High-efficiency volume hologram recording with a pulsed signal beam

We present an efficient photorefractive volume hologram recording technique with a pulsed signal beam and continuous reference-beam illumination. The grating envelope can be simply controlled by manipulation of the duty cycle of the signal beam. Thus, for any grating coupling strength and different initial reference-signal intensity ratios, the diffraction efficiency can be maximized with this technique and can be greatly increased in comparison with that of the conventional recording technique.     

Experimental evidence of photoinduced valence change of Fe3+ in BaTiO3 and mechanism for growth of new grating in depleted pump condition: An EPR investigation

With a view to understanding the role of photo-induced valence changes of impurities in BaTiO₃ in the phenomena of photorefraction, EPR experiments were conducted under in situ He-Ne laser illumination. These experiments gave evidence for photoinduced valence change of Fe in BaTiO₃ at room temperature. The EPR signal due to trivalent iron was found to reduce in intensity with laser illumination The kinetics of the valence change has been investigated. Under large fringe width condition, the time constant of the decay is identified as the dielectric relaxation time τ_d. The changes in line shape on laser illumination to Dysonian form, appeared most predominantly in mechanically poled crystal compared to electrically poled single domain crystals. This demonstrated the possible role of domain walls and the defects there, as source or sinks of charge carriers on photo excitation. It is observed, that there is transient growth of Fe³⁺, when the laser illumination was put on, before its decay. This was attributed to charge transfer between electrons in oxygen vacancies and Fe⁴⁺. This predicted the growth of a transient grating under depleted pump condition in a two beam coupling experiment. This was experimentally proved by following the diffracted signal of the reading beam under the depleted pump condition.     

Spatial light modulation with an azobenzene-doped polymer by use of biphotonic holography

Spatial light modulation by use of a biphotonic holographic grating has been demonstrated in an azobenzene-doped polymer film. The mechanism of incoherent-to-coherent optical conversion originates from the photoinduced trans <--> cis isomerization of azobenzene molecules by two color lights. It is also found that when the intensity of the blue beam is varied the output replica of the input information changes sign.     

Reversible light-induced capacitance switching of azobenzene ether/PMMA blends

Fully reversible light-induced switching of the capacitance with changes larger than 50% has been achieved by using photo-active thin polymer films. The films consist of PMMA polymer blended with high concentrations of branched azobenzene ether dye molecules. Polymer thin sheet capacitors can be easily produced by spin coating and drop casting in various thicknesses ranging from 100 nm to 8 μm, respectively. Reversible capacitance switching is induced by illumination with UV and visible light, respectively. The capacitance change is correlated to the trans to cis isomerization of the azobenzene ether molecules and the alignment of the molecular dipoles in the film. Switching times depend on chromophore concentration, film thickness and light intensity used. The influence of wavelength of the light, of the temperature as well as of an electrical field applied during illumination on the capacitance change is addressed.     

一种新的双偶氮染料掺杂聚合物薄膜的光致双折射及偏振全息的研究

作为一种有可能作为永久信息存储的材料,合成出一种新的双偶氮材料(BA1)。当样品被波长为532nm的光激发时,几乎大部分BA1分子从反-反态转化到顺-顺态,产生了光致双折射。因此,研究了BA1分子掺杂的PMMA薄膜的光致双折射和透过信号与入射光强度的关系。实验结果表明:透过信号强度随着泵浦光的增强而增强。通过偏振态互相平行(SS)和垂直(SP)的两束偏振光用来研究偏振全息存储,结果表明SP光栅形成的衍射信号要比SS光栅的衍射信号强很多。     

Enhancing near-field photolithography by surface plasmon polaritons excited from negative-refractive-index nanofilms

We report a novel method for enhancing near-field nanophotolithography using negative-refractive-index slabs. In our method, a negative-refractive-index-metal film (NIF) is attached to the plane surface of an aplanatic solid immersion lens (SIL), and the polymer layer is coated on the negative-refractive-index film. The intensity of the transmitted light is enhanced by the surface plasmon polaritons excited in the NIF-polymer composite layer. The numerical results based on the vector diffraction theory show that under the illumination of the SIL-NIF-polymer composite by a sharp focused radially polarized beam, the composite configuration under investigation can substantially increase the spot intensity, suppress the spot expansion along the distance from the interface, and improve the depth of focus, in comparison with conventional focusing systems using bare SILs. In the case where a photoresist film is placed near the composite configuration, a subtle deep-etching pattern with high resolution can be achieved.     

金纳米颗粒光散射提高InAs单量子点荧光提取效率

采用光学方法确定InAs/GaAs单量子点在样品外延面上的位置坐标, 利用AlAs牺牲层把含有量子点的GaAs层剥离并放置在含有金纳米颗粒或平整金膜上, 研究量子点周围环境不同对量子点自发辐射寿命及发光提取效率的影响. 实验结果显示, 剥离前后量子点发光寿命的变化小于13%, 含有金纳米颗粒的量子点发光强度是剥离前的7倍, 含有金属薄膜的量子点发光强度是剥离前的2倍. 分析表明在金纳米颗粒膜上的量子点荧光强度的增加主要来自于金纳米颗粒对量子点荧光的散射效应, 从而提高量子点发光的提取效率.     

中皮秒二波耦合及其应用

本文报道在InP:Fe中皮秒二波耦合的实验研究.文中测量了瞬态二波耦合增益随泵浦光与探测光相对延迟时间改变而变化的曲线,分析了瞬态二波耦合中各种过程的特性,最后还利用皮秒二波耦合实现了低开启光强、高信噪比,透过率达17%的皮秒光开关.     

线偏振高斯光束通过复合型衍射光栅的传输特性

提出一种集合浮雕和折射率周期分布调制的复合型衍射光栅,并利用角谱表示和严格的模式理论研究了线偏振高斯光束通过这种复合型衍射光栅的传输特性.数值计算结果表明,在光栅的同一透射深度处复合型衍射光栅光强的起伏频率要比浮雕型光栅光强的起伏频率小.最后使用复合型衍射光栅模型,研究了亚波长体积相位全息光栅的表面起伏和入射光束的束腰宽度对衍射效率的影响. 关键词： 复合型衍射光栅 亚波长体积相位全息光栅 高斯光束 严格模式理论     

Direct Fabrication Methods of Surface Relief Electro-Optic Gratings in Azo-Polymer Films

Two fabrication methods of surface relief electro-optic (EO) gratings in azo-polymer films are described: surface deformation by a continuous wave laser and laser ablation by a pulse laser. In the fabrication process, the periodic intensity modulation of laser beams is formed by an interference fringe or a phase mask. The surface relief EO grating is fabricated by poling the surface relief grating (SRG) after the SRG fabrication process. The linear dependence of an applied voltage in the modulation efficiency of the first-order diffraction beam is observed. The proposed methods are effective for highly-efficient surface relief EO grating fabrication.     

Nanosphere Templated Metallic Grating Assisted Enhanced Fluorescence

In this paper, enhanced fluorescence from a silver film coated nanosphere templated grating is presented. Initially, numerical simulation was performed to determine the plasmon resonance wavelength by varying the thickness of the silver film on top of a monolayer of 400 nm nanospheres. The simulation results are verified experimentally and tested for enhancing fluorescence from fluorescein isothiocyanate whose excitation wavelength closely matches with the plasmon resonance wavelength of the substrate with 100 nm silver film over nanosphere. The 12 times enhancement in the intensity is attributed to the local field enhancement in addition to the excitation of surface plasmon polaritons along the surface.     

Feedback method of the noise suppression in wave-mixing amplifiers based on non-linear materials with photorefractive response in a reflection grating configuration

In the photorefractive wave-mixing system, fluctuation in the signal beam intensity of the photorefractive output with a reflection grating has been analyzed by employing pump feedback method. In this method, fluctuations of the photorefractive wave-mixing process not only induce the intensity fluctuation of the mixing waves but also induce phase fluctuation of the mixing waves. Thus, the phase of the pump and signal beams at the output surface fluctuates in time around a mean value. Using such a positive feedback method of a pump beams, the relative fluctuation in the photorefractive output signal beam intensity with respect to its mean intensity can be minimized significantly without reducing its mean intensity. The factors that control the fluctuation in the signal beam intensity, such as the phase fluctuation of the output pump beam, absorption strength of the material and the feedback reflectivity of the cavity mirrors, on the relative fluctuation of the output signal intensity in the photorefractive wave-mixing systems have been studied in detail. It has been found that the fluctuation of the output signal intensity relative to its mean intensity in the photorefractive wave-mixing system can be suppressed to larger extent by taking lower value of feedback reflectivity of the cavity mirrors which could exist at a higher value of absorption strength of the photorefractive materials.     

Effect of photoconductivity and dielectric constant of the photorefractive materials on two-beam coupling gain and phase-shifts for a single unidirectional photorefractive ring resonator

Photoconductive dependence of two-beam coupling between the pump beam and the signal beam in photorefractive materials have been analyzed in case of non-degenerate wave mixing under the undepleted pump approximation method. During the two-wave mixing in photorefractive materials, steady state amplification of the signal beam and oscillation characteristics of a single unidirectional ring resonator has been studied. The domination of the two-beam coupling gain over the combined absorption and resonator losses such as Fresnel reflections from the crystal and imperfect mirrors builds up unidirectional oscillation. The buildup of such an oscillation leads to a saturation of the gain, which can be explained in terms of the photorefractive phase-shift. The existence of this phase-shift between the photorefractive index grating and the illumination intensity pattern, which is of characteristic of the photorefractive effect, leads to an energy transfer between the two beams. For a single unidirectional ring resonators, the effects of photoconductivity of the materials, two-beam energy coupling coefficient, dielectric constant, crystal thickness, and material's absorption coefficient on amplification of the two-beam coupling gain and photorefractive phase-shifts of the signal beam have also been studied in detail. It has been found that amplification of the signal beam and phase-shift can be enhanced by taking the photorefractive crystal having higher photoconductivity and lower dielectric constant, which improves performance of the resonators.     

在光致聚合物中采用脉冲激光记录全息图的记录条件研究

在蓝绿敏光致聚合物材料中,用532 nm激光器作为光源,在等光强不同重复频率和等重复频率不同光强条件下分别进行全息记录,用红光进行动态读出,获得两种记录条件下的衍射效率曲线.实验结果表明,在近连续脉冲光下进行全息记录时,较低重复频率和较低光强的脉冲激光能够明显提高光致聚合物光栅的最高衍射效率,通过优化脉冲激光的重复频率和平均光强,可以在材料中记录衍射效率最高的全息图.     

Optical bistability based on surface plasmon coupled between a noble metal film and a metal grating involving Kerr materials

A phenomenon about optical bistability is successfully investigated in a layered structure consisting of a silver film with Kerr medium and a silver grating sandwiched between semi-infinite linear dielectrics.This type of structure can lead to the optical bistability phenomena occurring in reflection and transmission.There exists an optimal thickness of the metal grating that can cut off the effect of the near-field enhancement and may have the lowest effect on conversion from surface plasmon to light.This structure can realize the functions of the beam splitter and the polarization splitter and will be essential for future classical and quantum information processing.     

Polarization properties in two wave mixing with moving grating in photorefractive BSO crystals. Application to dynamic interferometry

The polarization properties of the volume gratings in photorefractive B₁₂SiO₂₀ crystals permit to increase the contrast and signal to noise ratio in the image plane of a large size vibrating object structure. The experiment is based on the nearly degenerate two wave mixing of a pump beam with a low intensity signal beam due to the object structure (moving grating recording with an externally applied field in the 110 crystallographic direction). For a correct orientation of the incident recording beam polarizations, the pump beam emerging from the crystal is linearly polarized, while the signal, after two beam coupling is elliptically polarized. The selective attenuation of the direct pump and scattered beams is obtained with a polarizer. Improved contrasts in the interferogram plane of the vibrating object are demonstrated.     

Polarization storage by two-photon-induced anisotropy in bisazobenzene copolymer film

A copolymer containing bisazobenzene chromophores, poly[(methylmethacrylate)-co-4-{(2-methacryloyloxyethyl)oxy}-4′-(4-nitro-phenylazo)azobenzene)] (poly(MMA-co-M2BAN)), was synthesized and used for two-photon-induced polarization storage. Based on two-photon-induced optical anisotropy resulting from photoselective isomerization of bisazochromophores, the data have been recorded by linearly polarized Ti:Sapphire laser (wavelength: 800 nm, pulse duration: 80 fs, and repetition rate: 80 MHz) and read by reflection confocal laser scanning microscope. The recorded bits are dark dots when the polarization of the reading beam is parallel to the recording beam, but the recorded bits become bright dots when the polarization of the reading beam is perpendicular to the recording beam. Two letters can be encoded in the same region of a given layer by separating the two polarization directions of recording beams at an angle of 45°. The relationships between the readout intensity of the recorded bits and the recording power under two different readout modes were investigated and the results showed that there are two opposite change trends for parallel readout and perpendicular readout, respectively, and the readout threshold value is around 12 mW.     

High repetition rate mid-infrared generation with singly resonant optical parametric oscillator using multi-grating periodically poled MgO:LiNbO3

A high repetition rate mid-infrared singly resonant optical parametric oscillator (OPO) using MgO-doped multi-grating periodically poled LiNbO₃ (MgO:PPLN) is demonstrated. A 1064 nm Q-switched Nd:YVO₄ laser at 10 kHz repetition rate and pulse width of 17.8 ns was used to pump the OPO. The period of the quasi-phase matched (QPM) grating in the multi-grating MgO:PPLN chip varied from 25.5 to 31.5 μm in steps of 0.5 μm. This corresponds to the generation of a signal beam from 1.37 to 1.64 μm and an idler beam from 3.0 to 4.8 μm, respectively. A maximum signal power of 250 mW and idler power of 140 mW has been obtained with an input pump beam of power 1.92 W, for a grating period of 30.5 μm. A maximum optic-optic conversion efficiency of 20% and 7.4% in the idler has been observed. It has been observed that the output power increases as the period of the grating increases.     

Temperature and orientation dependence of surface relief gratings based on dye-doped polymer film with the interface of nematic liquid crystals

The formation of surface relief grating on dye-doped polymer film with the interface of nematic liquid crystals has been investigated by means of the holographic technique. The first-order diffraction efficiency of surface relief grating depends on the temperature and the orientation of molecular director in the interface of nematic liquid crystals. The diffraction efficiency is roughly independent of thermal fluctuations of molecular director in the most part of nematic temperature range and apparently drops near the transition temperature. The morphology of surface relief grating demonstrates that the surface modulation is larger for molecular director parallel to the groove direction. The experimental result also shows that the first-order diffraction efficiency is dependent on the surface modulation of surface relief grating.     

Formation of positive and negative anisotropic holographic gratings depending on recording energy in photoreactive liquid crystalline copolymer films

Anisotropic diffractive gratings formed by birefringent molecular alignment are fabricated by a holographic technique using a photo-cross-linkable liquid crystalline copolymer (PLCCP). Depending on the exposure energy used for holographic recording, bright or dark regions of the interference fringe can be chosen for the molecular-aligned region, analogous to positive- and negative-type photoresist processing in photolithography. In conjunction with simultaneously generated surface relief gratings, the anisotropic refractive index modulation inherent in the PLCCP film exhibits different diffraction properties depending on the incident beam polarization. The anisotropic refractive index grating profiles are quantitatively presented for two cases determined by the exposure energy, based on the Fourier analysis of diffractive optics. The selection of different grating formations selected by varying only the exposure energy could provide a strategy for the fabrication of functionalized diffractive optical elements.