Surface plasmon resonance spectroscopy on rotated sub-micrometer polymer gratings generated by UV-laser based two-beam interference

Two-beam interference method was applied to generate gratings having periods of 416 nm and 833 nm by the forth harmonic of a Nd:Yag laser on thin poly-carbonate films spin-coated onto silver layer-covered substrates. The dependence of the modulation depth on the fluence and number of laser pulses was investigated by atomic force microscopy. A secondary pattern appeared on very thin polymer layers thanks to the “p” polarized laser beam illumination induced self-organized processes. The conditions of the emergence of grating-coupling caused additional plasmon resonance peak were determined for the sub-micrometer periodic polymer gratings. Surface plasmon resonance measurements were performed in attenuated total reflection arrangement to determine the effect of the angle between the plasmon propagation direction and the polymer groves on the grating-coupling. The effect of the modulation depth on the grating-coupling caused additional resonance minimum was also analyzed. We found coupling phenomena according to our calculations, the differences between the measured and theoretically predicted resonance curves were explained by the scattering on the complex surface structure.     

Atomic force microscopical and surface plasmon resonance spectroscopical investigation of sub-micrometer metal gratings generated by UV laser-based two-beam interference in Au-Ag bimetallic layers

Metal films containing silver and gold layers having different thicknesses were evaporated on glass substrates. Two-beam interference technique was applied to irradiate the surfaces by the fourth harmonic of a pulsed mode Nd:YAG laser. The atomic force microscopical study showed that surface relief grating having a period of 900 nm corresponding to the interference pattern was developed on the metallic films. The modulation amplitude of the laser-induced gratings was increasable by enhancing the number of laser pulses at constant fluence, and a groove depth commensurable with the film thicknesses was generated at the average fluence of 39.5 mJ/cm² on bimetallic layers. The surface structure was more regular, and the modulation amplitude was larger in case of bimetallic films containing thicker gold layers. The threshold fluences of the phase transitions were determined by numerical temperature model calculations for different metal layer compositions, and a good agreement was found between the calculated and experimentally observed threshold values. The division of the metal stripes into droplets and the development of holes were explained by the melting of the entire metal layers and by the vaporization of silver at higher fluences. The angle-dependent surface plasmon resonance spectroscopy realized in Kretschmann arrangement proved that the laser-induced grating formation was accompanied by the change in the optical thickness and by the modification of the structure of the bimetallic films. Broad side wings appeared on the resonance curves caused by grating-coupling in case of appropriate rotation angle and sufficiently large modulation depth of the grating's grooves, according to our calculations. The coupling on deep gratings developed on bimetallic films containing the thinnest gold layer and on monometallic silver films resulted in separated secondary resonance minimum development. The periodic adherence of native streptavidin on the metallic gratings was detected by tapping mode AFM, and based on the shift of the secondary resonance peak.     

Plasmonic structure generation by laser illumination of silica colloid spheres deposited onto prepatterned polymer-bimetal films

Plasmonic structures are prepared on bimetal films evaporated onto glass substrates applying a multi-step process, and atomic force microscopy is utilized to study the structures after each step. Sub-micrometer gratings are generated on polycarbonate films spin-coated onto silver-gold bimetal layers by interference lithography (IL) applying the fourth harmonics of a Nd:YAG laser. These polymer gratings are used as prepatterned templates in order to deposit silica colloid spheres by spin-coating. It is shown that the conditions of periodic silica sphere-array formation along the template valleys are sufficiently large grating modulation depth, appropriate ratio of silica sphere diameter to grating period, and optimized speed of spinning. The periodic silica sphere arrays are illuminated by a homogeneous KrF excimer laser beam, and periodically arrayed sub-wavelength holes are drilled into bimetal films via colloid sphere lithography (CSL). The characteristic dimensions of the resulted plasmonic structures are defined by the polymer grating period and by the silica colloid sphere diameter. Attenuated total reflection spectroscopy is performed exciting plasmons on different metal-dielectric interfacial structures by the second harmonic of a continuous Nd:YAG laser. The polar and azimuthal angle dependent grating-coupling and scattering effects of the complex periodic structures on the resonance characteristic of plasmons is demonstrated.     

Surface plasmon scattering on polymer-bimetal layer covered fused silica gratings generated by laser induced backside wet etching

Large amplitude fused silica gratings are prepared by combining the UV laser induced backside wet etching technique (LIBWE) and the two-beam interference method. The periodic patterning of fused silica surfaces is realized by s-polarized fourth harmonic beams of a Nd:YAG laser, applying saturated solution of naphthalene in methyl-methacrylate as liquid absorber. Atomic force microscopy is utilized to analyze how the modulation amplitude of the grating can be controlled by the fluence and number of laser pulses. Three types of plasmonic structures are prepared by a bottom-up method, post-evaporating the fused silica gratings by gold-silver bimetal layers, spin-coating the metal structures by thin polycarbonate films, and irradiating the multilayers by UV laser. The effect of the bimetal and polymer-coated bimetal gratings on the surface plasmon resonance is investigated in a modified Kretschmann arrangement allowing polar and azimuthal angle scans. It is demonstrated experimentally that scattering on rotated gratings results in additional minima on the resonance curves of plasmons excited by second harmonic beam of a continuous Nd:YAG laser. The azimuthal angle dependence proves that these additional minima originate from back-scattering. The analogous reflectivity minima were obtained by scattering matrix method calculations realized taking modulation depths measured on bimetal gratings into account.     

Fabrication of 550 nm gratings in fused silica by laser induced backside wet etching technique

A series of 550 nm spacing gratings were fabricated in fused silica by laser induced backside wet etching (LIBWE) method using the fourth harmonic of a Q-switched Nd:YAG laser (wavelength: λ = 266 nm; pulse duration: FWHM = 10 ns). During these experiments we used a traditional two-beam interference method: the spatially filtered laser beam was split into two parts, which were interfered at a certain incident angle (2θ = 28°) on the backside surface of the fused silica plate contacting with the liquid absorber (saturated solution of naphthalene-methyl-methacrylate c = 1.85 mol/dm³). We studied the dependence of the quality and the modulation depth of the prepared gratings on the applied laser fluence and the number of laser pulses. The surface of the etched gratings was characterized by atomic force microscope (AFM). The maximum modulation depth was found to be 180-200 nm. Our results proved that the LIBWE procedure is suitable for production of submicrometer sized structures in transparent materials.     

Sudan IV dye based poly(alkyloxymethacrylate) films for optical data storage

The polymer poly{1-[2′-methyl-4′-(2″-methylphenylazo) phenylazo]-2-(m-methacryloyloxyoctyloxy}naphthalene, where m = 6, 8, 10, is synthesized by free radical addition polymerization method for holographic optical data storage. Characterization of the polymers is done by formation of the holographic grating. A study of the dependence of diffraction efficiency of the grating formed on various parameters is presented. Surface relief gratings on these polymer films are created upon exposure to argon ion laser beams at 514.5 nm without any subsequent processing steps. The surface structure of the relief gratings has been investigated by atomic force microscopy. The depth of surface relief in a typical case is found to be around 40 nm.     

基于电光聚合物的表面等离激元调制器

基于电光聚合物,提出了一种结构简单,尺寸小,效率高的表面等离激元(SPP)调制器.该调制器采用M-Z干涉仪结构的金属波导,金属周围是均匀极化后的电光聚合物,通过在金属波导两臂间加电压对聚合物折射率进行调制,折射率调制再通过M-Z干涉仪结构转化为对金属波导中SPP强度的调制. 通过求解金属波导附近的电场分布,并结合SPP场分布的特点,在理论上说明了这种结构可以通过外加电压有效地调制金属波导输出端SPP的强度,调制所需的半波电压约为2.8V. 关键词： 表面等离激元 电光调制 电光聚合物     

Fabrication of holographic diffraction gratings based on As<Subscript>2</Subscript>S<Subscript>3</Subscript> Layers

The formation of holographic diffraction gratings based on As₂S₃ layers is investigated. The variation of the groove profiles with exposure is studied by atomic force microscopy. The spectral curves of the diffraction efficiency are taken, and a relationship between these curves and grating surface relief is analyzed.     

Processing of Diamond for Integrated Optic Devices Using Q-Switched Nd:YAG Laser at Different Wavelengths

In the present investigation, a Q-switched Nd:YAG laser is used to study the various aspects of diamond processing for fabricating integrated optic and UV optoelectronic devices. Diamond is a better choice of substrate compared to silicon and gallium arsenide for the fabrication of waveguides to perform operations such as modulation, switching, multiplexing, and filtering, particularly in the ultraviolet spectrum. The experimental setup of the present investigation consists of two Q-Switched Nd:YAG lasers capable of operating at wavelengths of 1064 nm and 532 nm. The diamond cutting is performed using these two wavelengths by making the “V”-shaped groove with various opening angle. The variation of material loss of diamond during cutting is noted for the two wavelengths. The cut surface morphology and elemental and structural analysis of graphite formed during processing in both cases are compared using scanning electron microscopy (SEM) and laser Raman spectroscopy. Both the Q-Switched Nd:YAG laser systems (at 1064 nm and 532 nm) show very good performance in terms of peak-to-peak output stability, minimal spot diameter, smaller divergence angle, higher peak power in Q-switched mode, and good fundamental TEM₀₀ mode quality for processing natural diamond stones. Less material loss and minimal micro cracks are achieved with wavelength 532 nm whereas a better diamond cut surface is achieved with processing at 1064 nm with minimum roughness.     

Controllable grating fabrication by three interfering replicas of single femtosecond laser pulse

The controllable periodic M-shape gratings are fabricated on the surface of silica glass by three coplanar interfering beams from a single femtosecond pulse. The configuration of the M-shape periodic structure is characterized by optical microscopy and atomic force microscopy. The experimental results and the theoretical simulation show that the period and the modulation depth ratio between the neighboring grooves of the fabricated gratings can be controlled by adjusting the collision angles and pulse energy of the three beams, respectively.     

Structural and surface analysis of thin-film ZnTe formed with pulsed-laser deposition

Nanosecond pulses of a Nd:YAG laser have been employed to deposit thin-film zinc telluride (ZnTe) on silicon (Si) and glass without heating these substrates. We present and discuss the structural and surface properties of films deposited at 1064 nm and 532 nm. X-ray diffraction and analysis of the surface roughness with atomic force microscopy reveal that the material texture and surface morphology depend on the ablation laser line used rather than on the substrate. The observations contribute to improved understanding of pulsed-laser deposition and provide tools to optimize the optoelectronic and photonic properties of ZnTe thin-films as well as their incorporation into Si-based technologies in order to fabricate cost-effective and functional optoelectronic devices and all-optical laser digitizer.     

单脉冲纳秒激光诱导硅表面微结构

利用Nd:YAG纳秒脉冲激光(波长532nm)在空气中对单晶硅表面进行单脉冲辐照,研究了激光能量密度和光斑面积变化对微结构的影响。通过场发射扫描电子显微镜和原子力显微镜(AFM)对样品表征,并对纳秒激光辐照硅的热力学过程进行分析。结果显示:当脉冲激光的能量密度接近硅的熔融阈值且光斑直径小于8μm时,形成尖峰微结构;随着能量密度或光斑面积增大,尖峰结构消失,形成边缘隆起和弹坑微结构。通过流体动力学模型得到微结构形貌的解析解,模拟得到的微结构形貌与实验测得的AFM数据一致。研究表明微结构的形成主要是由于表面张力引起的熔融硅流动。表面张力与表面温度和表面活性剂的质量浓度有关。温度梯度引起的热毛细流作用和表面活性剂浓度引起的毛细作用共同影响下形成尖峰、边缘隆起和弹坑微结构。     

Direct Fabrication of Surface Relief Holographic Diffusers in Azobenzene Polymer Films

We propose a direct fabrication of light diffusers using azobenzene polymer films. Holographic recordings of surface relief diffusers are formed on the polymer film by exposure to an Nd:YAG laser (532 nm) through source diffusion materials. No post-treatment is needed, and it can be erased by heating or irradiating uniform laser beam. Diffusion pattern can be controlled by the polarization of the laser. Transmittance of over 88% is obtained at the wavelength of 633 nm.     

Laser-induced periodic surface structures on different poly-carbonate films

Laser-induced periodic surface structures (LIPSS) were generated on oriented and amorphous thick, as well as on spin-coated thin, poly-carbonate films by polarized ArF excimer laser light. The influence of the film structure and thickness on the LIPSS formation was demonstrated. Below a critical thickness of the spin-coated films the line-shaped structures transformed into droplets. This droplet formation was explained by the laser-induced melting across the whole film thickness and subsequent de-wetting on the substrate. The thickness of the layer melted by laser illumination was computed by a heat-conduction model. Very good agreement with the critical thickness for spin-coated films was found. The original polymer film structure influences the index of refraction of the thin upper layer modified by the laser treatment, as was proven by the dependence of the structure’s period on the angle of incidence both for ‘s’- and ‘p’-polarized beams. The effect of the original surface roughness – grains in thick films or holes in thin films – was studied using atomic force microscopy. It was shown that the oblique incidence of ‘s’-polarized beams results in an intensity confinement in the direction of the forward scattering and in asymmetrical interference pattern formation around these irregularities. A new, two-dimensional grating-like structure was generated on spin-coated films. These gratings might be used as a special kind of mask. Received: 10 July 2001 / Accepted: 23 July 2001 / Published online: 30 August 2001     

Surface studies on bimetallic thiocyanate ligand based single crystals of MnHg(SCN)4, CdHg(SCN)4 and ZnCd(SCN)4

Bimetallic SCN ligand based single crystals of manganese mercury thiocyanate (MMTC), cadmium mercury thiocyanate (CMTC) and zinc cadmium thiocyanate (ZCTC) are grown by slow solvent evaporation technique. The growth mechanism and surface features are investigated by optical microscopic techniques such as scanning electron microscopy (SEM) and atomic force microscopy (AFM). The laser induced surface damage measurements were carried out using a Q-switched Nd:YAG laser at 1064 nm with laser beam of 1.0 Hz and pulse duration 25 ps. The laser damage threshold values of MMTC, CMTC and ZCTC are found to be 15.9, 22.9 and 19.7 GW/cm², respectively. The SEM analysis of MMTC reveals the formation of elongated dendrite growth pattern caused by the fluctuations of Mn and Hg metal ligands when thiocyanate (SCN) bridges them. The etching study indicates the occurrence of different types of etch pit patterns like terraced triangles, pillars, pyramids and rods. The AFM images confirm the formation of three major hillocks with cavities in MMTC. The measured roughness values for CMTC crystal are very much lower than that of MMTC.     

Microstructural investigation of CaxCo4Sb12 films prepared by pulsed laser deposition

Ca_xCo₄Sb₁₂ skutterudite thin films have been synthesized by pulsed laser deposition from a Nd:YAG laser working at 532 or 355 nm. The influence of deposition temperature, laser fluence and working atmosphere on the structure and morphology of the films has been studied. The characterization has been carried out by X-ray diffraction, atomic force microscopy and scanning electron microscopy. Laser wavelength proved to be the most significant parameter to produce the skutterudite phase. PACS 81.15.Fg; 68.55.Jk; 81.05.Bx     

Surface treatment of titanium by Nd:YAG laser irradiation in the presence of nitrogen

This work presents the surface treatment of commercial titanium alloy by means of a Nd:YAG (1.064 7m) laser in the presence of nitrogen. The treated surface was characterised by using scanning electron microscopy, atomic force microscopy, X-ray diffraction, secondary ion mass spectrometry, UV-visible spectrophotometry and microindentation. Experimental results show the formation of a nitrogen-rich layer, 500 nm thick, with a surface morphology characterised by the presence of polygonal structures which suggest that melting occurred under the action of the laser. The ' phase of titanium nitride was identified, in addition to a nitrogen in !-titanium solid solution. The reflectance spectrum of the yellow-golden sample is similar to that of deposited titanium nitride thin films. The hardness of the treated surface was measured by microindentation and found to be 14 GPa, a value three times higher than that of the titanium alloy. In scratch tests the nitrided layer detached at a load of 0.9 N.     

不同衬底温度下PLD法制备的氧化锌薄膜的特性

利用GCR-170型脉冲激光器Nd:YAG的三次谐波(355nm),以蓝宝石Al2O3(0001)为衬底,在不同温度下采用脉冲激光沉积法制备了ZnO薄膜.通过原子力显微镜、Raman谱、光致发光谱、红外透射谱、霍尔效应和表面粗糙度分析仪对制备的ZnO薄膜进行了测试.分析了在不同衬底温度下薄膜的表面形貌、光学特性,同时进行了薄膜结构和厚度的测试.研究表明:衬底温度对ZnO薄膜的表面形貌、光学特性、结构特性都是重要的工艺参量,尤其在500℃时沉积的ZnO薄膜致密均匀,并表现出较强的紫外发射峰.     

Solid-state dye laser with moiré gratings

We fabricated distributed-feedback (DFB) solid-state dye lasers with moiré gratings. We formed a moiré grating pattern by superimposing two grating patterns with a rotation angle. The moiré gratings were fabricated by an “etchless” process utilizing a two-beam interference method. The gratings were coated with solidified rhodamine-B. The five laser devices were pumped with the second harmonic generation of a Nd:YAG laser and we obtained narrow-banded laser oscillations at 590, 600, 610, 620, and 630 nm wavelength. Full widths at half maximum of laser spectra were less than 0.5 nm. The results indicated that a moiré fringe can function as a resonator of DFB solid-state dye lasers.     

Comparative study of ZnO layers prepared by PLD from different targets at various oxygen pressure levels

Two series of polycrystalline zinc oxide (ZnO) layers, from Zn or ZnO targets, were grown on silicon (1 1 1) substrates by pulsed laser deposition (PLD) at ambient oxygen pressure levels, stepwise increased from 1 to 35 Pa. For ablation of targets, a pulsed Nd:YAG laser was used. The structural and morphological properties of the layers were investigated by scanning electron microscopy (SEM), atomic force microscopy (AFM), and secondary ion mass spectrometry (SIMS). The SEM images of ZnO layers in SE mode show a uniform granular structure and modified surface morphology, depending on oxygen pressure. The mean grain size in height and lateral directions decreases with an increase of oxygen pressure from 1 to 5 Pa, while a subsequent rise of oxygen pressure from 5 to 35 Pa will cause an increase in the grain size. The AFM measurement revealed that the surface structures of zinc oxide layers grown from different targets were similar, and the layers formed at an ambient oxygen pressure of 5 Pa exhibited the smallest values of calculated roughness and granularity. SIMS depth profiling analyses confirmed that the ZnO composition was homogenous across the layer, up to the abrupt change of chemical composition at the interface between the ZnO layer and the Si substrate.