Luminescence and radiation effects in lithium triborate crystals

Abstract

LUMINESCENCE AND RADIATION EFFECTS IN LITHIUM TRIBORATE CRYSTALS V. A. Maslov, I. N. Ogorodnikov, L. A. Ol'khovaya, I. N. Antsygin, V. Yu. Ivanov, A. V. Kruzhalov, and A. Yu. Kuznetsov UDC 535.37 In recent years the advent of new nonlinear optical media based on monocrystals of wide-band dielectrics has promoted the development of laser systems with a high radiation density. Their successful use depends, in particular, on resolving the problem of radiation-optical stability under extreme radiation conditions. It is a known fact that the formation of defects due to laser radiation worsens the nonlinear properties of crystals. These processes are rather complicated and are caused by the great number of primary events of formation and evolution of elementary point defects. Even for the best known and most widely used optical media no clear knowledge exists so far on this issue. On the other hand, the present-day level of development of laser technology dictates a search for new highly effective and radiation-resistant nonlinear crystalline media. Thus in late 1980s for the first time bulk crystals of lithium triborate LiB30 5 (LBO) were been synthesized [1-3], which surpass known crystals in threshold optical damage (a 4.1-fold increase in KTP, a 2.25-fold increase in KDP) at comparable nonlinear coefficients. This has placed LBO crystals among highly effective converters of laser radiation, in particular, of the radiation of A12Oa:Ti- lasers [4, 5 ]. By now the main physicochemical [1-3] and nonlinear [4, 5] properties of LBO crystals are studied; however data on radiation-induced defects and their properties are still absent. The appearance of bulk LBO monocrystals and ever-increasing interest in them have necessitated research studies along this line. The present work is devoted to investigation of the radiation defects, recombination processes, and spectral- luminescent properties of LBO crystals. LBO monocrystals of optical quality were grown by the modified solution-melt method with seed pulling in platinum crucibles in one- and two-zone furnaces. To increase the viscosity of the mett, Li2MoO 4 was added to the system Li20-B20 3 [6, 7 ]. Crystals were grown in the temperature range 384-810 ~ with seeding orientations {001} and {110} and the rotational speed of the seeding did not exceed 10 rpm. The mean size of the grown crystals amounted to 50 “ 40 “ 25 mm. LBO crystals were identified by x-ray analysis. The main crystallographic parameters of LBO were consistent with those in [8 ]. The luminescence of LBO crystals and its kinetics were investigated using an automated setup intended for analysis of the radiative-optical properties of solids (an MDR-2 monochromator, an FEU-106 photomultiplier tube operating in a regime of counting photons) [9 ] upon excitation by x-radiation (a URS-55 unit, Cu-anticathode, U = 40 kV, I = 10 mA) or by an electron beam (an MIRA-2D device, I = 1 A.cm -2, Zpuls e -- 10 nsec). Thermoactivation studies within the temperature range 77-600 K of practical importance were conducted using an automated unit [10 ] with x-radiation excitation of crystals. EPR spectra were measured in a modified X-band radiospectrometer RE- 1302 at two fixed temperatures (77 and 290 K). As an excitation source, use was made of a URS-55 x-ray unit and a RADAN-600 pulsed electron accelerator. The spectral-luminescent measurements showed that LBO crystals at 300 K, unlike other nonlinear crystals (e.g., KTP, KDP), show marked shortwave luminescence upon excitation by corpuscular and x-radiation. For example, the spectrum of x-ray luminescence is represented by a wide band with a peak at 4.2 eV and a weak shoulder at 3.4 eV (Fig. 1). The main band is elementary and well approximated by a Gaussian function (the halfheight line Ural Polytechnic Institute, Ekaterinburg Institute of General Physics, Russian Academy of Sciences, Moscow. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 59, Nos. 3-4, pp. 293-298, September-October, 1993. Original article submitted December 25, 1992.