Measurement of laser-induced refractive index change of inverted ferroelectric domain LiNbO3

Optical nonlinearities of periodically poled LiNbO(3) crystals were investigated by the single beam Z-scan technique with a continuous wave (cw) laser beam at 532 nm. The nonlinear optical absorption coefficient and refractive index change are determined to be 8.1 x 10(-6) cm/W and 2.6 x 10(-4) at 0.5 MW/cm(2) light intensity, respectively. Both sign and magnitude of the measured refractive nonlinearity are considerably different from the Z-scan results in congruent LiNbO(3). The nonlinearities in the periodically poled LiNbO(3) induced by 532 nm continuous waves are believed to be mainly due to the photorefractive effect.     

Absorption characteristic and nonvolatile holographic recording in LiNbO3:Cr:Cu crystals

The absorption characteristic of lithium niobate crystals doped with chromium and copper (Cr and Cu) is investigated. We find that there are two apparent absorption bands for LiNbO3:Cr:Cu crystal doped with 0.14 wt.% Cr2O3 and 0.011 wt.% CuO; one is around 480 nm, and the other is around 660 nm. With a decrease in the doping composition of Cr and an increase in the doping composition of Cu, no apparent absorption band in the shorter wavelength range exists. The higher the doping level of Cr, the larger the absorbance around 660 nm. Although a 633 nm red light is located in the absorption band around 660 nm, the absorption at 633 nm does not help the photorefractive process; i.e., unlike other doubly doped crystals, for example, LiNbO3:Fe:Mn crystal, a nonvolatile holographic recording can be realized by a 633 nm red light as the recording light and a 390 nm UV light as the sensitizing light. For LiNbO3:Cr:Cu crystals, by changing the recording light from a 633 nm red light to a 514 nm green light, sensitizing with a 390 nm UV light and a 488 nm blue light, respectively, a nonvolatile holographic recording can be realized. Doping the appropriate Cr (for example, NCr = 2.795 x 10(25) m(-3) and NCr/NCu = 1) benefits the improvement of holographic recording properties.     

Storage density of shift-multiplexed holographic memory

The storage density of shift-multiplexed holographic memory is calculated and compared with experimentally achieved densities by use of photorefractive and write-once materials. We consider holographic selectivity as well as the recording material's dynamic range (M/#) and required diffraction efficiencies in formulating the calculations of storage densities, thereby taking into account all major factors limiting the raw storage density achievable with shift-multiplexed holographic storage systems. We show that the M/# is the key factor in limiting storage densities rather than the recording material's thickness for organic materials in which the scatter is relatively high. A storage density of 100 bits/mum(2) is experimentally demonstrated by use of a 1-mm-thick LiNbO(3) crystal as the recording medium.     

Optimal recording wavelength for maximum diffraction efficiency of thermal fixing in LiNbO3:Fe

Optimal recording wavelength for maximum diffraction efficiency of thermal fixing in LiNbO(3):Fe crystal is investigated. Holographic gratings are recorded using three typical recording wavelengths including 488, 514, and 633 nm. Optimal switching from recording to thermal fixing is taken into consideration. The fixed holograms are developed by an original recording setup. Diffraction efficiencies of recording and thermal fixing are measured by a two-wave coupling technique. The theoretical and experimental results are analyzed and compared. With a blue beam, the nonvolatile hologram with maximum fixing efficiency is achieved. This work can obtain high persistent diffraction of the nonvolatile holographic storage in LiNbO(3):Fe crystals.     

Tunable diffraction-limited light at 488 nm by single-pass frequency doubling of a broad area diode laser

A laser system that is based on second-harmonic generation of a broad area laser diode and provides 23.2 mW of diffraction-limited light with narrow bandwidth is described. It is tunable from 487.4 to 489 nm. The broad area laser diode is frequency stabilized in an external cavity that yields 800 mW of diffraction-limited light. This infrared light is converted into the visible by use of a 1 cm periodically poled MgO:LiNbO(3) bulk crystal with a measured single-pass conversion efficiency of up to 3.6%/W x cm.     

Self-stabilized recording of fixed gratings at high temperature in LiNbO3:Fe

A high quality fixed holographic grating was recorded in a photorefractive LiNbO(3):Fe crystal at about 100 degrees C in a homemade temperature-controlled vacuum chamber. The recording was carried out using self-stabilization techniques with lambda=532 nm beams guided onto the crystal by polarization maintaining fibers. The diffraction efficiency of the fixed grating was eta=0.44 when measured in the recording setup using the same lambda=532 nm recording beams. A compatible eta was measured with lambda=633 nm in an auxiliary setup, and a 1 mrad angular Bragg selectivity at FWMH was estimated, thus demonstrating the uniformity and good quality of the fixed grating.     

Three-dimensional optical bit-memory recording and reading with a photorefractive crystal: analysis and experiment

We analyze the three-dimensional refractive-index distribution that is induced locally when a laser beam is focused onto a very small region in a photorefractive crystal. The formation of the index distribution is deduced from the temporal behavior of the electron density distribution in the crystal under non-steady-state conditions. The density distribution is computed by the use of a set of the recurrence relations that was derived from Kukhtarev's equations, which describe the transport of electrons in time. In particular, we calculated the index distribution formed in Fe-doped LiNbO(3) crystals. To verify the validity of our analysis, we read, by using a phase-contrast microscope, refractive-index dots that were recorded in Fe-doped LiNbO(3) crystals. A good agreement was obtained between experimental results and the calculated phase-contrast images when the characteristics of the imaging system are taken into account. We also found that the induced index change is largest when the c axis of the LiNbO(3) crystal is oriented parallel to the polarization direction of the reading beam. Under this optimal condition, we succeeded in recording up to 10 layers of readable data in a LiNbO(3) crystal.     

Linear electro-optic effect in the organic crystal 4-aminobenzophenone

The linear electro-optic effect in single crystals of 4-aminobenzphenone (ABP) is reported together with calibration data on LiNbO(3). For ABP the linear electro-optic coefficients r(22) and r(32) at 488 nm were found to be 2.12 and 5.05 pm/V, respectively, with the corresponding reduced half-wave voltages being 49.4 +/- 0.1 and 9.3 +/- 0.1 kV. For LiNbO(3) the half-wave voltage was found to be 4.0 +/- 0.1 kV at 632.8 nm and 2.4 +/- 0.1 kV at 488 nm.     

High-density perpendicular recording media with large grain separation

The possibility of 300-500 Gbit/in/sup 2/ perpendicular recording using granular recording media has been investigated through micromagnetic simulation based on the Langevin equation. Writability and thermal stability in 10 years were obtained changing media parameters such as the grain size D, the grain separation d, and the thickness of the recording layer t/sub mag/ for proper combination of the grain saturation magnetization M/sub s-grain/ and the grain perpendicular anisotropy energy K/sub u-grain/. It was found that high-density recording is realized under the large grain separation, the large grain saturation magnetization, and the large grain anisotropy energy. The read/write calculation using ordered medium with D of 4.2 nm, d of 2.3 nm, t/sub mag/ of 12.0 nm, M/sub s-grain/ of 1313 emu/cm/sup 3/, and K/sub u-grain/ of 7.0 Merg/cm/sup 3/ confirmed the possibility of 1303 kFCI and 1954 kFCI perpendicular recording, leading to 325 and 488 Gbit/in/sup 2/ with 250 kTPI (track pitch of 102 nm).     

Monolithic bulk shear-wave acousto-optic tunable filter

We demonstrate a monolithic bulk shear-wave acousto-optic tunable filter combining a piezoelectric transducer array and the acoustic interaction medium in a single crystal. An X-propagating acoustic longitudinal wave is excited in the "crossed-field" scheme by an RF-E/sub y/-field in a chirped acoustic superlattice formed by domain-inversion in lithium niobate (LiNbO/sub 3/). The acoustic longitudinal wave is efficiently (97.5%) converted at a mechanically free boundary into a Y-propagating acoustic slow-shear wave that couples collinearly propagating e- and o-polarized optical waves. A relative conversion efficiency of 80%/W was measured at 980 nm.     

Three-dimensional optical memory with a photorefractive crystal

We propose a three-dimensional optical-memory device in which refractive dot data are recorded directly into a photorefractive crystal. To record a single bit of datum, one focuses a laser beam with an objective lens onto a specific spot in a crystal, thereby changing its refractive index locally as a result of photorefraction. To record in three dimensions, one keeps the objective lens stationary while the crystal is translated. The beam-spot intensity is modulated with a beam shutter according to the logic state of the data point. The recorded data points are read with a phase-contrast microscope objective lens. We present experimental results of three-dimensional recording and reading with a LiNbO(3) crystal. The distribution of the refractive index formed by a focused beam is also analyzed with the charge-transport model.     

Influence of composition on the photorefractive centers in pure LiNbO3 at low light intensity

The influence of composition on the photorefractive effect in pure LiNbO3 crystals at low light intensity was investigated. The experimental results indicate that different defects dominate the photorefractive centers of pure LiNbO3 with various compositions. Bipolarons are considered to be responsible for the enhanced photovoltaic field in reduced near-stoichiometric LiNbO3, and their bulk photovoltaic constant kappa is estimated to be approximately 6.95 x 10(-32) m3/V. Q polarons (composed of two bipolarons) are introduced to explain the photorefractive effect of congruent LiNbO3 at both low and high light intensities.     

Ti:Fe:LiNbO_3晶体的生长及其光折变性能的研究

采用熔体提拉法生长了不同掺杂浓度的Ti：Fe：LiNbO3晶体．研究了掺杂杂质离子浓度变化对晶体光折变性能的影响，测定了晶体经热化学还原处理前后的透射谱．用ESR方法证实，未经还原处理时，Ti：Fe：LiNbO3晶体中Ti离子以Ti4 形式存在．与Fe：LiNbO3和Ti：LiNbOa相比，Ti、Fe复合掺杂，通过电荷补偿效应，使未经还原处理的晶体中Fe2 增加，从而使光吸收增强；可以通过改变Ti、Fe掺杂浓度的方法来控制晶体中Fe2 离子的浓度，达到控制并改善晶体光折变性能的目的．本文还对Ti：Fe：LiNbO3晶体的全息性能进行了研究，测得Ti：Fe：LiNbO3晶体响应时间缩短，衍射效率高达90％以上．Ti：Fe：LiNbO3晶体是一种优质的光折变材料．     

Characteristics of recording and thermal fixing in lithium niobate

We present a theoretical model in which the band-transport equations and the coupled-wave equations are considered to study the two thermal-fixing methods (simultaneous fixing and postfixing) in Fe:LiNbO(3). We found that, in simultaneous fixing, the existing ionic-grating affects the writing of the electronic grating by reduction of the coupling gain, and the grating envelope of the fixed-index grating is quite uniform inside the photorefractive crystal in comparison with the method of postfixing. The resulting diffraction efficiency of the fixed-volume grating is dependent mainly on the initial intensity modulation of the two writing beams. A set of experiments is also presented.     

The study of self-erasing phenomenon in reduced Zn:Fe:LiNbO3 crystal

It is experimentally demonstrated that the self-erasing phenomenon, diffraction efficiency grows gradually to its maximum and then decays a great extent during the recording process, existed in reduced Zn:Fe:LiNbO₃ crystal. To analyze this phenomenon, the absorption spectra were measured by UV–Visible Spectrophotometer in the region of 200–700 nm, and the nonlinear dependence of photoconductivity on light intensity were studied. The cooperative action of the two kinds of carriers and their own shallow or deep traps was considered to explain the self-erasing phenomenon in reduced Zn:Fe:LiNbO₃ crystal.     

脉冲激光沉积金刚石基c轴取向LiNbO3压电薄膜

采用等化学计量比的LiNbO3多晶陶瓷为靶材,利用脉冲激光沉积技术在以非晶SiO2为缓冲层的金刚石/Si衬底上制备c轴取向LiNbO3薄膜。研究了靶材与衬底之间的距离对LiNbO3薄膜的结晶质量和c轴取向性的影响,发现在靶材与衬底之间的距离为4.0cm时获得了具有优异结晶质量的完全c轴取向LiNbO3压电薄膜。采用扫描电子显微镜和原子力显微镜对最佳条件下制备的薄膜进行了分析,结果表明制得的薄膜呈与衬底垂直的柱状结构,且薄膜表面光滑,晶粒均匀致密,表面平均粗糙度约为9.5 nm。     

Nonvolatile holographic storage of near-stoichiometric LiNbO3:Cu:Ce with green light

The nonvolatile holographic storage of a near-stoichiometric LiNbO(3):Cu:Ce crystal with green light was investigated. With an increase in composition, improved nonvolatile holographic performance was obtained. The sensitivity S? of the near-stoichiometric LN(49.57):Cu:Ce crystal is 1 order of magnitude larger than the congruent crystal. And according to our research, Ce ions should be the deep centers of the LiNbO(3):Cu:Ce crystal.     

Ce:Eu:LiNbO3晶体制备及光折变性能

为了昨到光折变性能优良的晶体材料，在LiNbO3中掺进CeO2和Eu2O3，生长Ce:LiNbO3和Ce:Eu:LiNbO3单晶体，对晶体进行极化和氧化还原处理，并利用XRD、UV-VIS及二波耦合光路测试了晶体的晶格常数、吸收光谱、指数增益系数和响应时间。结果表明，Ce:Eu:LiNbO3晶体的晶格常数比Ce:LiNbO3晶体小，其吸收边比CeLiNbO3晶体红移更多，指数增益系数比Ce:LiNbO3晶体大，而响应时间比Ce:LiNbO3晶体的要短，Ce:Eu:LiNbO3是优良的光折变晶体材料。     

Determination of threshold exposure and intensity for recording holograms in thick green-sensitive acrylamide-based photopolymer

For optical data storage applications, it is essential to determine the lowest intensity (also known as threshold intensity) below or at which no data page or grating can be recorded in the photosensitive material, as this in turn determines the data capacity of the material. Here, experiments were carried out to determine the threshold intensity below which the formation of a simple hologram--a holographic diffraction grating in a green-sensitized acrylamide-based photopolymer--is not possible. Two main parameters of the recording layers--dye concentration and thickness--were varied to study the influence of the density of the generated free radicals on the holographic properties of these layers. It was observed that a minimum concentration per unit volume of free radicals is required for efficient cross-linking of the created polymer chains and for recording a hologram. The threshold intensity below which no hologram can be recorded in the Erythrosin B sensitized layers with absorbance less than 0.16 was 50 μW/cm(2). The real-time diffraction efficiency was analyzed in the early stage of recording. It was determined that the minimum intensity required to obtain diffraction efficiency of 1% was 90 μW/cm(2), and the minimum required exposure was 8 mJ/cm(2). It was also determined that there is an optimum dye concentration of 1.5 × 10(-7) mol/L for effective recording above which no increase in the sensitivity of the layers is observed.     

Single beam one-way imaging through a thick dynamic turbulent medium

A new method to record an image through a thick dynamic phase distorting medium by using a photorefractive LiNbO3 crystal is demonstrated. The method uses only one beam of light, the object light. By making use of the photorefractive fanning effect, gratings are formed in the LiNbO3 crystal through the interference between the object light and its own fanning light. Because the time scale of the distorting fluctuation is much shorter than the writing time of the LiNbO3 crystal, the fluctuated light does not induce any fanning. Therefore only the static portion of the image is recorded in the crystal, and the intensity distribution of the image can then be reconstructed at any later time.