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1.
Anomalous behaviour of long-time increase in diffraction efficiency of photorefractive grating in Ce:BaTiO3 in dark 下载免费PDF全文
Anomalous long-time increase of the diffraction efficiency is observed in dark-decay experiments of photorefractive gratings in Ce:BaTiO3. It is deduced that a phase-conjugate beam is induced by the writing beam at acute angle to the +c axis of the crystal and it interferes with the other writing beam to form a second grating which is perpendicular to the first grating formed by the interference between two writing beams. The rising behaviour of the diffraction efficiency results from the different decay rates of these two photorefractive gratings. Furthermore, a simplified model of two gratings, both induced by two deep traps, is proposed to account for this phenomenon and the fitting results agree well with the experimental results. 相似文献
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M.Y. Goulkov T. Granzow U. Dörfler T. Woike M. Imlau R. Pankrath 《Applied physics. B, Lasers and optics》2003,76(4):407-416
We study light-induced scattering (beam-fanning) in the photorefractive crystal SBN:Ce as a function of the polar structure
of the crystal. The spatial structure of the beam-fanning is measured at different externally applied electric fields, and
an optical hysteresis is found in the scattering. It is shown that the scattering hysteresis results from a polarization hysteresis
typical for ferroelectrics in the polar phase. New information about primary scattering in SBN is obtained, and a corresponding
model of its origin is proposed. It is shown that the intensity and angular distribution of the primary scattering strongly
depend on the polar structure of the crystal and can be affected by the subsequent action of an external field and coherent
illumination.
Received: 27 August 2002 / Revised version: 19 December 2002 / Published online: 26 March 2003
RID="*"
ID="*"Corresponding author. E-mail: mirco.imlau@uni-osnabrueck.de 相似文献
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We investigate diffraction of light at phase holographic-type gratings recorded by a piezoactive ultrasonic wave in gyrotropic cubic photorefractive crystals. We show that the efficiency of diffraction at a holographic grating can be increased substantially by applying a d.c. field to a crystal under the conditions of a longitudinal electrooptical effect; elliptical polarization of diffracted light is determined by electrically induced anisotropy of a crystal in the piezofield of a photorefractive grating, by the external electric field, detuning of phase synchronism, gyrotropy of the crystal, ultrasound frequency, and the time of recombination of charge carriers. 相似文献
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C. L. Woods C. L. Matson M. M. Salour 《Applied Physics A: Materials Science & Processing》1986,40(3):177-182
We report the first experimental measurement of fringe-angle applied electric field scaling of space charge growth and of crystallographic orientation effects in the initial development of the diffraction efficiency of thick holograms produced by the photorefractive effect in a bismuth silicon oxide (BSO) crystal. Diffraction efficiencies of holograms made by interfering two plane waves on the [¯110] face are measured as a function of the angle between the fringe pattern and the applied electric field. As the crystal is rotated relative to the interference fringes, the applied field may be scaled to yield identical space charge growth. Polarization-dependent diffraction measurements agree with the theory of a birefringent grating when optical activity is included as a separate, serial effect. Both the rotation-scaled applied electric fields and the crystallographic variations in the birefringent diffraction grating are consistent with charge transport processes in which the initial space charge fields are perpendicular to the interference fringes over growth times extending nearly into the steady state regime. 相似文献
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Based on the electro-optic and piezoelectric effects, the principle of a 2×2 bypass-exchange switch in photorefractive LiNbO3 crystal is discussed. In photorefractive volume grating, the Bragg condition can be changed by applying a field during readout. That is, by applying a specific field Es or zero, the diffraction efficiency will be 0 or 1, respectively and this property can be used to realize the bypass or exchange operation of the switch. In this paper, considering both the piezoelectric and electro-optic effects caused by the applied electric field, we analyzed relations of both the specific field and the incident intensity ratio with respect to the writing angles and polarizations of writing beams. 相似文献
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光轴方向任意时光折变晶体中体全息光栅的衍射性质 总被引:2,自引:2,他引:0
利用坐标旋转方法和Kogelnik耦合波理论,建立了光轴方向任意时单轴晶体中体光栅布拉格衍射的耦合波方程,分析了Li NbO3晶体的光轴方向对光折变体全息光栅的各向同性和各向异性布拉格衍射性质的影响。模拟计算表明,在给定光栅的结构参量时,通过适当选择光轴方向角可以使得光折变体光栅的各向同性和各向异性布拉格衍射的衍射效率达到最大,给出了相应类型的衍射效率取得最大值时晶体光轴的大致方向。这些理论分析为光折变体全息光学器件的优化设计和进一步广泛应用提供了很好的理论参考依据。 相似文献
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Fast photorefractive response in strongly reduced near-stoichiometric LiNbO(3) crystals 总被引:3,自引:0,他引:3
The photorefractive response time of LiNbO(3) crystal is of the order of minutes, and such a long response time limits the crystal's practical applications. We report the photorefractive properties of nominally pure near-stoichiometric LiNbO(3) crystal that is strongly reduced in vacuum. A short photorefractive response time of the order of 100 ms is measured at a wavelength of 514.5 nm, with incident light intensity of 1.6 W/cm (2) , and possible corresponding mechanisms are discussed. To our knowledge this is the first experimental evidence of a subsecond photorefractive response in pure LiNbO(3) crystals. The diffraction efficiency of a holographic grating written in this reduced crystal is low but can be enhanced by an externally applied electric field. 相似文献
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N.C. Deliolanis A.G. Apostolidis E.D. Vanidhis D.G. Papazoglou 《Applied physics. B, Lasers and optics》2002,75(1):67-73
We study the influence of the application of an external electric field on the grating formed in (1-10) and (111)-cut photorefractive
sillenite crystals. Optimum conditions for application of the bias field are examined, taking into account the shift of the
grating orientation for maximum diffraction efficiency along the crystal depth. The contribution of the piezoelectric effect
is considered both in the electro-optic tensor and space charge field calculations. An analysis on the diffractive properties
under these optimum conditions shows an increase of 30% in maximum diffraction efficiency in Bi12GeO20, and experimental verification for this is provided.
Received: 5 November 2001 / Revised version: 18 April 2002 / Published online: 8 August 2002 相似文献
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Diffraction efficiency as a function of the applied electric field for the non-linear regime has been calculated by solving numerically the beam coupling equations. The refractive index variation used in the beam coupling equations was calculated directly from the material rate equations via the total space charge field. The diffraction efficiency of the holograms recorded in photorefractive media is not only a function of the applied external electric field but also a function of crystal thickness, diffusion field, reduced fringe contrast modulation ratio and absorption coefficient of the materials. The effects of these parameters on the efficiency of the holograms have been studied in details. In the absence of the external applied field, it is found that the diffraction efficiency of the holograms could be maximized for a thinner photorefractive crystal having lower absorption coefficient and higher value of diffusion field, which could exist at a much lower value of reduced fringe contrast modulation ratio of the index grating. More efficient holograms can be recorded in the presence of the externally applied electric field as compared to the case of no external field. 相似文献
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Volume phase-hologram formation by the photorefractive effect in KNbO3 is accompanied by a stationary energy transfer between writing beams. The change in energy transfer by applying an electric
field on the reduced crystals is shown to be due to an efficient increase in migration length which can reach values comparable
or larger than the fringe spacing. It is demonstrated that photovoltaic contribution to the diffraction efficiency and energy
transfer is rather small in reduced KNbO3 and that diffusion of photogenerated free holes is the dominant charge transport for the photorefractive effect in unbiased
crystals. The experimental results for diffraction efficiency and energy transfer as a function of grating spacing, electric
field, light intensity and temperature is well described by a recent theory of Kukhtarev and Vinetskii. 相似文献
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S. Balasubramanian I. Lahiri Y. Ding M.R. Melloch D.D. Nolte 《Applied physics. B, Lasers and optics》1999,68(5):863-869
The photorefractive response to an applied electric field is measured in a photorefractive quantum well, providing evidence
in favor of the nonlinear transport in the device due to the hot electrons. The reduced mobility of the hot electrons limits
the drift length, and thereby limits fringe overshoot. Thus the nonlinear transport prevents the slowing down of the grating
writing rate for increasing fields which is common in bulk photorefractives. The photorefractive phase shift in transverse-field
photorefractive quantum wells is measured as a function of the frequency offset between two laser writing beams that generate
moving gratings. The two-wave mixing passes through a maximum at an optimum frequency which depends on the magnitude and the
sign of the applied dc electric field. The phase shift associated with the moving grating adds or subtracts from the static
phase shift induced by hot-electron transport in the semiconductor quantum wells, depending on the sign of the field and the
sign of the dominant photocarriers. We observe a linear relationship between the roll-off frequency and the power of the writing
beams.
Received: 26 November 1998 / Revised version: 22 January 1999 / Published online: 12 April 1999 相似文献
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We present a new method of light amplification based on vectorial two-wave mixing in cubic crystal via an unshifted photorefractive grating. The energy transfer between interacting waves is achieved in an experimental configuration that provides the amplitudes of modulation of refractive indices of opposite signs, Dn(1)=-Dn(2), for two orthogonal polarization modes of the interacting waves. The configuration demonstrates bidirectional amplification; i.e., the weak wave is always amplified, independent of its direction with respect to the strong pump beam. We present the results of numerical analysis along with the experimental results for a Bi(12)TiO(20) crystal with a dc applied electric field. 相似文献
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Self-Deflection of Dark Screening Spatial Solitons Based on Higher-Order Space Charge Field 下载免费PDF全文
The effects of higher-order space charge field on the self-deflection of dark screening spatial solitons in biased photorefractive crystals are numerically investigated under steady-state conditions. The expression for an induced space-charge electric field including higher-order space-charge field terms is obtained. Numerical results indicate that dark solitons possess a self-deflection process during propagation, and the solitons always bend in the direction of the c axis of the crystal. The self-deflection of dark solitons can experience considerable increase especially in the regime of high bias field strengths. 相似文献
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We report some electric field controlled photorefractive higher-order diffraction phenomena of a paraelectric phase potassium lithium tantalate niobate crystal doped with iron. In experiments, a p-polarized semiconductor laser (532 nm) was used to record grating at a small incident angle. Higher-order diffraction images were observed when the signal beam was focused behind and in front of the crystal. Then the higher-order diffraction images were reconstructed by a p-polarized He–Ne laser (632.8 nm) in the direction perpendicular to the surface. The higher-order diffraction images could be controlled by the external electric field. A theory about the higher-order diffraction images of the K and 2K grating is developed. The results show that the even order diffraction images of the K grating and the odd order diffraction of the 2K grating overlap each other. The odd order diffraction images of the K grating are diffracted in unattached direction. The electric field controlled higher-order diffraction image provides a useful method for optical information processing. 相似文献
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Mode splitting was observed when a He-Ne laser beam was reflected through a prism-coupled liquid-crystal slab waveguide under
an applied electric field. The splitting manifests itself as the imposed voltage reaches a critical level, and diminishes
when the voltage increases above a critical high value. If the applied voltage increases even further, mode splitting vanishes,
attributing to the result that almost all the directors of the liquid crystal turn upright to the surface. The multi-layer
matrix simulation can satisfactorily account for this phenomenon by exploiting the property of the anisotropic optical birefringence
of a liquid crystal under applied voltages.
Received: 2 December 2002 / Revised version: 24 January 2003 / Published online: 23 May 2003
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ID="*"Corresponding author. Fax: +886/3572-3052, E-mail: jtlue@phys.nthu.edu.tw 相似文献