高衍射效率亚波长结构Dammann光栅的设计

传统的Dammann光栅是基于标量衍射理论设计的二值相位等光强分束器件，其衍射效率的典型值为80%左右.基于严格耦合波分析理论和遗传算法，提出了一种设计亚波长结构Dammann光栅的新方法，且该新型二值相位光栅具有较高的衍射效率.同时利用自编的仿真程序包设计了多个光栅，并分析了制作误差对其衍射效率和光强均匀性的影响.仿真结果表明，用该方法所设计的Dammann光栅的衍射效率超过92%.     

Comparative study of sub-micrometer polymeric structures: Dot-arrays, linear and crossed gratings generated by UV laser based two-beam interference, as surfaces for SPR and AFM based bio-sensing

Two-dimensional gratings are generated on poly-carbonate films spin-coated onto thin gold-silver bimetallic layers by two-beam interference method. Sub-micrometer periodic polymer dots and stripes are produced illuminating the poly-carbonate surface by p- and s-polarized beams of a frequency quadrupled Nd:YAG laser, and crossed gratings are generated by rotating the substrates between two sequential treatments. It is shown by pulsed force mode atomic force microscopy that the mean value of the adhesion is enhanced on the dot-arrays and on the crossed gratings. The grating-coupling on the two-dimensional structures results in double peaks on the angle dependent resonance curves of the surface plasmons excited by frequency doubled Nd:YAG laser. The comparison of the resonance curves proves that a surface profile ensuring minimal undirected scattering is required to optimize the grating-coupling, in addition to the minimal modulation amplitude, and to the optimal azimuthal orientation. The secondary minima are the narrowest in presence of linear gratings on multi-layers having optimized composition, and on crossed structures consisting of appropriately oriented polymer stripes. The large coupling efficiency and adhesion result in high detection sensitivity on the crossed gratings. Bio-sensing is realized by monitoring the rotated-crossed grating-coupled surface plasmon resonance curves, and detecting the chemical heterogeneity by tapping-mode atomic force microscopy. The interaction of Amyloid-β peptide, a pathogenetic factor in Alzheimer disease, with therapeutical molecules is demonstrated.     

Two-dimensional non-spatial filtering based on holographic Bragg gratings

The filter made up of two gratings performs as a two-dimensional non-spatial filtering. This paper reports that the volume Bragg gratings are fabricated by interfering two collimated coherent laser beams in photopolymer. Diffraction efficiency of a single grating is up to 78% in Bragg's condition, then a two-dimensional non-spatial filter, which consists of two volume Bragg gratings and a half-wave plate, enables the laser beam filtered in two dimensions with the diffraction efficiency of 54%. The Bragg's condition and effect of polarisation on performances of the two-dimension filter are also discussed.     

折射率周期分布取样光纤Bragg光栅的快速分析方法

基于Chebyshev恒等关系,提出了一种分析折射率周期分布取样光纤Bragg光栅(Sampled Fiber Bragg Grating,SFBG)的快速方法.这类SFBG由许多结构一致的折射率分布周期构成,对应计算过程中多次重复的幺模矩阵运算,计算效率低；利用Chebyshev关系可将传输矩阵的幂运算简化为一次矩阵运算,因而这种快速算法能有效地简化计算.应用此方法分析两种典型的周期性结构光栅（周期啁啾SFBG和二元相位SFBG）,数值结果充分验证了其计算速度上的优势.     

Linear and nonlinear propagation of light beams in two-dimensional photorefractive photonic lattices formed in lithium niobate

Special features of linear and nonlinear propagation of coherent light beams in two-dimensional photorefractive photonic lattices optically induced in the lithium niobate crystal doped with iron are experimentally investigated. The two-dimensional gratings are formed by consecutive recording of two one-dimensional photorefractive gratings in the crystal. Experiments were performed using He-Ne laser irradiation with a wavelength of 633 nm. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 58–62, September, 2006.     

Computer-generated holographic optical elements produced by photolithography

We report the creation of novel computer-generated holographic optical elements. The method of computing diffraction gratings producing a continuous focal pattern in the form of a line and also gratings that have N orders in a row of equal energy is described. The method has also been developed for coding the filter-complex transmittance functions in order to form and analyze the transverse modes. All the above elements are realized as glass-substrate binary-phase refractive holograms by projection photolithography by using photoreduction. The minimum line width is 2·5 mkm. These elements may also be fabricated as reflective holograms.

Phot-masks and the results of an investigation of the intensity distribution produced by the elements are presented. The investigation results confirm the synthesized elements' high quality.     

Quasi-Dammann grating with proportional intensity array spots

A quasi-Dammann grating is proposed to generate array spots with proportional-intensity orders in the far field. To describe the performance of the grating, the uniformities of the array spots are redefined. A two-dimensional even-sampling encode scheme is adopted to design the quasi-Dammann grating. Numerical solutions of the binary-phase quasi-Dammann grating with proportional-intensity orders are given. The experimental results with a third-order quasi-Dammann grating, which has an intensity proportion of 3:2:1 from zero order to second order, are presented.     

Femtosecond laser-fabricated microstructures in bulk poly(methylmethacrylate) and poly(dimethylsiloxane) at 800 nm towards lab-on-a-chip applications

Laser direct writing technique is employed to fabricate microstructures, including gratings (buried and surface) and two-dimensional photonic crystal-like structures, in bulk poly(methylmethacrylate) (PMMA) and poly(dimethylsiloxane) (PDMS) using ∼100 femtosecond (fs) pulses. The variation of structure size with different writing conditions (focussing, speed and energy) was investigated in detail. Diffraction efficiencies of the gratings were calculated and the changes in diffraction efficiency (DE) as a function of period, energy and scanning speed were evaluated. Highest diffraction efficiencies of 34% and 10%, for the first order, were obtained in PMMA and PDMS respectively. Heat treatment of these gratings demonstrated small improvement in the diffraction efficiency. Several applications resulting from these structures are discussed. Fs modification in PMMA and PDMS demonstrated emission when excited at a wavelength of 514 nm. We attempted to prepare buried waveguides in PMMA with higher refractive index at the core. We have successfully fabricated branched and curved structures in PMMA and PDMS finding impending applications in microfluidics.     

Imaging of flames and cold flows in air by diffraction from a laser-induced grating

Nonresonant laser-induced gratings are created in gases employing the second-harmonic output of a Nd: YAG laser in a degenerate four-wave mixing beam geometry. The diffraction efficiency of the gratings has been investigated as a function of laser intensity and gas pressure. Single-shot images of a helium flow in ambient air illustrate that diffraction of light from a laser-induced grating has the potential for remote, two-dimensional diagnostics of gas mixing processes. In addition, this coherent technique is used to image a sooty flame.     

Dependence of surface enhanced Raman scattering on the plasmonic template periodicity

Surface enhanced Raman scattering has been investigated from rhodamine 6G molecules embedded in polymethyl methacrylate (R6G+PMMA) and coated on one-dimensional and two-dimensional gold-dielectric gratings fabricated by laser interference lithographically. The Raman signals from these plasmonic templates are 200 to 400 times larger than the signal from R6G+PMMA coated on plain gold films. The enhancement of the Raman signal varies almost periodically with the period of the grating. Finite-difference time-domain simulations show that large electromagnetic near fields occur at the metallic edges due to the resonant excitation of localized surface plasmon of the gold patches by the pump laser. These give rise to large enhancements of the Raman signal. The dependence on period is due to the combined effects of the localized surface plasmon and the periodic grating that couples the pump laser to the surface plasmon polariton.     

Femtosecond laser color marking of metal and semiconductor surfaces

Color marking of rough or smooth metal (Al, Cu, Ti) and semiconductor (Si) surfaces was realized via femtosecond laser fabrication of periodic surface nanorelief, representing one-dimensional diffraction gratings. Bright colors of the surface nanorelief, especially for longer electromagnetic wavelengths, were provided during marking through pre-determined variation of the laser incidence angle and the resulting change of the diffraction grating period. This coloration technique was demonstrated for the case of silicon and various metals to mark surfaces in any individual color with a controllable brightness level and almost without their accompanying chemical surface modification.     

Photofabrication of periodic microstructures in azodye-doped polymers by interference of laser beams

Volume holographic gratings and two-dimensional periodic microstructures in azodye-doped polymethylmethacrylate were fabricated, respectively, by interference of two coherent beams of a femtosecond laser and by interference of three coherent beams of a nanosecond laser. The dependence of the first-order Bragg diffraction efficiency and the photoinduced refractive-index modulation of the gratings on the intensity of the writing light was investigated. The measurements of the absorption spectra before and after irradiation with the writing light suggest that the photoinduced gratings were refractive-index-modulated gratings, which arose from a photoinduced decomposition reaction of the azodye molecules through multiphoton absorption. In the experiments involving the interference of three beams, the period of the two-dimensional periodic microstructures was changed by adjusting the angle between the three writing beams. Received: 10 July 2002 / Revised version: 5 September 2002 / Published online: 20 December 2002 RID="*" ID="*"Corresponding author. Fax: +81-774/955206, E-mail: jhsi@photon.jst.go.jp     

利用干涉光刻技术制备LED表面微纳结构

为了制备大面积周期性微纳米结构以提高LED的发光效率,建立了劳厄德(Lloyd)干涉光刻系统。简单分析了该干涉光刻系统的工作原理,并介绍了利用干涉曝光工艺制备一维光栅、二维点阵、孔阵列等纳米结构图形的具体实验过程。最后对纳米图形进行结构转移,制备出了金属纳米结构。实验结果表明:利用劳厄德干涉光刻系统,可以在20 mm×20 mm大小的ITO衬底上稳定制备出周期为450 nm的均匀光栅或二维点阵列图形结构,它们的占空比也是可以调节变化的。     

Colorizing silicon surface with regular nanohole arrays induced by femtosecond laser pulses

We report on the formation of one- and two-dimensional (1D and 2D) nanohole arrays on the surface of a silicon wafer by scanning with a femtosecond laser with appropriate power and speed. The underlying physical mechanism is revealed by numerical simulation based on the finite-difference time-domain technique. It is found that the length and depth of the initially formed gratings (or ripples) plays a crucial role in the generation of 1D or 2D nanohole arrays. The silicon surface decorated with such nanohole arrays can exhibit vivid structural colors through efficiently diffracting white light.     

Optical characterization of PMMA phase gratings written by a 387 nm femtosecond laser

We report on the fabrication of optical Bragg type phase gratings in polymethyl methacrylate substrates by a femptosecond Ti: Sapphire laser. As for their optical characterization, a spatially resolved microscopy interferometric technique is used to investigate the two-dimensional distribution of the refractive index change produced by the irradiation process. The technique gives a direct and quantitative two-dimensional profile of the index of refraction in irradiated PMMA, providing information on how the fabrication process depends on the laser irradiation.     

Formation of two-dimensional periodic nanostructures on fused quartz,polyimide, and polycrystalline diamond by pulsed four-wave interference laser modification

Atomic force microscopy is used to examine the topography of submicron periodic structures formed on the surfaces of synthetic polycrystalline diamond and polyimide films. The films are deposited on fused quartz substrates by four-wave interference modification using a pulsed 308-nm UV XeCl excimer laser. It is demonstrated that a two-dimensional periodic relief with a submicron period can be formed on the diamond surface directly by laser evaporation in the absence of a photoresist. Depending on the exposure, two mechanisms of polyimide film modification are observed. At exposures less than 100 mJ/cm², the relief is formed due to swelling at the positions of interference maxima. At exposures greater than 100 mJ/cm², holes are formed in the films. A periodic relief on the fused quartz surface is formed by using a UV photoresist exposed to pulsed interference laser radiation and subsequent Ar ion etching.     

High resolution patterning of sapphire by F<Subscript>2</Subscript>-laser ablation

The ablation behavior of single crystalline sapphire with nanosecond laser pulses at 157 nm wavelength is investigated. Ablation rates of about 10 to 100 nm/pulse are obtained at fluences ranging from 1 to 9 J/cm². At moderate fluences, incubation behavior is observed, i.e. ablation starts after material modification by a number of laser pulses. The ablation can be utilized to fabricate sapphire micro-optics. The capability of creating lenses or gratings on the tip of sapphire fibers is demonstrated. Multilevel diffractive optical elements and high resolution gratings with 1 μm period are fabricated on planar sapphire substrates.     

Circular Dammann grating

A circular Dammann grating that can produce circular equal intensities at various orders in the far field is described. A set of parameters such as order, circular number, uniformity, and diffraction efficiency has been defined to describe the novel diffractive phase elements. Numerical solutions of binary-phase (0, pi) circular Dammann gratings are given. The results of experiments with a four-order circular Dammann grating made by a lithographic technique are presented. This novel diffractive optical element should be highly interesting in a wide variety of practical applications.     

Heterodyne polarimetry for mapping defects in lithography masks

A new optical technique based on the heterodyne polarimetry is developed for fast inspection of uniformity of lithography masks in semiconductor industry. Sub-wavelength periodical structure of a sample acts as a wire-grid polarizer, making both the amplitude and phase of the reflected laser beam dependent on geometrical dimensions and optical properties of the mask pattern. The heterodyne technology based on the cross-polarized two-frequency Zeeman laser is used to simultaneously measure the amplitude and the phase of the reflected laser beam. A two-dimensional map of spatial variations can be obtained via point-by-point scanning of the sample. The technique is applicable not only to exact periodical structures like diffraction gratings, but also to double-periodical patterns consisting of large number of periodically distributed small areas of sub-wavelength gratings. Theoretical background, simulation, and experimental results are presented.     

Digital volume gratings

An optical element constructed by stacking a set of binary-phase grating sheets can simulate the functions of optically recorded volume gratings. Our electromagnetic numerical study also shows that if one of the grating sheets is replaced by another one with different grating period, power spectrum of the diffracted wave changes completely with extra diffraction orders. This property will claim strong advantage in security document applications. Analysis of alignment error reveals interesting phenomena concerning to how misalignment affects diffraction efficiency.