pulsed Recording of Dynamic Holograms in Bismuth Silicate Crystals
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GRAPHY
Photoinduced Absorption and Pulsed Recording of Dynamic Holograms in Bismuth Silicate Crystals I. G. Dadenkova, A. L. Tolstika, *, Yu. I. Miksyukb, and K. A. Saechnikovb aBelarusian bBelarusian
State University, Minsk, 220030 Belarus State Pedagogical University, Minsk, 220030 Belarus *e-mail: [email protected]
Received March 15, 2020; revised March 15, 2020; accepted May 20, 2020
Abstract—The dynamics of photoinduced absorption and holographic-grating recording in photorefractive crystals of bismuth silicate is studied. It is shown that, with the use of nanosecond laser pulses and of the intensity on the order of 1 MW/cm2 or higher, the induced absorption due to population of the short-lived trapping levels, with characteristic relaxation times about several milliseconds or tens of milliseconds, is the case. Recording of dynamic holograms has been realized in these conditions in bismuth silicate crystals. Two mechanisms of holographic-grating recording, with the lifetimes differing by three orders of magnitude, are established. At relatively low intensities, about 1 MW/cm2 or lower, the medium response is determined by a photorefractive mechanism of nonlinearity, with relaxation times of several seconds. At the intensities exceeding 5 MW/cm2, one can observe a fast (ms relaxation times) component that may be associated with population of the short-lived traps. It is shown that the contribution of each mechanism is greatly dependent on the intensity of laser radiation and, for the intensities above 10–15 MW/cm2, the short-lived traps having millisecond lifetimes play the decisive role. Keywords: holography, dynamic holograms, photoinduced absorption, sillenites, bismuth silicate DOI: 10.1134/S0030400X20090052
INTRODUCTION One of the significant advantages of cubic photorefractive crystals from the sillenites family (Bi12SiO20, Bi12TiO20, Bi12GeO20) is the real-time formation of dynamic holograms. This property enables the use of these crystals in adaptive interferometers, in systems of associative memory, optical image amplification, holographic recording, data storage and processing, and the like [1–6]. The physical processes of hologram recording in photorefractive crystals are based on spatial redistribution of the charges in a field of interfering light beams over the numerous centers having different nature and characteristics. By their structure, photorefractive crystals are classified with wide-band dielectrics. Of great importance are impurities and structural defects of the crystal lattice, which lead to the formation of donor and acceptor energy levels in the forbidden band [5, 7]. A distinctive feature is the fact that both the long-lived (seconds, hours) and short-lived (micro- and milliseconds) traps can exist simultaneously [8–13]. The transitions from a level in the forbidden band to the conduction band result in the formation of mobile charge carriers. It should be noted that such transitions are feasible when using radiation with a considerably longer wavelength than in the case of a direct
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