Fabrication of an integrated optical Mach-Zehnder interferometer based on refractive index modification of polymethylmethacrylate by krypton fluoride excimer laser radiation

It is known that deep ultraviolet (UV) radiation induces a refractive index increase in the surface layer of polymethylmethacrylate (PMMA) samples. This effect can be used for the fabrication of integrated optical waveguides. PMMA is of considerable interest for bio and chemical sensing applications because it is biocompatible and can be micromachined by several methods, e.g. structuring by photolithography, ablation and hot embossing. In the presented work direct UV irradiation of a common PMMA substrate by a krypton fluoride excimer laser beam through a contact mask has been used to write integrated optical Mach-Zehnder interferometers (MZI). MZI are used as sensitive bio and chemical sensors. The aim was to determine contact mask design and laser irradiation parameters for fabricating single-mode MZI for the infrared region from 1.30 μm to 1.62 μm. Straight and curved waveguides have been generated and characterized to determine the optical losses. The generation of channel waveguide structures has been optimized by a two step irradiation process to minimize the lithographic writing time and optical loss. By flood exposure to UV laser radiation in the first step the optical absorption of PMMA can be increased in the irradiated region. The required refractive index profile is then achieved with a second lithographic irradiation. The spectral behaviour of an unbalanced, integrated optical MZI fabricated by this excimer laser based contact mask method is shown for the first time. Further the optical intensity at the output port of a MZI has been measured while the optical path length difference was tuned by creating a temperature difference between the two arms of the MZI.     

Femtosecond laser microstructuring: an enabling tool for optofluidic lab‐on‐chips

This paper provides an overview of the rather new field concerning the applications of femtosecond laser microstructuring of glass to optofluidics. Femtosecond lasers have recently emerged as a powerful microfabrication tool due to their unique characteristics. On the one hand, they enable to induce a permanent refractive index increase, in a micrometer‐sized volume of the material, allowing single‐step, three‐dimensional fabrication of optical waveguides. On the other hand, femtosecond‐laser irradiation of fused silica followed by chemical etching enables the manufacturing of directly buried microfluidic channels. This opens the intriguing possibility of using a single laser system for the fabrication and three‐dimensional integration of optofluidic devices. This paper will review the state of the art of femtosecond laser fabrication of optical waveguides and microfluidic channels, as well as their integration for high sensitivity detection of biomolecules and for cell manipulation.     

Photo and thermally induced properties change in Bi/Ag/Se trilayer thin film

In this article, we have demonstrated the optical and structural properties change in Bi/Ag/Se trilayer thin films by the influence of thermal and photon energy. The trilayer films prepared by thermal evaporation technique were annealed and laser irradiated at room temperature. The X-ray diffraction study revealed the Ag₂Se phase formation and the surface morphology change is being studied by Field emission scanning electron microscopy. The optical properties of the studied films were characterized by using FTIR spectrophotometer in the wavelength range 400–1200?nm. The reduction of optical band gap by both thermal and laser irradiation is being discussed on the basis of chemical disorderness, defect states and density of localized states in the mobility gap. The Raman shift due to annealing and irradiation supports the changes in the film. The large change in optical band gap in thermal annealing is useful for memory device and waveguide fabrication.     

Fabrication of two- and three-dimensional periodic submicron structures by holographic lithography with a 635~nm laserand matched photopolymer

2D and 3D submicron periodic structures are first fabricated by red-induced photopolymerization using a common 635 nm semiconductor laser and specially developed red-sensitive polymer material. The principle of this new photo-polymer material fabrication is explained and the absorption spectra of the material are measured. This fabrication technique allows a deeper penetration into volume and larger interference irradiation area which is more than 1 cm². The optical design, theoretical calculations and experimental results including diffraction patterns verifying the formation of periodic structures are presented. Compared with other fabrication technologies using high-power lasers, this approach has greatly reduced the demand for laser apparatus. Therefore, it is much more accessible to most laboratories and potentially usable in holographic fabrication of photonic crystals and devices in micro electro-mechanical systems (MEMS).     

Fabrication of microstructures on paraffin substrate using laser heating

In this paper, a technique of forming convex microstructures on a paraffin substrate using laser heating is described. This technique makes use of a phenomenon that solid paraffin, melted by continuous laser irradiation, grows into a pillar-shaped structure with a high aspect ratio in a liquid. Two-dimensional structures, such as line- and wall-shaped structures, are available by scanning irradiation. Also, it is possible to fabricate oblique structures with an inclined laser irradiation. In addition, some characteristics of this fabrication method, such as the laser power and the material temperature, are investigated. Furthermore, the mechanism of this fabrication method is presented. PACS 07.10.Cm; 47.55.Pf; 65.40.De; 79.20.Ds; 81.05.-t     

Enhancement of pulsed laser removal of metal oxides by electrochemical control

Pulsed laser irradiation of oxidized metallic surfaces in an electrolytic cell under proper voltage conditions is demonstrated to be a promising new approach for effective removal of oxide films. Systematic measurements on simulated corrosion-product films by optical reflectance profile and energy dispersive X-ray spectroscopy are used to study the physical mechanisms of this novel phenomenon, the physical conditions for its observation and its possible generality. It was observed that the utilization of a basic electrolyte solution and the imposition of a certain cathodic potential prior to laser irradiation is an essential requirement for a high removal efficiency. This new technique has potential applications in metallurgy, semiconductor fabrication technology and decontamination of nuclear power plans and is suitable for maskless patterning of oxidized surfaces.     

Laser backwriting process on glass via ablation of metal targets

Ablation of metal targets onto pyrex glass substrates, using a Q-switched Nd:YAG laser working at 355 nm, was used to study the potential of a laser backwriting process for the fabrication of optical waveguides via an index of refraction change. Metal foils of stainless steel, aluminum, copper, brass and gold have been used as blanks and irradiated by focusing the laser beam through a cylindrical lens under continuous movement in a direction perpendicular to the irradiation. An horizontal setup was found suitable to improve the effect of the plume in the sample. Results were obtained for two different configurations. Transversal profiles were analysed using a contact profilometer, comparing results obtained for the different configurations, traverse speeds and metal targets used. Two ablation regimes were identified, which are related to a critical laser fluence value of 2.7 J/cm². Surface micrographs obtained by scanning electron microscopy are discussed, together with the characteristics of the structures attained, taking into account the optical and thermal properties of the ablated metal blanks.     

Effect of electron beam on structural,linear and nonlinear properties of nanostructured Fluorine doped ZnO thin films

Electron beam induced effects on Fluorine doped ZnO thin films (FZO) grown by chemical spray pyrolysis deposition technique were studied. The samples were exposed to 8 MeV electron beam at different dose rate ranging from 1 kGy to 4 kGy. All films exhibit a polycrystalline nature which shows an increase in crystallanity with irradiation dosages. The electron beam irradiation effectively controls the films surface morphology and its linear optical characteristics. Z-Scan technique was employed to evaluate the sign and magnitude of nonlinear refractive index and nonlinear absorption coefficient using a continuous wave laser at 632.8 nm as light source. Enhancement in the third order nonlinear optical properties was were noted due to electron beam irradiation. Tailoring the physical and NLO properties by electron beam, the FZO thin films becomes a promising candidate for various optoelectronic applications such as phase change memory devices, optical pulse compression, optical switching and laser pulse narrowing.     

Control of the refractive index change in fused silica glasses induced by a loosely focused femtosecond laser

We demonstrate waveguide fabrication using of a loosely focused femtosecond laser that induces both nonlinear ionization and nonlinear propagation in silica glasses without any scanning process. The numerical aperture is chosen to be 0.007 to avoid spatial splitting of the laser pulses during the nonlinear propagation of the femtosecond laser pulses inside the fused silica glass. It also enables a uniform cylindrical refractive index change, which acts as an optical waveguide, to be induced. We found that the induction of irregular refractive index changes is related to the free electron density of the focused area and is controlled by decreasing the NA. The length, position, and core diameter of the fabricated waveguide can be controlled by the pulse-width, energy, and irradiation time of the incident laser. By using this technique, we varied the length of the fabricated waveguides between 20 m and 6 mm, while keeping the core diameter at around 5 m. PACS 42.70.Ce; 42.82.Et; 61.80.Ba     

Temperature-controlled UV-laser induced fabrication of planar polymeric waveguides

Integrated-optical waveguides may be written directly into the surface of a planar polymeric substrate by UV-excimer laser irradiation. The loss rate is relatively high due to the refractive index depth profile of waveguides produced in this way. The loss rate can be reduced significantly by use of a temperature-controlled fabrication process: during the UV-laser irradiation the polymeric substrate is heated in a controllable way by a hot plate. An explanation is given for this temperature-dependent effect.     

Analysis of UV-Laser induced oxidation of implanted silicon by optical reflectivity measurements

The oxidation of ion implanted silicon induced by a repetitive excimer laser working in liquid phase regime has been monitored by a simple in situ technique. It consists to follow the optical reflectivity at the wavelength 633 nm of the silicon samples under irradiation. The influence of implantation and laser irradiation conditions on the oxidation process has been investigated by this technique. The results obtained have been compared using infrared absorption data. The role of the Si/SiO₂ interface roughness on the oxide film quality has been studied.     

Femtosecond-laser nanofabrication onto silicon surface with near-field localization generated by plasmon polaritons in gold nanoparticles with oblique irradiation

Nanohole fabrication process with gold nanoparticles irradiated by femtosecond laser at different incident angles is investigated. Nanoparticles with diameter of 200 nm and laser irradiation with center wavelength of 800 nm are used in the present study. The analysis of the electromagnetic field distribution in the near-field zone of the particle is made by simulations based on finite-differential time domain (FDTD) method. It is shown that when gold nanoparticle is irradiated by laser pulse surface plasmon excitation can be induced, and associated with it, high-intensity near field is produced in a limited area around the particle. It is found that the change of the irradiation conditions by means of irradiation from various incident directions gives a possibility of laser nanoprocessing with tunable characteristics. Our results show that enhanced optical intensity is able to be induced on the substrate surface regardless of incident direction of the laser due to the image charge interaction with the substrate. Furthermore, the use of p-polarized laser irradiation at a certain angle gives a minimum of the spatial dimensions of the enhanced zone on the substrate which is about two times smaller than that obtained at normal incidence.     

低功率近红外光的偏振方向对光致折射率变化影响的实验研究

研究了利用低功率近红外光辐照In:Fe:LiNbO3晶体时写入光束的偏振方向对光致折射率变化(Δn)的影响.分别采用正常偏振(o光)和非常偏振(e光)的近红外细激光束,在In:Fe:LiNbO3晶体中进行了光折变实验.研究表明,两种偏振方向引起晶体的Δn实测曲线相似,但变化规律恰好相反,o光引起的折射率变化量约是e光的3倍左右.近红外光写入下两种偏振光束引起晶体的Δn分布规律都不同于可见光,尤其是e光辐照区域中心晶体的折射率升高.因此,通过选择不同偏振方向的近红外光可以在光折变晶体中制作不同折射率分布的非线性光学器件.     

Large area laser surface micro/nanopatterning by contact microsphere lens arrays

Laser surface micro/nanopatterning by particle lens arrays is a well-known technique. Enhanced optical fields can be achieved on a substrate when a laser beam passes through a self-assembled monolayer of silica microspheres placed on the substrate. This enhanced optical field is responsible for ablative material removal from the substrate resulting in a patterned surface. Because of the laser ablation, the microspheres are often ejected from the substrate during laser irradiation. This is a major issue impeding this technique to be used for large area texturing. We explored the possibility to retain the spheres on the substrate surface during laser irradiation. A picosecond laser system (wavelength of 515 nm, pulse duration 6.7 ps, repetition rate 400 kHz) was employed to write patterns through the lens array on a silicon substrate. In this experimental study, the pulse energy was found to be a key factor to realize surface patterning and retain the spheres during the process. When the laser pulse energy is set within the process window, the microspheres stay on the substrate during and after laser irradiation. Periodic patterns of nanoholes can be textured on the substrate surface. The spacing between the nanoholes is determined by the diameter of the microspheres. The depth of the nanoholes varies, depending on the number of laser pulses applied and pulse energy. Large area texturing can be made using overlapping pulses obtained through laser beam scanning.     

Three-dimensional optical data storage using third-harmonic generation in silver zinc phosphate glass

We demonstrate the possibility of three-dimensional optical data storage inside a specific zinc phosphate glass containing silver by using third-harmonic generation (THG) imaging. Information is stored inside the glass with femtosecond laser irradiation below the refractive index modification threshold. We use the same laser for THG readout. The capability of storage with this technique is discussed.     

Planar Bragg gratings made by excimer-laser modification of ion-exchanged waveguides

The refractive indices of planar silver-ion-exchanged waveguides have been modified by UV excimer laser irradiation (lambda=193 nm) . The effective index changes of the fundamental modes of the waveguides after exposition are as large as 2x10(-2) . The procedure permits the fabrication of integrated optical components in a direct way, with neither the use of standard lithography nor previous sensitizing of the substrate. Optical characterization of the irradiated samples is presented. By the use of appropriate masks, we have fabricated planar Bragg gratings.     

激光超衍射加工机理与研究进展

随着纳米科技和微纳电子器件的发展,制造业对微纳加工技术的要求越来越高.激光加工技术是一种绿色先进制造技术,具有巨大的发展潜力,己广泛应用于不同的制造领域.为实现低成本、高效率、大面积尤其是高精度的激光微纳加工制造,研究和发展激光超衍射加工技术具有十分重要的科学意义和应用价值.本文首先阐述了基于非线性效应的远场激光直写超衍射加工技术的原理与国内外发展状况,包括激光烧蚀加工技术、激光诱导改性加工技术和多光子光聚合加工技术等;然后介绍了几种基于倏逝波的近场激光超衍射加工技术,包括扫描近场光刻技术、表面等离子激元光刻技术等新型超衍射激光近场光刻技术的机理与研究进展;最后对激光超衍射加工中存在的问题及未来发展方向进行了讨论.     

Photothermally enabled lithography for refractive-index modulation in SU-8 photoresist

We present a new lithographic technique based on a hybrid photothermal process to modulate the refractive index in commercial SU-8 photoresist. Owing to a difference in cross-linking, the refractive index of unexposed SU-8 cross-linked by thermally induced polymerization is 0.0072 higher than that of SU-8 cross-linked by UV exposure and postbaking. Making use of this property, we fabricated two thick, flat-topped index-modulated diffractive optical elements (DOEs) that contain different phase distributions and measured their wavefront reconstruction. The good experimental reconstructions of the index DOEs demonstrate the potential to extend the refractive-index modulation technique for the fabrication of three-dimensional optical elements without needing a development step.     

Modelling of diffractive structures in photorefractive Er/Yb–co-doped glass waveguides

The photorefractive properties of Ge-doped glasses are routinely exploited for the production of Bragg gratings in optical fibres. Today, the most common fabrication procedure relies on the phase-mask technique, which allows to produce (through UV irradiation) a periodical refractive index variation in Ge-doped fibre cores. The same methodology can be applied to integrated optical components in photorefractive glass planar substrates. If rare earth ions (such as erbium and ytterbium) are included in the glass matrix, both active and photorefractive properties can be combined in the same material. This fact allows for the possibility of realizing complex integrated optical components, such as Bragg-reflection lasers and amplifiers. In this work, a numerical analysis of a waveguide Bragg reflector is presented based on a homemade modelling technique.     

New speckle control technique using high-speed electro-optic modulators with resonant electrode and polarization-reversed structures

We propose a new technique for reducing speckle noise in laser displays utilizing a high-speed optical phase modulator for expanding the laser beam spectrum. By adopting standing-wave resonant electrodes and polarization-reversed structures to LiTaO₃ electro-optic modulators, high-efficiency phase modulation with a large modulation index at high microwave frequency ranges is obtainable for different color laser beams at the same time. This enables us to expand laser spectra to over 100 GHz and reduce speckle noise. The device design, fabrication, and experimental demonstration for speckle control are reported.