Synthesis of nanoparticles and suspensions by pulsed laser ablation of microparticles in liquid

Recent studies demonstrated that the process to produce metal and oxide nanoparticles by laser ablation of consolidated microparticles is a convenient and energy-efficient way to prepare nanoparticles. In this work, the novel process is applied to nanoparticle synthesis in the liquid environment and the results are compared with those by the gas-phase process. Metal and oxide nanoparticles are synthesized by pulsed laser ablation of the compacted metal microparticles using a Q-switched Nd:YAG laser in water. It is shown that the process is effective for preparing nanoparticle suspensions having relatively uniform size distributions. While the laser fluence and the degree of compaction strongly influence the size of the produced nanoparticle in air, the sedimentation time is shown to be the most critical factor to determine the mean size of the suspended particles.     

向列相液晶中光学相位共轭技术的研究进展

对向列相液晶中光学相位共轭技术的研究进展进行了介绍,分析总结了向列相液晶中光学相位共轭过程的机理与实验方法,并对向列相液晶中的光学相位共轭技术的应用前景进行了展望。     

Non-reciprocal transmission via phase conjugation in multimode optical fibres

We demonstrate phase conjugate correction of modal distortion of a single mode beam on double-passing a passive highly multimode fibre. Phase conjugation is achieved in a self-pumped photorefractive BaTiO₃ crystal which conjugates only the e-polarised input component. We show that only a small fraction of the multimode fibre output is required to faithfully conjugate the single mode input beam. This technique enables the demonstration of a novel non-reciprocal transmission device, which for example has immediate application in gain-grating holographic resonator designs with multimode fibres as the active gain medium.     

Optical phase conjugation by resonant degenerate four-wave mixing in liquid phase medium

In this paper, frequency-doubled Nd:YAG laser's phase conjugation has been investigated experimentally using resonant degenerate four-wave mixing (DFWM). The iodine solution was used as resonant medium. In this way, the energy of the DFWM phase conjugation beam arrived at 43μJ/pulse while the total pump beam's energy reached 8mJ/pulse. The relation between the pulse energy of the DFWM phase conjugation beam and that of the pump beam was investigated. We also measured the optical field distributions of pump beam and DFWM phase conjugation beam, from which it can be noticed that the DFWM phase conjugation can improve the laser beam quality. At the same time, we observed the influence of the pump beam's disturbance on the DFWM signal.     

Optical information storage using refresh via phase conjugation

In this paper we discuss the possibility for realizing an optical memory using dynamic refreshment. Via phase-correct back-coupling by means of nonlinear optical phase-conjugation, the information stored in a photorefractive crystal is incessantly read out, transmitted into an auxiliary memory and from this back into the crystal again and in this way refreshed. Practical realizations and first results are presented.     

Optical phase conjugation in the hybrid polymer liquid crystal panel

In this paper, we present experimental results of the optical phase conjugation (PC) process in nematic liquid crystal cell with a photoconducting polymer layer (PVK doped with TNF). We analyze and discuss the influence of a few parameters (applied voltage, power of the laser beams, and geometry of an experiment) on conjugation efficiency. We also present experimental results showing possible application of the optical phase conjugation phenomenon in correction of phase distortion and aberration.     

Phase conjugation by reflection gratings in electrooptic crystals

Phase conjugation by reflection gratings in electrooptic crystals in investigated in a nonlinear model proposed by Kukhtarev and Semenets [1] which takes into account not only photoconductivity but also dark conductivity. The coupled-wave equations of the model can be solved analytically. To take the boundary conditions into account, numerical calculations are unavoidable. These can be performed, however, without any approximation. Phase mismatch and self-oscillation effects are dealt with in the undepleted pump approximation.Two of us (KHR and GT) would like to dedicate this paper to Prof. Dr. H. Salecker on the occasion of his 65^th birthday     

Efficient and fast optical phase conjugation by use of two-photon-induced gratings in the orientation of angular momentum

Optical phase conjugation (OPC) is of interest for many applications. The generation of squeezed light, phase-conjugate mirrors, optical correlation, and turbulence correction would all benefit from improved OPC performance. Using Zeeman sublevels and cross-circularly polarized light in rubidium vapor, we demonstrate an OPC process that uses very low power (30 mW) but is still very fast (60 ns) and efficient (gain of 20). This process is generic enough to be applicable to almost any resonant medium.     

Diagnostics and doppler tomography of liquid flows with ultrasonic phase conjugation

Acoustical Physics - The potential of ultrasonic velocimetry and mapping for liquid flows in a model and real biological media is demonstrated experimentally and theoretically using the technique...     

Phase conjugation via unequal amplitude multiple gratings in photorefractives: A simple shooting method for numerical solution

We present a simple shooting method to obtain numerical solutions of the nonlinear coupled-wave equations for degenerate four-wave mixing in photorefractive crystals when four different types of the index gratings of unequal amplitudes are responsible for phase-conjugation. Our analysis includes the effects of pump depletion and absorption in the medium. Intensities of the four beams, both inside and at the output surface, are obtained as a function of four unequal coupling strengths and absorption coefficient. Intensity of the generated phase-conjugate beam is shown as a function of both the small and wide ranges of signal beam intensity. It is found that the multigrating operation with a careful choice of coupling constants and their combinations may improve the phase-conjugate wave generation. Also the presence of absorption reduces the phase-conjugate intensity more significantly when all the gratings are operative than when a single grating is allowed. Numerical results obtained from the computer calculations are presented in graphical form.     

Mechanism of phase conjugation via stimulated Brillouin scattering in narrow band gap semiconductors

We develop a theoretical model to study optical phase conjugation via stimulated Brillouin scattering (OPC-SBS) in narrow band gap transversely magnetized semiconductors. Threshold value of pump electric field and reflectivity of the image radiation for the onset of OPC-SBS are estimated. The analysis is applied to both cases viz. centrosymmetric (CS) and non-centrosymmetric (NCS) crystals. Numerical estimates made for n-type InSb crystal at liquid nitrogen temperature duly irradiated by nanosecond pulsed 10.6 μm CO₂ laser shows that high OPC-SBS reflectivity (90%) can be achieved in NCS crystals at moderate pump electric fields if the crystal is used as an optical waveguide with relatively large interaction length (L = 5 mm) which proves its potential in practical applications such as fabrication of phase conjugate mirrors.     

Switchable gratings by spatially periodic alignment of liquid crystals via patterned photopolymerization

Spatially periodic patterning of the anchoring condition of a nematic liquid crystal (NLC) within a polymer matrix via a patterned photopolymerization affords a novel and facile method to prepare electrically switchable diffraction gratings. UV irradiation through a photomask of two comonomers, with opposite tendencies to align the NLC and also with different reactivity ratios, leads to definition of areas with either homeotropic or planar alignment of the NLC. Photopolymerization-induced diffusion of the monomers accounts for the spatial distribution of the concentration of these monomers. The resulting diffraction gratings are switchable under low electric fields and possess structural stability offered by the polymer matrix.     

Rewritable self-assembled long-period gratings in photonic bandgap fibers using microparticles

We report the demonstration of rewritable self-assembled long-period gratings in air-core photonic bandgap fibers. The long-period gratings are written by filling the air-core region of the fiber with a solution containing polystyrene microspheres. The microspheres are self-assembled into a periodic structure as the liquid inside the fiber evaporates, forming the long-period grating. The formed grating can then be easily erased for rewriting purposes by connecting the end of the fiber to a microfluidic pump through a microfluidic channel and washing out the microparticles with an appropriate liquid. The same microfluidic system can also be used to fine control the writing process of the gratings. This technique creates possibilities for writing various different filtering functions in air-core photonic bandgap fibers by varying the composition and/or the shape of the used microparticles. The presented technique is potentially applicable to solid-core photonic crystal fibers as well.     

Properties of higher-order phase transitions

Experimental evidence for the existence of strictly higher-order phase transitions (of order three or above in the Ehrenfest sense) is tenuous at best. However, there is no known physical reason why such transitions should not exist in nature. Here, higher-order transitions characterized by both discontinuities and divergences are analysed through the medium of partition function zeros. Properties of the distributions of zeros are derived, certain scaling relations are recovered, and new ones are presented.     

Quality of phase conjugation in silicon

The quality of phase conjugates generated by real-time holography in silicon is assessed interferometrically. The conjugate phase is found to be degraded by about a half wave distortion due to self-defocussing. We infer that the nonlinearly- induced gratings decay at a rate much faster than the value suggested previously.     

Self-pumped phase conjugation in a liquid crystal light valve with a tilted feedback mirror

We report self-pumped phase conjugation in a photorefractive liquid crystal light valve with a tilted feedback mirror. This is a new scheme for the self-generation of a phase-conjugate replica and can in general be applied to thin nonlinear layers, opening the way to distortion-correction applications of thin media.     

高密度相位光栅的偏振选择性

高密度光栅具有与传统光栅不同的性质,其衍射特性往往是偏振相关的。本文针对1550nm波长TE/TM偏振入射光和0.5的光栅占空比,利用严格耦合波分析数值计算了不同光栅周期下0级及-1级的衍射效率。研究表明,相比周期为1550nm的光栅,当周期为1200nm时,偏振相关衍射效应明显增强,当光栅周期为890nm时,TE偏振光的衍射效率随着光栅深度呈正余弦变化,而TM偏振光的衍射效率始终集中在0级,具有偏振选择性。通过模式方法,利用模式中的有效折射率概念,研究了不同周期下被入射光所激发的两种光波模式通过光栅区域传播所累积的相位差;基于双光束干涉,模拟了0级和-1级的衍射效率。结果表明,利用严格耦合波分析的数值计算结果符合模式方法的理论预期,对于高密度相位光栅的偏振选择性给予了合理的物理机制解释。