Holographic storage of information and peculiarities of light diffraction in birefringent electro-optic crystals

Using birefringent crystals such as LiNbO₃: Fe, reading and writing of holograms at different wavelengths and so nondestructive readout of information may be accomplished.     

Two-wave mixing of phase-modulated beams in GaP under a dc electric field

Two-wave mixing of phase-modulated light beams in crystals of cubic symmetry is analyzed on the basis of the vectorial theory of light diffraction. We derive an analytical expression for phase-to-intensity transformation in crystals of the 43m point group of symmetry, which differs from the previously obtained solution based on the scalar approach. The most effective transformation is achieved when the amplitude of the space-charge-field grating is equal to the quarter-wave field. It is shown that the space-charge-field grating created in GaP semi-insulating crystal at the wavelength of 632 nm is much smaller than can be predicted from the one-level band-transport model.     

Measuring tablet porosity using multispectral imaging system

A multispectral imaging system with computer controlled light source of 16 light emitting diodes is applied for measuring of tablet porosity. The system is based on a subspace vector model of surface reflection. The measured spectral data are compressed on the stage of measurement and used directly for the discrimination of tablets with different porosity. The experimental results demonstrate that the multispectral imaging system is a potential method for tablet porosity measurement.     

Nonlinearity of a response in photorefractive crystals with a square-wave applied field

The space-charge-field formation in a photorefractive crystal illuminated by a one-dimensional light pattern is studied when an alternating square-wave electric field is applied to the crystal. We derive the general nonlinear equation for the time-averaged distribution of the space-charge field and employ the simplified versions of this equation to analyze the space-charge-field formation in the case of a Gaussian beam and interference light pattern. The borders of the applicability of simplified equations and analytic expressions to describe a self-action of light beams and the large modulation effects under two-beam coupling are estimated from the numerical calculations. Received: 18 November 1998 / Revised version: 26 January 1999 / Published online: 12 April 1999     

Amplitude autocorrelation of femtosecond laser pulses using linear photogalvanic effect in sillenite crystals

We demonstrate excitation of the linear photogalvanic current in a Bi₁₂TiO₂₀ crystal by two orthogonally polarized femtosecond laser pulses with detecting the electrical current via charge accumulation on the sample electrodes. Such a setup was used to implement an interferometric autocorrelation technique for characterization of ultrashort light pulses. Integration of the detected current in femtosecond time domain leads to vanishing of a bipolar component of the photogalvanic current which arises due to a pulse chirping. The advantage of the proposed technique is that it produces the electric field correlation function directly without the need for data processing using a compact, robust, and non-expensive detector in the form of a photoconductive cell of a non-centrosymmetric crystal.     

Highly efficient hologram enhancement in Bi12TiO20 fibre-type crystal

Experimental results for two-wave mixing experiments in a single Bi₁₂TiO₂₀ fibre-type crystal at a wavelength of 632.8 nm under transverse a.c. electric field are reported. An exponential gain factor =4.7 cm^-1, which is much higher than the absorption coefficient (0.6 cm^-1), is obtained. It is observed that the signal beam enhancement is in fact signal beam depletion for spatial frequencies higher than F _B, which is determined by fibre geometry. An unusually strong dependence of response time on the recording beams intensities ratio reflects the nonlinearities of the multi-two-wave mixing process.     

Growth and characterization of photorefractive Bi12TiO20 single crystals

Photorefractive Bi₁₂TiO₂₀ single crystals of high optical quality were grown in a resistive heating furnace from high temperature nonstoichiometric (10:1) solutions of Bi₂O₃ and TiO₂ at pulling rates 0.3–0.8 mm/h at rotation 20–30 rpm along the <001> and <011> axis. Powder X-ray analysis, Laue method, and electron-probe microanalysis were used for characterization. BTO crystals have the bcc structure of sillenite type with a₀ = 10.178(8) Å. The chemical composition of the crystals can be written down as Bi_{12.1 ± 0.2}Tio_{0.96 ± 0.09}O_20.1. Natural optical activity ρ of BTO crystals is 6.3 ± 0.2 deg/mm at λ = 0.633 μm and 11.9 ± 0.2 deg/mm at λ = 0.5145 μm, optical absorption coefficient α = 0.42 ± 0.04 cm⁻¹ at λ = 0.633 μm and linear electro-optic coefficient r₄₁ = r₅₂ = r₆₃ = 5.3 pm/V. Fanning effect in the “fiber-like” BTO sample was studied and double phase conjugation with conversion efficiency up to 8% was observed in a wide range of incidence angles of the pump at λ = 0.633 μm for 2 × 3 mW input light power.     

Photorefractive effects in long, narrow BSO crystals with applied electric field

In a photorefractive Bi₁₂SiO₂₀ crystal with high applied electric ac field of square-wave shape a fast two-wave coupling response (less than 1 s) and a slow hologram readout decay (minutes) was found for a wavelength of 633 nm. This can be explained by electron–hole transport with two trap levels. An intensity dependence of the slower complementary grating was found. Illuminating with the readout wave without applied electric field leads to a very slow grating decay (many hours). Received: 18 November 1998 / Revised version: 3 February 1999 / Published online: 7 April 1999     

Fast photorefractive response in B(12)SiO(20) in the near infrared

The polarization self-modulation effect was applied for effective measurement of the characteristic response time of nominally pure Bi(12)SiO(20) (BSO) at wavelengths of 810 and 980 nm. Owing to oxygen deficiency in the crystal lattice, the BSO crystals showed unusual photorefractive sensitivity and remarkable operation speed in the near-infrared spectral region. A response time of 130 ms was measured at 810 nm, and a response time of 540 ms was measured at 980 nm, with incident intensities of 110 and 200mW/cm(2), respectively. To our knowledge, this is the first experimental evidence of a subsecond response in the infrared for a nonsemiconductor photorefractive material.