Super-Resolution of Interference Pattern with Independent Laser Beams

We propose a scheme to achieve super-resolution of interference pattern with independent laser beams. We perform an experimental observation of a double-slit interference with two orthogonally polarized laser beams. The resolution of the interference pattern measured by a two-photon detection is doubled provided the two beams illuminate the double-slit with certain incident angles. The scheme is simple and can favour both high intensity and perfect visibility.     

Intensity clamping during dual-beam interference

One has prospected to overcome the intensity clamping phenomenon during femtosecond laser filamentation by superimposing two beams. This experimental scheme is thus studied in our work. We have realized the interference of two ultrashort laser beams and one of them creates a filament in air. The formation of a dynamic plasma grating induced by the interference pattern is observed. However, qualitative analysis of the experimental results indicates that intensity clamping still governs the intensity evolution during the dual-beam interaction. The expectation of greatly increasing laser intensity by dual beams superposition might not be realistic.     

三光束激光干涉光刻的实现方法

分析了多光束空间分布产生的误差对图形的影响,通过计算优化得到三光束产生的干涉图形在整个面内有着更好的图形稳定性。利用氦镉激光光源通过特定的光学系统形成空间分布近似旋转对称的三束光, 对光致抗蚀剂进行干涉曝光,制作出了周期600 nm、高度350 nm的蜂窝状点阵,测量结果表明该系统具有很好的图形重复性和稳定性,同时降低了对于光学光路的精密性要求。     

Laser surface patterning using a Michelson interferometer and femtosecond laser radiation

We report on a simple method to obtain surface gratings using a Michelson interferometer and femtosecond laser radiation. In the optical setup used, two parallel laser beams are generated using a beam splitter and then focused using the same focusing lens. An interference pattern is created in the focal plane of the focusing lens, which can be used to pattern the surface of materials. The main advantage of this method is that the optical paths difference of the interfering beams is independent of the distance between the beams. As a result, the fringes period can be varied without a need for major realignment of the optical system and the time coincidence between the interfering beams can be easily monitored. The potential of the method was demonstrated by patterning surface gratings with different periods on titanium surfaces in air.     

在聚合物表面刻写方格子的新方法及理论研究

为了提高制作偶氮苯微结构的实验技术,在偶氮苯聚合物的光致异构和光致取向特性的基础上,利用两激光束的新实验方案,运用比较简便的分步制作的方法,成功的在偶氮苯功能化聚合物薄膜表面“刻写”出二维的正方格子结构。提出了一个新的光异构取向场理论：在两束偏振方向互相垂直的偏振光照射下,偶氮苯聚合物表面光栅的形成是光异构取向场的干涉的结果,并对实验现象进行了解释。用计算机对实验结果进行了模拟,将模拟结果与实验结果进行了比较,结果表明计算机模拟将会更加有利于偶氮苯聚合物微结构的设计和制作。     

Peculiarities of spontaneous grating generation in light-sensitive waveguide layers during holographic grating formation

The generation of spontaneous (noise) gratings during the simultaneous formation of a holographic grating in thin (on the order of the cutoff thickness of the TE₀ waveguide mode) AgCl-Ag films by two laser beams with λ = 532 nm and polarization vectors that make an angle of 45° with the plane of incidence have been studied. The electron microscopy images and diffraction patterns have revealed a significant difference of the spontaneous-grating structure from the structure obtained under irradiation by one laser beam. The spontaneous gratings have a significant spread in the directions of their wave vector, and the diffraction pattern (recorded using a probe beam with λ = 337 nm) has the form of a bundle of diverging arcs that intersect at one point. This difference is caused by self-diffraction from the holographic grating, which is responsible for the main diffraction peaks with odd orders, the growth of spontaneous gratings because of the interference of the beams diffracted by the holographic grating with the waveguide TE₀ modes scattered in the film, and silver transport to the interference minima. The diffraction patterns are quantitatively analyzed and the period of the new (formed under two-beam irradiation) spontaneous gratings is calculated. The calculation results are in good agreement with the experimental data.     

Designing of interference pattern in ultra-short pulse laser processing

We have studied material processing by using an interfering ultra-short pulse laser. In this process, an interference pattern is transferred to material surface, and a periodic structure can be generated in a single shot of laser irradiation. Here, it is well known that the period of the interference pattern can be controlled by correlation angle and wavelength. In addition, a composite, doubled in density or designed interference pattern can be formed by controlling the phase shift and variation of amplitude between the interfering laser beams. In this paper, basic patterns of interference are summarized in the case of four-beam correlation.     

双激光束干涉条纹处理方法的研究

在分析了双激光束干涉条纹特点的基础上 ,提出了基于遗传算法的最小二乘非线性曲线拟合法 ,以实现对双激光束干涉条纹的高精度细分定位。该算法以高斯调制余弦平方函数为拟合模型 ,为克服目标函数在参数空间上存在多极 (小 )值现象 ,引入了具有全局优化能力的遗传算法以实现对目标函数寻优。仿真及实验研究表明 :该方法用于双光束干涉条纹处理具有很好的鲁棒性和自适应能力     

Periodic sub-micrometric structures using a 3D laser interference pattern

A method to obtain three-dimensional sub-micrometric periodic structures is presented. The experimental set-up consists in a pulsed UV laser beam source (λ = 355 nm) coming into an interferometer in order to generate four beams converging inside a chamber. According to the directions, to the relative intensities and to the polarizations of these four beams, a 3D interference pattern can be obtained inside the overlapping volume of these four beams; the characteristics of the four laser beams have been optimized in order to obtain a maximal contrast of intensity. In order to visualize the interference pattern, its contrast and its stability at each laser pulse, a video camera coupled to an oil immersion microscope objective has been installed above the interferometer. By suppressing the central beam, it is also possible to generate a bidimensional interference pattern which defines an hexagonal structure in the (1 1 1) plane with a period of 377 nm.This optical set-up has been used to obtain 3D sub-micrometric periodic structures in negative photoresists. Experiments consist in a one- or multi-pulse irradiation of the photoresist followed by a development procedure which leads to a sub-micrometric face-centred cubic structure cut in a (1 1 1) plane with a cell parameter of 650 nm. The optimization of the experimental conditions is presented for two kinds of photoresists; the role of the substrate according to its reflectivity at the laser wavelength and its influence on the interference pattern is also discussed.     

Change of interference pattern using fundamental and second-harmonic wavelengths by phase shift of a beam

Interference pattern of two different wavelengths, forming a four-sided pyramid consisting of eight beams, is discussed. Synchronization between two wavelengths having a multiple relationship, which corresponds to mixing of fundamental and second-harmonic generation wavelengths, forms a stationary interference pattern successfully. Phase shift variation of a beam changes the basic pattern of interference. This technique can be a new scheme for multi-wavelength material processing with periodic structure and will increase the variation of meta-atoms fabricated by interfering femtosecond laser processing.     

无衍射光的干涉实验与理论分析

分析了两束无衍射光的干涉场分布形式和干涉条纹轨迹。将一束单色光入射两小孔产生的两束相干光照射轴锥镜,在轴锥镜后将产生两束无衍射光。根据单束倾斜光入射轴锥镜的无衍射理论,分析出这两束无衍射光产生的干涉场为每束无衍射光的无衍射场的线性叠加。利用零阶贝塞尔函数的零点公式,推导出两束无衍射光的干涉条纹的轨迹为双曲线。计算结果表明,干涉场中两中心的间距与两孔实际的间距和干涉场距轴锥镜的距离成正比。实验结果与理论仿真相一致。     

Driving and analysis of micro-objects by digital holographic microscope in microfluidics

We propose an optical configuration in which floating particles in a microfluidic chamber can be characterized by an interference microscopy configuration to obtain quantitative phase-contrast maps. The configuration is simply made by two laser beams from the same laser source. One beam provides the optical forces for driving the particle along appropriate paths, but at same time works as the object illumination beam in the holographic microscope. The second beam plays the role of the reference beam, allowing recording of an interference fringe pattern (i.e., the digital hologram) in an out-of-focus image plane. The system and method are illustrated and experimental results are offered for polymeric particles as well as for in vitro cells with the aim to demonstrate the approach.     

三光束飞秒激光干涉在GaP,ZnSe表面诱导二维复合纳米-微米周期结构

报道了三光束飞秒激光干涉在GaP和ZnSe晶体表面诱导二维复合纳米-微米周期结构.改变三束光的偏振组合方式,可以得到不同的纳米-微米复合结构.理论计算了相应偏振条件下光场强度分布、椭偏度分布和偏振方向分布.实验和理论计算结果表明,烧蚀斑上的微米长周期结构是由三光束干涉的强度花样决定,短周期纳米结构是由光场的偏振干涉花样决定.这些研究在纳米材料制备、超高密度光存储以及材料特性周期性调制等方面有很大的应用前景.     

非线性变周期全息光栅的制作

叙述一种用发散光束来的制作变周期光栅的方法,简单的系统保证了两束形成全息干涉的光强相等这一点对获得高反衬结构的光栅图形是重要的。在显影过程中用He-Ne激光作为衍射效率的在位监控。     

Second-order temporal interference of two independent light beams at an asymmetrical beam splitter

The second-order temporal interference of classical and nonclassical light at an asymmetrical beam splitter is discussed based on two-photon interference in Feynman's path integral theory. The visibility of the second-order interference pattern is determined by the properties of the superposed light beams, the ratio between the intensities of these two light beams, and the reflectivity of the asymmetrical beam splitter. Some requirements about the asymmetrical beam splitter have to be satisfied in order to ensure that the visibility of the second-order interference pattern of nonclassical light beams exceeds the classical limit. The visibility of the second-order interference pattern of photons emitted by two independent single-photon sources is independent of the ratio between the intensities. These conclusions are important for the researches and applications in quantum optics and quantum information when an asymmetrical beam splitter is employed.     

Fabrication of magnetic nanostructures by direct laser interference lithography on supersaturated metal mixtures

We present the formation of magnetic nanostructures through the annealing of PLD-deposited Fe-Cr and Co-Ag thin film mixtures by the interference of pulsed laser beams. Both these materials systems have a large miscibility gap under equilibrium conditions. The structures obtained were studied with atomic force and magnetic force microscopy. A pattern of magnetic dots was successfully fabricated on the Fe-Cr sample, but attempts to produce a similar pattern on the Co-Ag film were unsuccessful. The conclusion is drawn that the observed transformations of the film materials under laser heat treatment took place through a local melting of the film in the maxima of the interference pattern. The magnetic properties of the resulting structure are considered to be defined by the liquid state miscibility of the film components.     

Spatial coherence properties of monochromatic electromagnetic beams and of laser modes

It is shown that, contrary to common belief, monochromatic light beams are, in general, not spatially completely coherent, i.e., they will, in general, not produce fringes of unit visibility in a Young's double pinhole interference experiment. We cite experiments with laser modes which confirm this result.     

Oscillatory mode coupling and electrically strobed gratings in photorefractive quantum-well diodes

Oscillatory mode coupling between two coherent laser beams is produced when an interference pattern moves against a quasi-static electrically strobed grating in a photorefractive AlGaAs/GaAs multiple-quantum-well diode operated in the quantum-confined Stark geometry. The oscillation frequency is equal to the frequency difference between the two laser beams and provides a method to measure high-frequency Doppler shifts or large surface displacements for laser-based ultrasound. Combined photorefractive gains (normally forbidden by symmetry in the Stark geometry) and absorptive gains approach 1200cm(-1)during two-wave mixing using moving gratings.     

两路光纤激光器自组织相干合成实验研究

 建立了两路光纤激光器自组织相干合成的理论模型，并进行实验验证。结果表明，通过能量的相互注入，可以实现光纤激光器的自组织相干合成，合成效率达到92%。当外界基本没有干扰时，远场光斑为稳定、高对比度的干涉条纹，激光器阵列输出模式为异相模，与理论分析的结果一致；当外界环境存在轻微干扰时，实验出现条纹移动现象，输出模式也不再是恒定的异相模。对外界干扰对自组织相干合成的影响进行了数值分析，结果表明：外界干扰的幅度与频率越大，两束激光的相位差的变化幅度与频率也就越大；当外界干扰超过一定程度，两束激光之间的相位差将不能被锁定，始终处于变化状态。