Monitoring the Heat of a Material during the Laser Formation of Defects
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toring the Heat of a Material during the Laser Formation of Defects V. P. Dresvyanskya, *, A. V. Kuznetsova, Sodnom Enkbatb, and E. F. Martynovicha aInstitute
of Laser Physics (Irkutsk Branch), Siberian Branch, Russian Academy of Sciences, Irkutsk, 664033 Russia of Physics and Technology, Mongolian Academy of Sciences, Ulaanbaatar, 210651 Mongolia *e-mail: [email protected]
bInstitute
Received February 14, 2020; revised March 16, 2020; accepted March 27, 2020
Abstract—Results are presented from studying the role of material heating as defects form under conditions of the multiple filamentation of repeated laser pulses in lithium fluoride crystals. A way of monitoring local heating at the sites of light filament transmission is proposed, based on an analysis of the ratio of intensities of absorption bands of simple and aggregate color centers in the spectra of irradiated crystals. DOI: 10.3103/S1062873820070084
INTRODUCTION New quantum systems that absorb and emit optical radiation are created in transparent crystalline media under the action of the femtosecond laser radiation of the near IR region of the spectrum [1–4]. These quantum systems are point defects of the crystalline structure. For example, the defects induced by femtosecond laser pulses in the wide-band gap crystals of lithium fluoride (Eg ~ 14 eV) are the simple and aggregate color centers (CC) characteristic of the radiation coloration [5] of the given type of crystals. The creation of similar luminescent structural defects proceeds at high intensities of laser radiation and is accompanied by a number of highly nonlinear optical phenomena. Among these are the self-focusing and the multiple filamentation of the active laser radiation [6, 7], the generation of a supercontinuum [8], and the formation of electron-hole plasma [9]. The interaction between intense laser radiation and transparent media has been the subject of many modern studies. The mechanism of color center creation under the action of femtosecond laser radiation includes the highly nonlinear generation of electron-hole pairs in the region of filament transmission; their recombination with the creation of anion excitons; the decay of excitons into Frenkel defects according to the Lushchik–Vitol–Hersh–Puli mechanism; and their recharging, migration, and aggregation [10, 11]. The efficiency of laser defect formation depends on the condition of interaction between the laser radiation and the crystal. It was shown in [12] that the efficiency of laser defect formation in cubic crystals of lithium fluoride is a periodic function of the azimuth angle that determines the orientation of the electrical
vector of the exciting radiation with respect to the direction of a cube’s edge. In addition to the geometry of an experiment, the nature of the processes that occur in a crystalline medium is determined by characteristics of the femtosecond radiation and the conditions of excitation (i.e., the focusing circuit and the wavelength, energy, duration, and rate of pulse repetition of the laser radiation).
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