Proceedings Volume 7589

Frontiers in Ultrafast Optics: Biomedical, Scientific, and Industrial Applications X

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Proceedings Volume 7589

Frontiers in Ultrafast Optics: Biomedical, Scientific, and Industrial Applications X

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Volume Details

Date Published: 16 February 2010
Contents: 11 Sessions, 28 Papers, 0 Presentations
Conference: SPIE LASE 2010
Volume Number: 7589

Table of Contents

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Table of Contents

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  • Front Matter: Volume 7589
  • Special Session: Ultrafast Lasers in Ophthalmology
  • Multiphoton Imaging
  • fs Lasers for Cell Manipulation
  • Femtosecond Lasers and Components
  • Modeling and Measurement
  • fs Laser-Induced Waveguides
  • Ultrashort Pulse Micromachining: Joint Session with Conference 7584
  • Femtosecond Laser Nanoprocessing: Joint Session with Conference 7584
  • Three-Dimensional Direct Writing: Joint Session with Conference 7584
  • Poster Session
Front Matter: Volume 7589
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Front Matter: Volume 7589
This PDF file contains the front matter associated with SPIE Proceedings Volume 7589, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.
Special Session: Ultrafast Lasers in Ophthalmology
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Femtosecond lasers in ophthalmology: clinical applications in anterior segment surgery
Tibor Juhasz, Zoltan Nagy, Melvin Sarayba, et al.
The human eye is a favored target for laser surgery due to its accessibility via the optically transparent ocular tissue. Femtosecond lasers with confined tissue effects and minimized collateral tissue damage are primary candidates for high precision intraocular surgery. The advent of compact diode-pumped femtosecond lasers, coupled with computer controlled beam delivery devices, enabled the development of high precision femtosecond laser for ophthalmic surgery. In this article, anterior segment femtosecond laser applications currently in clinical practice and investigation are reviewed. Corneal procedures evolved first and remain dominant due to easy targeting referenced from a contact surface, such as applanation lenses placed on the eye. Adding a high precision imaging technique, such as optical coherence tomography (OCT), can enable accurate targeting of tissue beyond the cornea, such as the crystalline lens. Initial clinical results of femtosecond laser cataract surgery are discussed in detail in the latter portion part of the article.
High resolution macroscopy (HRMac) of the eye using nonlinear optical imaging
Moritz Winkler, Bryan E. Jester, Chyong Nien-Shy, et al.
Non-linear optical (NLO) imaging using femtosecond lasers provides a non-invasive means of imaging the structural organization of the eye through the generation of second harmonic signals (SHG). While NLO imaging is able to detect collagen, the small field of view (FoV) limits the ability to study how collagen is structurally organized throughout the larger tissue. To address this issue we have used computed tomography on optical and mechanical sectioned tissue to greatly expand the FoV and provide high resolution macroscopic (HRMac) images that cover the entire tissue (cornea and optic nerve head). Whole, fixed cornea (13 mm diameter) or optic nerve (3 mm diameter) were excised and either 1) embedded in agar and sectioned using a vibratome (200-300 um), or 2) embedded in LR White plastic resin and serially sectioned (2 um). Vibratome and plastic sections were then imaged using a Zeiss LSM 510 Meta and Chameleon femtosecond laser to generate NLO signals and assemble large macroscopic 3-dimensional tomographs with high resolution that varied in size from 9 to 90 Meg pixels per plane having a resolution of 0.88 um lateral and 2.0 um axial. 3-D reconstructions allowed for regional measurements within the cornea and optic nerve to quantify collagen content, orientation and organization over the entire tissue. We conclude that NLO based tomography to generate HRMac images provides a powerful new tool to assess collagen structural organization. Biomechanical testing combined with NLO tomography may provide new insights into the relationship between the extracellular matrix and tissue mechanics.
Multiphoton Imaging
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Brain plasticity and functionality explored by nonlinear optical microscopy
L. Sacconi, L. Allegra, M. Buffelli, et al.
In combination with fluorescent protein (XFP) expression techniques, two-photon microscopy has become an indispensable tool to image cortical plasticity in living mice. In parallel to its application in imaging, multi-photon absorption has also been used as a tool for the dissection of single neurites with submicrometric precision without causing any visible collateral damage to the surrounding neuronal structures. In this work, multi-photon nanosurgery is applied to dissect single climbing fibers expressing GFP in the cerebellar cortex. The morphological consequences are then characterized with time lapse 3-dimensional two-photon imaging over a period of minutes to days after the procedure. Preliminary investigations show that the laser induced fiber dissection recalls a regenerative process in the fiber itself over a period of days. These results show the possibility of this innovative technique to investigate regenerative processes in adult brain. In parallel with imaging and manipulation technique, non-linear microscopy offers the opportunity to optically record electrical activity in intact neuronal networks. In this work, we combined the advantages of second-harmonic generation (SHG) with a random access (RA) excitation scheme to realize a new microscope (RASH) capable of optically recording fast membrane potential events occurring in a wide-field of view. The RASH microscope, in combination with bulk loading of tissue with FM4-64 dye, was used to simultaneously record electrical activity from clusters of Purkinje cells in acute cerebellar slices. Complex spikes, both synchronous and asynchronous, were optically recorded simultaneously across a given population of neurons. Spontaneous electrical activity was also monitored simultaneously in pairs of neurons, where action potentials were recorded without averaging across trials. These results show the strength of this technique in describing the temporal dynamics of neuronal assemblies, opening promising perspectives in understanding the computations of neuronal networks.
A novel flexible clinical multiphoton tomograph for early melanoma detection, skin analysis, testing of anti-age products, and in situ nanoparticle tracking
Martin Weinigel, Hans Georg Breunig, Axel Gregory, et al.
High-resolution 3D microscopy based on multiphoton induced autofluorescence and second harmonic generation have been introduced in 1990. 13 years later, CE-marked clinical multiphoton systems for 3D imaging of human skin with subcellular resolution have first been launched by JenLab company with the tomography DermaInspect®. This year, the second generation of clinical multiphoton tomographs was introduced. The novel multiphoton tomograph MPTflex, equipped with a flexible articulated optical arm, provides an increased flexibility and accessibility especially for clinical and cosmetical examinations. Improved image quality and signal to noise ratio (SNR) are achieved by a very short source-drain spacing, by larger active areas of the detectors and by single photon counting (SPC) technology. Shorter image acquisition time due to improved image quality reduces artifacts and simplifies the operation of the system. The compact folded optical design and the light-weight structure of the optical head eases the handling. Dual channel detectors enable to distinguish between intratissue elastic fibers and collagenous structures simultaneously. Through the use of piezo-driven optics a stack of optical cross-sections (optical sectioning) can be acquired and 3D imaging can be performed. The multiphoton excitation of biomolecules like NAD(P)H, flavins, porphyrins, elastin, and melanin is done by picojoule femtosecond laser pulses from an tunable turn-key femtosescond near infrared laser system. The ability for rapid high-quality image acquisition, the user-friendly operation of the system and the compact and flexible design qualifies this system to be used for melanoma detection, diagnostics of dermatological disorders, cosmetic research and skin aging measurements as well as in situ drug monitoring and animal research.
fs Lasers for Cell Manipulation
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Functional enucleation of porcine oocytes for somatic cell nuclear transfer using femtosecond laser pulses
K. Kuetemeyer, A. Lucas-Hahn, B. Petersen, et al.
Cloning of several mammalian species has been achieved by somatic cell nuclear transfer over the last decade. However, this method still results in very low efficiencies originating from biological and technical aspects. The highly-invasive mechanical enucleation belongs to the technical aspects and requires considerable micromanipulation skill. In this paper, we present a novel non-invasive method for combined oocyte imaging and automated functional enucleation using femtosecond (fs) laser pulses. After three-dimensional imaging of Hoechst-labeled porcine oocytes by multiphoton microscopy, our self-developed software automatically determined the metaphase plate position and shape. Subsequent irradiation of this volume with the very same laser at higher pulse energies in the low-density-plasma regime was used for metaphase plate ablation. We show that functional fs laser-based enucleation of porcine oocytes completely inhibited further embryonic development while maintaining intact oocyte morphology. In contrast, non-irradiated oocytes were able to develop to the blastocyst stage without significant differences to control oocytes. Our results indicate that fs laser systems offer great potential for oocyte imaging and enucleation as a fast, easy to use and reliable tool which may improve the efficiency of somatic cell clone production.
Fs-laser cell perforation using gold nanoparticles of different shapes
Markus Schomaker, Holger Fehlauer, Willem Bintig, et al.
The resulting effects of the interaction between nanoparticles and laser irradiation are a current matter in research. Depending on the laser parameters as well as the particles properties several effects may occur e.g. bubble formation, melting, fragmentation or an optical breakdown at the surface of the nanoparticle. Besides the investigations of these effects, we employed them to perforate the membrane of different cell lines and investigated nanoparticle mediated laser cell perforation as an alternative optical transfection method. Therefore, the gold nanoparticles (GNP) of different shapes were applied. Furthermore, we varied the methods for attaching GNP to the membrane, i.e. co-incubation of pure gold nanoparticles and bioconjugation of the surface of GNP. The optimal incubation time and the location of the GNP at the cell membrane were evaluated by multiphoton microscopy. If these GNP loaded cells are irradiated with a fs laser beam, small areas of the membrane can be perforated. Following, extra cellular molecules such as membrane impermeable dyes or foreign DNA (GFP vectors) are able to diffuse through the perforated area into the treated cells. We studied the dependence of the laser fluence, GNP concentration, GNP size and shape for successful nanoparticle mediated laser cell perforation. Due to a weak focusing of the laser beam a gentle cell treatment with high cell viabilities and high perforation efficiencies can be achieved. A further advantage of this perforation technique is the high number of cells that can be treated simultaneously. Additionally, we show applications of this method to primary and stem cells.
Dual-beam optical trapping of cells in an optofluidic device fabricated by femtosecond lasers
N. Bellini, F. Bragheri, K. C. Vishnubhatla, et al.
We present design and optimization of an optofluidic monolithic chip, able to provide optical trapping and controlled stretching of single cells. The chip is fabricated in a fused silica glass substrate by femtosecond laser micromachining, which can produce both optical waveguides and microfluidic channels with great accuracy. Versatility and three-dimensional capabilities of this fabrication technology provide the possibility to fabricate circular cross-section channels with enlarged access holes for an easy connection with an external fluidic circuit. Moreover, a new fabrication procedure adopted allows the demonstration of microchannels with a square cross-section, thus guaranteeing an improved quality of the trapped cell images. Optical trapping and stretching of single red blood cells are demonstrated, thus proving the effectiveness of the proposed device as a monolithic optical stretcher. We believe that femtosecond laser micromachining represents a promising technique for the development of multifunctional integrated biophotonic devices that can be easily coupled to a microscope platform, thus enabling a complete characterization of the cells under test.
Femtosecond Lasers and Components
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Applications of femtosecond laser induced Bragg gratings in silica and non-silica based optical fibers
Stephen J. Mihailov, Dan Grobnic, Christopher W. Smelser, et al.
Since its development in 2003, the technique of Bragg grating inscription using an ultrafast infrared laser and a phase mask has proven to be far more versatile than the standard ultraviolet laser approach. The ultrafast IR laser-based process allows for the creation of grating structures in glassy and crystalline material waveguides that are not typically UV-photosensitive, thereby creating new applications for Bragg gratings where the use of UV-photosensitive silica fibers is not possible. In this paper we will review the studies that have been performed at the Communications Research Centre Canada on the applications of the ultrafast laser technique to fabricate gratings in various optical fibers and waveguides.
Mode selective fiber Bragg gratings
Focussing ultrashort laser pulses allows for inscribing fiber Bragg gratings (FBGs) directly into rare earth doped fiber cores - without prior photosensitivity treatment. High reflective FBGs can be written into active Large Mode Area (LMA) Fibers with 20 micron core diameter using a phase mask scanning technique. Here, we demonstrate fiber Bragg gratings (FBGs), which cover only a fraction of the core. With this additional degree of freedom it is possible to taylor the reflectivity of individual modes. We show for example how those FBGs can be used in few mode LMA fibers to suppress reflections into higher order modes.
Compact 60fs multigigawatt diode-pumped laser using postcompression technique
Antoine Courjaud, Eric Mével, Eric Constant, et al.
We report on a compact and simple system delivering pulses as short as 60fs with an energy up to 330μJ at 2kHz and 200μJ at 5kHz. High energy pulses delivered by a diode-pumped ultrafast laser with 500fs pulse duration are injected in a hollow fiber filled with nitrogen, experiencing spectral broadening through self-phase modulation, and are subsequently compressed to sub-100fs pulse duration using negative dispersion mirrors. The overall power efficiency of 40% and the temporal compression factor larger than 8 leads to a peak power increase of more than 3, corresponding to 5GW peak power in the multi-kilohertz regime.
Compression of idler pulses with an identical positive dispersive media to signal pulse stretcher in ultrafast optical-parametric chirped-pulse amplification
Yutaka Akahane, Kanade Ogawa, Koichi Tsuji, et al.
We have proposed and demonstrated a very simple and robust femtosecond optical-parametric chirped-pulse amplification scheme in which an even order dispersion of an idler pulse is compensated by passing through an identical positive dispersive material used for temporal stretching a signal pulse. By compressing the idler pulses having a negatively chirp in this manner, high power sub-100 fs pulses were successfully obtained with only a transparent glass block used for the stretcher and compressor.
Broadband Yb:CaF2 regenerative amplifier for millijoule range ultrashort pulse amplification
Sandrine Ricaud, Martin Delaigue, Antoine Courjaud, et al.
We report on a diode-pumped regenerative amplifier based on Yb:CaF2 material, delivering pulses up to 1.8mJ pulse energy at a repetition rate of 100Hz. The crystal is pumped at the zero line at 978 nm with a 10W continuous wave (CW) fiber coupled laser diode. The pulses have a spectral bandwidth of 16nm centered at 1040 nm, which indicates a good potential for millijoule range sub 100fs pulse duration after compression. It is also a good candidate for seeding higher energy diode-pumped ytterbium lasers.
Modeling and Measurement
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Measuring complex and visible ultrashort pulses
Frequency-Resolved Optical Gating (FROG) and its variations are the only techniques available for measuring complex pulses without a well-characterized reference pulse. We study the performance of the FROG generalized-projections (GP) algorithm for retrieving the intensity and phase of very complex ultrashort laser pulses in the presence of noise. Also, we show that a highly simplified version of FROG, GRENOUILLE, can easily measure visible pulses. By tuning a thick crystal, it can cover the entire visible spectrum, which is typically generated from commercial Optical Parameter Amplifications (OPAs)
Modeling the propagation of ultrashort pulses through optical systems
Frank Wyrowski, Christian Hellmann, Rene Krieg, et al.
The propagation of harmonic fields through arbitrary optical components is the fundamental task in optical modeling. Unified optical modeling by field tracing uses different techniques for different components in order to ensure the best compromise between simulation effort and accuracy. This approach can be extended to non-harmonic fields. With a set of harmonic fields modeling partial coherence of stationary sources is enabled. The same approach can be applied to model the propagation of fully coherent ultrashort pulses through optical systems, which may include for instance lenses, gratings and micro-optical components. For that we can rely on field tracing with its numerous sophisticated propagation techniques for a single harmonic field. Methods to reduce frequency domain sampling are presented. They allow a convenient pulse modeling in practice. Several examples are presented using ultrashort pulse modeling with VirtualLab™.
Surface-enhanced Raman scattering hot spot isolation using surface-enhanced multiphoton lithography
In this Manuscript, we present the fabrication and spectroscopic characterization of a large-area surfaceenhanced Raman scattering (SERS) substrate, as well as a method for improving femtomole-level trace detection (109 molecules) using this substrate. Using multiphoton-induced exposure of a commercial photoresist, we physically limit the available molecular adsorption sites to only the electromagnetic "hot spots" on the substrate. This process prevents molecules from adsorbing to sites of weak SERS enhancement, while permitting adsorption to sites of extraordinary SERS enhancement. For a randomly adsorbed submonolayer of benzenethiol molecules the average Raman scattering cross-section of the processed sample is 27 times larger than that of an unprocessed SERS substrate.
fs Laser-Induced Waveguides
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Curved-waveguide fabrication using femtosecond laser processing with glass hologram
Jun'ichi Suzuki, Yasunori Arima, Masahiro Yamaji, et al.
We report on a curved waveguide fabrication using femtosecond laser processing with a glass hologram. We design and produce a glass hologram that transforms femtosecond laser beam into a half-ring beam. The half-ring beam generated by the glass hologram is patterned inside fused silica with one laser shot. The guided light whose bending radius is larger than 1mm is observed at wavelength of 635nm. As a simple application, a directional coupler consisting of a straight-line waveguide and a half-ring waveguide is fabricated by two laser shots. Its basic functionality as a coupler is confirmed. We also develop a hologram that simultaneously produces a straight-line and a half-ring. Using it, we demonstrate a directional coupler fabrication inside crown glass with one laser shot.
Direct femtosecond laser written waveguides in bulk Ti3+:sapphire
We report on direct inscription of type-II waveguides in bulk titanium-doped sapphire with an ultrafast chirpedpulse oscillator. Ti3+:Sapphire is of particular interest due to its large emission bandwidth which enables a broadband tunability and generation of ultra-short pulses. However, its lasing threshold is high and powerful high brightness pump sources are required. The fabrication of a waveguide in Ti3+:Sapphire could thus enable the fabrication of low-threshold tunable lasers and broadband fluorescence sources. The latter are of interest for optical coherence tomography where the obtainable resolution scales with the bandwidth of the light source. The fabricated waveguides are formed in-between two laser induced damage regions. This technique has been applied to other crystalline materials (e.g. LiNbO3) but not in Ti3+:Sapphire, yet. The size of the structural changed regions is strongly dependent on the writing laser polarization. These damage regions of changed structure cause a stress-field inside the crystalline lattice which consequently increases the refractive index to form a waveguide. The written structures exhibit a strong birefringence and two waveguides that support orthogonal polarized modes are formed between each pair of damage lines. Linearly polarized light parallel to the crystal's surface is guided between the two damage regions while a waveguide for the orthogonal polarization is formed underneath. The propagation properties of the waveguides are characterized by their near-field profiles and insertion losses with respect to the writing parameters. Further the fluorescence output power is measured and the emission spectra of the waveguides are compared to the bulk material.
Soliton formation near topological defects in waveguide arrays
Matthias Heinrich, Robert Keil, Alexander Szameit, et al.
We report on the impact of topological defects on the formation of discrete spatial solitons in waveguide arrays. The influence of defects, i.e. waveguides with detuned effective refractive index, is well understood within such systems. They have been shown to support linear bound states and thus influence the formation of spatial solitons in the surrounding sites. We show numerically and demonstrate experimentally how the presence of topological defects caused by junctions within the otherwise periodical system similarly has a strong influence on the surrounding sites.
Ultrashort Pulse Micromachining: Joint Session with Conference 7584
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Ultrafast laser-based tools enhance solar cell efficiencies
Finlay Colville
Opportunities for process equipment incorporating ultrafast laser types within the solar industry are numerous and varied. Historic trends and legacy laser adoption provide some perspective to the challenges facing tool suppliers seeking to enter the marketplace. Understanding the different cell concepts produced is one of the first requirements for tool suppliers, with specific process flows within each production line. Thereafter, it is important to consider the limitations of alternative laser types - especially nanosecond lasers - and what if any cost benefit is available to cell manufacturers when switching to ultrafast laser sources. Opportunities are then enabled by appropriate choice of laser processing conditions and tool supply which yields the greatest return-on-investment to the cell and panel producers.
Picosecond laser patterning of NiCr thin film strain gages
This paper presents results of ablation experiments of NiCr layers with thicknesses ranging from 23nm to 246nm on Al2O3 substrates. Investigated parameters are fluence, number of pulses, film thickness and substrate roughness. The influence of the parameters on the removal threshold is analyzed in order to identify stable processing parameters. Patterned NiCr thin films as an essential component for the measurement of mechanical stress are required for the development of sputtered thin film strain gages. With this new approach strain sensors will be resistant against creeping or swelling through changing ambient conditions unlike conventional strain gages.
High repetition rate femtosecond laser processing of metals
Joerg Schille, Robby Ebert, Udo Loeschner, et al.
Previously, in high repetition rate femto second laser processing novel laser matter interacting effects were reported, such as heat accumulation and particle shielding. In this study, high repetition rate laser processing was investigated to discuss and understand the impact of laser repetition rate and accompanied accumulative laser material interacting effects. Therefore, a high repetition rate femto second fibre laser setup joint together with galvo scanner technology was applied in laser micro machining of metals (copper, stainless steel, aluminium). High repetition rate laser processing of aluminium and stainless steel lead to considerably lowered ablation thresholds accompanied with higher ablation rates. Laser ablation behaviour of copper was almost independent of the repetition rate with neither considerable lower ablation thresholds nor higher ablation rates. For explanation, heat accumulation caused by higher repetition rates were assumed as mainly ablation behaviour influencing effect, but thermal material properties have to be considered. Furthermore laser machining examples demonstrate the possibilities and limits of high repetition rate laser processing in 3d micro structuring. Thus, by using innovative scanning systems and machining strategies very short processing times were achieved, which lead to high machining throughputs and attract interest of the innovative laser technology in Rapid Micro Tooling. For discussion, high repetition rate processing results are evaluated by means of comparative machining examples obtained with 1 kHz femto second laser system.
Femtosecond Laser Nanoprocessing: Joint Session with Conference 7584
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Brighter incandescent light sources from the black metal
Anatoliy Y. Vorobyev, Chunlei Guo
In this work, by applying the femtosecond laser blackening technique directly to a tungsten incandescent lamp filament, we dramatically brighten the tungsten lamp and enhance its emission efficiency by 60%. This process in fact leads the tungsten incandescent light bulb to approach the 100% emission efficiency. A comparison study of emission and absorption for the structured metal surface shows that Kirchhoff's law is applicable for the black metal. More importantly, we demonstrate that we can even obtain partially polarized light as well as control the spectral range of the optimal light emission from the laser-blackened tungsten lamp.
Three-Dimensional Direct Writing: Joint Session with Conference 7584
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Birefringent elements based on femtosecond laser-induced nanogratings
Lourdes Patricia R. Ramirez, Matthias Heinrich, Sören Richter, et al.
We report on the fabrication of birefringent optical components based on so-called nanogratings. These selforganized nanostructures with sub-wavelength periodicity are formed during femtosecond laser processing of transparent materials, resulting in characteristic birefringent modifications. Nanogratings provide the means for the direct inscription of customized birefringent elements with position-dependent retardation. We present our investigations on the formation process of nanogratings in fused silica and the influence of fabrication parameters, thereby identifying ways to systematically control the structural properties of the gratings. Consequently, we were able to fabricate nanograting-based birefringent elements with specific retardations in bulk fused silica.
Femtosecond laser written embedded diffractive optical elements and their applications
Femtosecond laser direct writing (FLDW) has been widely employed to create volumetric structures in transparent materials that are applicable as various photonic devices such as active and passive waveguides, couplers, gratings, and diffractive optical elements (DOEs). The advantages of fabrication of volumetric DOEs using FLDW include not only the ability to produce embedded 3D structures but also a simple fabrication scheme, ease of customization, and a clean process. DOE fabrication techniques using FLDW are presented as well as the characterization of laserwritten DOEs by various methods such as diffraction efficiency measurement. Fresnel zone plates were fabricated in oxide glasses using various femtosecond laser systems in high and low repetition rate regimes. The diffraction efficiency as functions of fabrication parameters was measured to investigate the dependence on the different fabrication parameters such as repetition rate and laser dose. Furthermore, several integration schemes of DOE with other photonic structures are demonstrated for compact photonic device fabrication.
Poster Session
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Observations of the intense soft x-ray emissions from ultrathin Au films irradiated with high contrast laser pulses
Masahiko Ishino, Masataka Kado, Masaharu Nishikino, et al.
Observation of soft x-ray emissions from laser produced plasmas using ultra thin film targets has been carried out. Au ultra thin films deposited on silicon nitride membranes were irradiated with a high contrast Nd:glass laser pulses. The spectral properties of emitted soft x-rays were monitored with an x-ray spectrograph from the membrane side. The observed emission intensities had a clear dependence on the Au film thickness. The results suggest that most of the laser energy irradiated is absorbed by the Au films and few of the energy goes into the silicon nitride membranes, which means an efficient laser energy deposition to the ultra thin Au film target and a high energy conversion rate from laser to x-rays.
Nonlinear lithographic properties by femtosecond laser pulses using a low-NA lens
M. Mizoshiri, H. Nishiyama, J. Nishii, et al.
Refractive indices of a chemically amplified photoresist were changed by femtosecond (fs) laser irradiation without post exposure bake (PEB) treatment. We have proposed a combined process of fs nonlinear lithography and plasma etching for the fabrication of functional photonic devices with 3-D surfaces of inorganic materials. In this study, we report the nonlinear lithographic properties using fs laser pulses focused by a low-NA objective lens. Diffraction gratings were written directly inside the pre-baked resists by fs laser nonlinear absorption. From diffraction efficiencies using He-Ne laser light, refractive indices were changed by 8 × 10-3 without PEB treatment which was required for cross-linking reaction. In contrast, no changes of refractive index were observed in the case of ultraviolet light exposure (i-line). Considering this large refractive index change and the threshold intensity of nonlinear absorption of the resist, self-guiding of fs laser pulses can occur due to the optical confinement in the radial direction. In fact, filamentary patterns which were optical-axially asymmetric and longer than the focal depth could be obtained without translating the focal spot using this lithographic property.
Laser microsintering of tungsten in vacuum
Robby Ebert, Frank Ullmann, Lars Hartwig, et al.
Laser microsintering of tungsten powder is investigated as a function of laser output power, pulse interval and vacuum level. The intensities are calculated for the evaporation thresholds of tungsten powder particles of various sizes. In addition, the powder layer generation and the resulting layer thicknesses are calculated. The powder abrasion occurring during the process was taken into consideration. Polished sections and REM images were prepared in order to analyse the experimental outcomes. The dependence of sinter density on the parameters is discussed.