Bi12TiO20 Crystals Research Articles

Charge–carrier diffusion length in photorefractive crystals computed from the initial hologram phase shift

The phase shift between the pattern of light onto a photorefractive crystal and the resulting hologram at the very beginning of the recording process in two-wave mixing is analyzed and measured as a function of the applied electric field. These data allow one to compute the diffusion length of photoexcited charge carriers and to evaluate the actual electric field inside the crystal. A diffusion length of 0.14 μm is measured in a nominally undoped photorefractive Bi12TiO20 crystal using a 532 nm wavelength laser illumination, in agreement with results obtained from other methods.

Experimental transient self-focusing in Bi12TiO20 crystal

An experimental study of both transient and steady state bi-dimensional photorefractive bright spatial solitons in Bi12TiO20 is presented. The results are compared to previous theoretical work, showing qualitative agreement conceming both the transient and the steady soliton widths.

Shape-asymmetry of the diffraction efficiency in Bi12TiO20 crystals: the simultaneous influence of absorption and higher harmonics

Studying moving holographic gratings in a Bi12TiO20 crystal with an applied external field, we have observed the appearance of a “shape-asymmetry” in the diffraction efficiency as a function of frequency detuning. The effect is mainly absorption-induced; for large modulation depth it is further enhanced by the influence of higher harmonics.

Laser-diode injection locking with a double phase-conjugate mirror in photorefractive Bi_12TiO_20 fiberlike crystal under external alternating voltage

Injection locking of two laser diodes operating at 681–684 nm with a double phase-conjugate mirror recorded in photorefractive Bi12TiO20 crystal under an external alternating electric field is studied. It was found that there are three different regimes of laser-diode coupling distinguished by the various coherences of the light emitted by the coupled laser diodes. The ability of the photorefractive double phase-conjugate mirror to recover the coherence length is experimentally demonstrated. Advantages of photorefractive Bi12TiO20 fiberlike samples over bulk BaTiO3 crystals to record the double phase-conjugate mirror are listed.

Grating oscillations in photorefractive crystals

A theoretical analysis is presented that describes wave-mixing processes in thin holographic gratings for phase-modulated beams. Experiments with photorefractive Bi(12)TiO(20) crystals in which the signals for Bragg diffraction and for the first non-Bragg diffraction order were measured support the analysis.

Optical and Photorefractive Characterization of BTO Crystals Doped with Cd, Ca, Ga, and V

Single crystals of Bi12TiO20 (BTO) are grown by a modified Czochralski technique. Doping with cadmium, calcium, gallium, and vanadium is performed by adding corresponding oxides to the melt. Optical parameters like refractive index, optical absorption, optical activity, light induced absorption, electrooptic coefficient, and photoconductivity are determined. Photorefractive properties of the samples are characterized with the help of a conventional two-beam interference setup. Diffraction efficiency and decay time constant of elementary holograms are determined. The influence of the utilized dopands is discussed.

Fast mutually pumped phase conjugation using transient photorefractive coupling

Unusually fast mutually pumped phase-conjugation (MPPC) is achieved in a Bi12TiO20 crystal by using transient photorefractive beam coupling under a dc electric field. Pulses of phase-conjugate waves with a buildup time of 4.5 ms and a width of 50 ms at an input intensity of 2×0.8 W/cm2 are obtained after applying a rising front of external electric field to the crystal. A phase-conjugation efficiency as high as 25% of the incident light power is measured.

Influence of the purity of the starting materials on the recording of dynamic holograms in Bi12TiO20 crystals

The parameters of Bi12TiO20 crystals grown with bismuth oxide of different degrees of purity were measured by a holographic recording technique in an external ac electric field and were compared with a crystal specially doped with chromium. Various crystal parameters such as the diffusion length of the photoexcited carriers and the Debye screening length were determined. It was found that the absorption spectrum of the material and the holographic recording efficiency were strongly influenced by light chromium doping and by insufficient purity of the starting mixture.

Holographic-current study in laser and photorefractive crystals

We modified the theory of the holographic current, describing both the steady-state and the transient contributions and accounting for the parameters of the measurement circuit. In the model of the transient photocurrent we connected the observables of the steplike modulation experiment with the carrier transport parameters such as carrier diffusion and screening length, carrier effective lifetime, and Maxwell relaxation time. Experimentally, we have shown that theoretically predicted dependences of the photocurrent amplitude and relaxation time on the grating period, the light intensity, and the depth of the modulation were observed not only in the photorefractive Sr1-xBaxNb2O6 and Bi12TiO20 crystals but in Er-doped YSGG laser crystals as well.

Photorefractive surface waves in a thin Bi12TiO20 crystal

A theoretical model considering total internal reflections of fanned beams from the side crystal surfaces and their coupling with the pump beam is proposed. The interaction of these beams results in the light intensity redistribution and in the generation of the photorefractive surface wave. An experimental observation of the light intensity self-concentration near the surface of a thin photorefractive Bi12TiO20 crystal is reported. It results in an acceleration of the photorefractive energy exchange process.

Real‐time holographic interferometry using optical phase conjugation in photorefractive materials and direct spatial phase reconstruction

The structure and operation of a holographic interferometer that uses photorefractive Bi12TiO20 crystals as reusable recording mate- rials and computer image processing for rapid and precise interpretation of the fringe patterns are discussed. Different types of two-wave and four-wave mixing were applied to optical testing by double-exposure ho- lographic interferometry. A very convenient matching was found in the phase-conjugation setup for the Bi12TiO20 crystals and usual He-Ne la- sers, when the exposures were of ;10 s for light energy densities of 4310 23 W/cm 2 . Simple algorithms and direct spatial phase reconstruc- tion were used for computer image processing of the fringe patterns. © 1996 Society of Photo-Optical Instrumentation Engineers. Subject terms: real-time holographic interferometry; optical phase conjugation; photorefractive materials; fringe image processing; direct spatial phase demodu- lation.

Effects of optical activity on photorefractive spatial solitons in a biased Bi_12TiO_20 crystal

It is shown that the propagation of steady-state spatial solitons in a bismuth titanium oxide (Bi12TiO20) photorefractive crystal is strongly affected by the presence of concomitant natural optical activity. We assume that the Bi12TiO20 crystal is (110) cut and that the external bias field is along the [001¯] axis. Such orientation introduces self-focusing in one polarization of an optical wave front, whereas it has no effect whatsoever on the component orthogonal to it. Under these conditions, the equations that describe the coupled evolution of the two polarizations of an optical planar beam are derived. Our numerical study of the interaction dynamics shows that, even though the optical beam experiences periodic diffraction in one of the polarization components, it can still recover significantly at particular distances within the crystal. Moreover, it is initially capable of maintaining its solitonlike form over a certain distance of propagation. Relevant examples are provided.

Multimode fiber-optic sensor using photorefractive double phase conjugator

An optical sensing scheme for measuring the mode phase shifts and mode amplitude modulation in a multimode fiber is proposed. The scheme uses a double phase conjugator based on photorefractive fiberlike Bi12TiO20 (BTO) crystal that allows to detect high speed changes of environmental conditions with adaptation for slow variations. An acoustic sensor based on the proposed scheme is used to demonstrate the scheme’s feasibility.

Phase-compensated holographic recording based on anisotropic photorefractive diffraction

We report a simple actively stabilized setup for holographic recording on sillenite photorefractive crystals that is based on the anisotropic diffraction properties of these materials. The method is much simpler than previously published ones, requires neither phase modulation nor synchronous phase-sensitive detection, and needs no external reference. We describe the successful operation of this stabilization procedure for a Bi(12)TiO(20) crystal in a practical holographic interferometry setup, using the 632.8-nm wavelength of a He-Ne laser.

Recording of a travelling interference pattern by a two-frequency field in a crystal

Theoretical and experimental investigations were made of hologram recording in a photorefractive Bi12TiO20 crystal. The application of an electric field of the type makes it possible to record an interference pattern travelling at a velocity .

Optical spectra of doped Bi12TiO20 crystals

Single crystals of Bi12TiO20. Both "pure" and doped with various concentrations of V, W, Mo, Nd and Cr, have been studied. Absorption spectra in the energy range 2.0-2.8 eV have been obtained from classical reflection and transmission measurements. The constant-photocurrent method has also been employed in order to study absorption spectra in a wider energy range (1.3-2.8 eV). A well expressed absorption decrease at energies hv>2.2 eV has been observed for Bi12TiO20: 0.056 wt% V and Bi12TiO20: 0.013 wt% Mo crystals. Also, a considerable absorption decrease has been measured at energies hv<2 eV for Bi12TiO20: 0.056 wt% V and Bi12TiO20: 0.008 wt% W. The origin of some new absorption bands observed in doped Bi12TiO20 crystals as well as some differences in the effect of impurities on the optical absorption of Bi12TiO20 and Bi12TiO20 crystals have been discussed.

Steady photo-emf in photorefractive crystals

Measurements were made of a constant current induced in Bi12TiO20 (BTO) and Bi12SiO20 (BSO) crystals exclusively by the motion of an interference pattern. A method for determining the parameters of photorefractive crystals was developed on the basis of this effect.

Information capacity of holograms in photorefractive crystals

From a single measurement of the signal-to-noise ratio of the image reconstructed from a hologram it is possible to estimate the information capacity of superimposed holograms and to predict the total number of superimposed holograms associated with the maximum information capacity. Information capacity densities of 3.7 x 10(6) bits/mm(3) for Bi(12)TiO(20) crystal samples and 2.3 x 10(8) bits/mm(3) for LiNbO(3) samples have been calculated.

Phase-sweep method for characterization of cw-laser-induced dielectric gratings

We present a new phase-sweep method for characterizing laser-induced complex dielectric gratings. This characterization requires four parameters: the amplitudes and the spatial phases of both the refractive-index and the absorption components. We apply this phase-sweep method to study a complex polarizability grating in a nominally undoped cubic photorefractive Bi(12)TiO(20) crystal. We determine the polarizability difference afe between alpha(fe) full and an empty photoexcitable deep trap at 633 nm in Bi(12)TiO(20) to be (2.5 - 8.3i +/- 1.0 +/- 3.3i) x 10(-40) Fm(2) in SI units or (2.3 - 7.5i +/- 0.9 +/- 3.0i) x 10(-24) cm(3) in Gaussian units.

Growth and characterization of undoped and doped Bi12 TiO20 crystals

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