Spectral dependence of absorption photoinduced in a Bi₁₂TiO₂₀crystal by 532-nm laser pulses

Abstract

The spectral dependences of absorption photoinduced in a pure bismuth titanium oxide crystal by 532-nm laser pulses are studied. It is shown that optical absorption in the crystal in the range from 492 to 840 nm increases with increasing exposure. The photoinduced absorption relaxes in the dark for more than 60 hours. A model of photoinduced absorption is proposed which assumes the population of two trap centres with the normal energy distribution law for the concentrations of electrons photoexcited from donors to the conduction band. This model well describes the spectral dependences of photoinduced absorption by using the average ionisation energies of the traps E 1 1X60 eV and E 2 2X57 eV. The model is used to estimate the increase in the photorefractive sensitivity of a bismuth titanium oxide crystal in the near IR region, which was earlier observed after exposing the crystal to visible radiation. It is predicted that the speed of response of dynamic holography devices based on BTO crystals exposed to green light can be increased.

Photoinduced Absorption and Pulsed Recording of Dynamic Holograms in Bismuth Silicate Crystals

“…Taking into account a great number of different impurities and defect centers characteristic for bismuth silicate crystals, in an effort to explain the observed divergences, we divide trapping level 3 into two sublevels 3 а and 3 b with different lifetimes. For the sublevel with a shorter lifetime, 3 а, a solution for equation (5), in the conditions of marked population of long-lived level 3 b, may be given as (12) For the sublevel with a longer lifetime, we have Based on equations ( 12), ( 13), the absorption factor associated with population of the indicated sublevels is of the form (14) where k 3a0 , k 3b0 -initial absorption factors associated with the corresponding sublevels, τ 3a and τ 3b -their lifetimes.…”

“…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 band-to-band transition, offering the possibility of operations in the visible and near IR spectral regions [5,14]. Due to the subsequent diffusion or electron drift in an external electric field, the trapping levels become populated, a field of the space charge is formed, and the refractive index is modulated in consequence of the Pockels effect.…”

Dynamics of Changes in Optical Absorption Induced By Nanosecond Laser Pulses in the Bi12Tio20:Al Crystal

“…The experiments carried out in [14] showed that for a lower intensity equal to ∼ 5 mJ/cm 2 the time of such return of the absorption to the initial level decreased to 70 ms. It will be observed the work [11], wherein was found that the exposure of the Bi 12 TiO 20 crystal by the laser pulses with the wavelength of 532 nm, duration of 30-50 ns and single impulse energy of 0.5 mJ leads to increase in the optical absorption in the spectral range 492-840 nm.…”

“…Photorefractive bismuth titanium oxide crystals are used both in basic elements of dynamic holography devices and in studies of new self-action effects of light beams due to advantageous combination of its characteristics such as low specific optical rotation, relatively high speed, good handling properties and resistance to external temperature and mechanical durability [1][2][3][4][5].The significant photoinduced changes in optical absorption (the photochromic effect) are observed in these crystals [6][7][8][9][10][11][12][13][14]. It should be taken into account when developing real devices and when interpreting the various phenomena of photorefractive nonlinear optics.…”

“…It should be taken into account when developing real devices and when interpreting the various phenomena of photorefractive nonlinear optics. The physical reason of the photochromic effect is usually considered the charge exchanges of defect centers with the different photoionization cross section under the influence of light [8][9][10][11][12][13], features of which depend on the limited timespan of the acting optical radiation [10][11][12][13][14]. The dynamic of changes in optical absorption Δα (t) in bismuth titanium oxide crystals wich related with switch-on and shut-down of continuous radiation with different wavelengths from the visible range [10,12] allowed to refine the energy diagram of the corresponding defective centers [12] and satisfactorily describe the experimental time dependences.…”