This PDF file contains the front matter associated with SPIE Volume 6460, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

Noninvasive and straightforward methods to inactivate selected proteins in the living cell with high spatiotemporal resolution are eagerly sought for elucidation of protein function in the post-genome-mapping era. Chromophore-assisted laser inactivation (CALI) facilitates inactivation of proteins by photochemically generated reactive oxygen species (ROS), but CALI using single-photon excitation thus far has presented several drawbacks, including its complex procedure, low efficiencies of inactivation with a certain chromophore, and photodamage effects. We here show that by application of multiphoton excitation to CALI using near-infrared femtosecond laser, enhanced green fluorescent protein (EGFP) can work as an effective chromophore for inactivation of a protein's function without nonspecific photodamage in the living cell.

The laser microtome (LMT) is designed to slice biological tissue and various materials with high precision. The cutting process is performed by an ultrafast laser, emitting infrared light with high repetition rate of about 10 MHz. Biological Tissue can be sliced without fixation and embedding. Therefore cutting of native tissue is possible. Due to the non contact procedure further investigations of the material like immuno histological tests can be performed. At present slices with a thickness of 5 - 100&mgr;m in different biological tissues like cartilage, kidney, lung and cornea has been shown. In conjunction with a 3D imaging system like optical coherence tomography, preparation of 3D tissue volumes is possible too.

We have developed novel techniques of protein crystallization and processing using femtosecond laser, all solid-state 193 nm laser, and solution stirring. Femtosecond laser technique enables us to extremely increase the crystallization probability, and to trigger the nucleation at low supersaturation of solution where spontaneous nucleation dose not occur. Since the laser-induced nuclei grow slowly in the low-range supersaturated solution, the crystals exhibit high crystallinity. The processing techniques using femtosecond laser and all solid-state 193 nm laser are effective for processing and manipulation of protein crystals without significant damage. Solution stirring technique also contributes the improvement of the crystal quality. We have succeeded in obtaining high-quality crystals of various proteins using the techniques, and revealing the precise structure of the protein molecules from the X-ray analysis. These techniques can accelerate structural biology and subsequent structure-based drug discoveries, resulting in important revelations in these fields.

In the past decades, many efforts have been made to replace mechanical tools in oral applications by various laser systems. The reasons therefore are manifold: i) Friction causes high temperatures damaging adjacent tissue. ii) Smear layers and rough surfaces are produced. iii) Size and shape of traditional tools are often unsuitable for geometrically complicated incisions and for minimum invasive treatment. iv) Mechanical damage of the remaining tissue occurs. v) Online diagnosis for feedback is not available. Different laser systems in the µs and sub-&mrg;s-pulse regime, among them Erbium lasers, have been tested in the hope to overcome the mentioned drawbacks and, to some extent, they represent the current state of the art with respect to commercial and hence practical application. In the present work the applicability of scanned ultrashort pulse lasers (USPLs) for biological hard tissue as well as dental restoration material removal was tested. It is shown that cavities with features superior to mechanically treated or Erbium laser ablated cavities can be generated if appropriate scan algorithms and optimum laser parameters are matched. Smooth cavity rims, no microcracks, melting or carbonisation and precise geometry are the advantages of scanned USLP ablation. For bone treatment better healing conditions are expected as the natural structure remains unaffected by the preparation procedure. The novelty of this work is represented by a comprehensive compilation of various experimental results intended to assess the performance of USPLs. In this context, various pulse durations in the picosecond and femtosecond regime were applied to dental and bone tissue as well as dental restoration materials which is considered to be indispensable for a complete assessment. Parameters like ablation rates describing the efficiency of the ablation process, and ablation thresholds were determined - some of them for the first time - and compared to the corresponding Erbium values. The morphology of the tissue surfaces remaining after laser preparation was investigated and the surface roughness was evaluateded. Selective ablation was stressed and the temperature impact induced by USPLs was analyzed. Due to the limited space only a selection of results can be presented.

We report a broad-band continuum light source with high power, low noise and a smooth spectrum centered at 1.15 &mgr;m for ultrahigh-resolution optical coherence tomography (OCT). The continuum is generated by self-phase modulation using a compact 1.059 &mgr;m femtosecond laser pumping a novel photonic crystal fiber, which has a convex dispersion profile with no zero dispersion wavelengths. The emission spectrum ranges from 800 to 1300 nm and results in a measured axial resolution of ~2.8 &mgr;m in air. We demonstrate ultrahigh-resolution OCT imaging of biological tissue using this light source.

In this article we investigate laser-assisted ablation of Hydroxy Ethyl Methacrylate (Hydro-gel) material using 30 femtosecond laser pulses delivered from a Ti:Sapphire multipass amplifier with a repetition rate of 1 kHz. Measurements of the crater depth, width and removed volume as a function of laser pulse energy and pulse number were made for stationary and translated ablation. Based on laser fluence, crater profile, collateral damage and translation speed, optimal laser parameters for efficient micromachining were defined. The results presented in this paper will be important for future developments in laser ophthalmology, where insight into the optimal laser parameters for corneallike surgery will decrease both processing time and collateral tissue damage.

Studies on corneal surgery and flap processing on enucleated porcine eyes have been performed using a dedicated femtosecond laser source based on Ytterbium technology. The influence of several laser parameters such as wavelength, energy, repetition rate and numerical aperture has been studied. Best parameters for ocular femtosecond laser surgery are discussed in terms of process efficiency and safety aspects. The 100 KHz laser system is a promising tool for corneal surgery

We demonstrate a directly diode-pumped Yb:KYW femtosecond chirped pulse laser amplifier with > 1 mJ pulse energy and pulse repetition rates higher than 5 kHz. We extracted up to 1.8 mJ at a pulse repetition rate of 2 kHz and 1.5 mJ at a pulse repetition rate of 5 kHz. After a high efficiency grating compressor having >80% transmission, we measured a pulse duration of 480 fs. The M² value was better than 1.2.

A microjoule level diode-pumped femtosecond Yb:KYW laser oscillator is demonstrated, delivering up to 1 &mgr;J pulse energies and 430 fs pulse duration, resulting in a pulse peak powers exceeding the MW level. The pulse repetition rate is about 10 MHz and the average power is on the 10 W. The laser setup is extremely compact and fits in a 60 x 30 cm footprint. Fiber-based pulse compression of the oscillator pulse train leads to 60-fs pulses of 0.42 &mgr;J pulse energy which corresponds to a peak power of 7 MW. This laser source is ideally suited for many micro- and nano-structuring applications where high-speed and precision are required.

Optical telecommunication systems constantly evolve toward higher bit rates, requiring the modulation and detection of higher-bandwidth optical waves. Commercial systems operating at 40 Gb/s are now available and research and development efforts are targeting higher bit rates for which optical pulses with picosecond-range duration are used. Chromatic dispersion, nonlinearities and amplified spontaneous emission from optical amplifiers are sources of transmission impairments that must be characterized and mitigated. Advanced modulation formats rely on the modulation of not only the amplitude of an optical wave (e.g., on/off keying), but also its phase (e.g., phase-shift keying) in order to optimize the transmission capabilities. The importance of the characterization of the properties of optical sources and components and the specificities of the optical telecommunication environment with respect to ultrafast optics are emphasized. Various diagnostics measuring the electric field of optical sources in the telecommunication environment are described. Sampling diagnostics capable of measuring eye diagrams and constellation diagrams of high-bit-rate, data-encoded sources are presented. Various optical pulse characterization techniques that meet the sensitivity requirements imposed by the telecommunication environment are also described.

Despite the growing number of biomedical and micromachining applications enabled by ultra-short pulse lasers in laboratory environments, realworld applications remain scarce due to the lack of robust, affordable and flexible laser sources with meaningful energy and average power specifications. In this presentation, we will describe a laser source developed at the eye-safe wavelength of 1552.5 nm around a software architecture that enables complete autonomous control of the system, fast warm-up and flexible operation. Our current desktop ultra-short pulse laser system offers specifications (1-5 microJ at 500 kHz, 800 fs-3 ps pulse width, variable repetition rate from 1 Hz to 500 kHz) that are meaningful for many applications ranging from medical to micromachining. We will also present an overview of applications that benefit from the range of parameters provided by our desktop platform. Finally, we will present a novel scalable approach for fiber delivery of high peak power pulses using a hollow core Bragg fiber recently developed for the first time by Raydiance and the Massachusetts Institute of Technology for operation around 1550 nm. We will demonstrate that this fiber supports single mode operation for core sizes up to 100 micron, low dispersion and low nonlinearities with acceptable losses. This fiber is a good candidate for flexible delivery of ultra-short laser pulses in applications such as minimally accessible surgery or remote detection.

In this contribution we report a high repetition rate optical parametric amplifier (OPA) pumped by a chirped pulse fiber amplifier system. Fiber CPA systems have demonstrated power scaling and open the route to OPAs at repetition rates in the 100 kHz-10MHz range. The OPA stage is seeded by a continuum generated in a Sapphire plate and more than 50 nm bandwidth is efficiently amplified, resulting in 3 &mgr;J, 29 fs pulses.

This document reports recent theoretical and experimental investigations of strong field ionization and high harmonic generation from mid-infrared lasers at 2 and 4 microns. Numerical solution of the time-dependent Schrodinger equation as well as Strong Field approximation calculations are reported. Photoelectron and high harmonic spectra are discussed. Preliminary experimental results are compared to the theoretical predictions.

We report the direct observation of a train of attosecond pulses by mean of autocorrelation method using nonresonant two photon processes in atoms and molecules in the xuv region. By analyzing the photoelectron spectra produced by two-photon above-threshold ionization in Ar, the chirp in the attosecond pulse train on the top of the pulse duration of 450 atttoseconds was determined. We also succeeded in observing interferometric fringes on the autocorrelation trace and reconstructed the electric field inside of attosecond pulses with ion signals produced by molecular Coulomb explosion.

We analyse recent experiments on momentum shearing interferometry of electron wave packets by using an optical analogy with shearing interferometry for optical waves. This analogy offers a convenient point of view to discuss the capabilities and difficulties of this technique used to access the phase of electron wave packets.

The evanescent coupling of femtosecond laser written waveguides with elliptical and circular shape is investigated in detail. Elliptical waveguides are used to investigate directional tuning of the coupling properties in a square array by tilting the elliptical waveguides. This allows to specifically pronounce diagonal coupling. In contrast, directional insensitive coupling is demonstrated in a circular waveguide array based on circular waveguides.

The use of ultrashort laser pulses has found widespread attention in the microstructuring of transparent materials. Specifically, the origin of refractive index changes in glasses and crystalline materials was extensively investigated. In LiNbO₃, which is an important material for nonlinear optical applications, the possibility of waveguide fabrication with fs laser pulses was also shown. Recently, two distinct types of waveguides were discovered in LiNbO₃ which show different thermal stability and optical properties. In one type, frequency doubling of 1064-nm radiation was demonstrated. Here, we discuss the different origins of the two waveguide types and present results of thermal annealing experiments. Furthermore, the influence of the processing parameters and the focussing on the properties of the waveguides was investigated. The electrooptic coefficient of the waveguide was measured and gives evidence that the nonlinear properties of the crystal are depleted by the laser structuring.

Over the last decade, Fiber Bragg Gratings (FBG) have become key components for optical telecommunication systems and sensor applications due to their low losses and narrow bandwidth filtering. Using conventional writing techniques based on UV absorption requires the use of photosensitive fiber material. However, this is problematic in active fibers and, therefore, causes problems when applying this technique to fiber lasers and amplifiers. In the last years, an alternative method based on the non-linear absorption of focused femtosecond pulses allowed the inscription of FBG into non-photosensitive fibers. We report here on the inscription of such gratings using IR femtosecond pulses and a phase-mask scanning technique to produce high reflectivity gratings in various non-photosensitive fibers. The specific issues associated with the femtosecond inscription like appropriate focusing and positioning techniques necessary for high quality phase-mask scanning will be discussed. We will discuss the application in fiber lasers based on rare-earth doped fibers with integrated Bragg reflectors.

Ultrashort laser pulses tightly focused provide intensity sufficient to initialize nonlinear ionization processes. Thus a plasma is generated in the focal region eventually resulting in optical breakdown. The deterministic character of this nonlinear interaction enables the generation of precise and highly reproducible material alteration. To gain better spatial precision applications have recently evolved strongly towards tight focusing of ultrashort pulses using microscope objectives as focusing units. The pulse energy required to generate optical breakdown was thus reduced to nanojoules or even below. The mechanical effects subsequent to plasma generation can be minimized to the very focus. Cell surgery with ultrashort pulses enables to precisely ablate cell organelles without observable hazardous effects to the surroundings or the entire cell. To numerically investigate the nonlinear interaction of ultrashort pulses with transparent media, a model including both nonlinear pulse propagation and plasma generation is introduced. The numerical code is based on a (3+1)-dimensional nonlinear Schrödinger equation describing the pulse propagation and the interaction with the density of free electrons that are generated in the focus. The nonlinear wave equation was derived taking into account both nonparaxial and vectorial effects to accurately include tight focusing at high numerical aperture. A multi rate equation model for dielectrics recently published by B. Rethfeld is used to simultaneously calculate the generation of free electrons. Numerical calculations based on this model are used to understand the dependence between size, geometry and density of optical breakdown plasmas in various focusing geometries of high numerical aperture. The code enables to use arbitrary initial conditions for the laser field in the focus. At high numerical aperture it is most important to start the simulation using realistic initial conditions. Especially the vectorial character of the electric field is most important to be considered. Thus a vectorial diffraction integral was used to calculate initial conditions at high numerical aperture. The code is applicable to any transparent Kerr medium, whose linear and nonlinear optical parameters are known. Within this work the code was applied to water as a model substance to biological soft tissue and cellular constituents.

Near infrared ultrafast pulsed laser is used to ablate pure metal and metal alloy targets in a vacuum chamber. We find that by optimizing the ablation conditions, as a direct result of ultrafast laser ablation, crystalline nanoparticles can be abundantly produced without intermediate nucleation and growth processes. Combining with different background gases, versatile structural forms can also be obtained for the nanocrystals. Using metal nickel as a sample material, we have produced Ni/NiO core/shell nanospheres and NiO nanocubes. We also study the production of alloy nanoparticles, which has been challenging in fabrication. We demonstrate production of nanoparticles containing up to three metal elements using ultrafast laser ablation. The laser ablation process is investigated using an ion probe in real-time. Nanoparticle samples are examined using atomic force microscopy and high resolution transmission electron microscopy for morphological, structural, and chemical analysis. This study provides a simple physical method for generating nanoparticles with a narrow particle size distribution, a high particle yield, versatile chemical compositions and structural forms.

Engineers have devised a novel ultra-short pulse laser lathe system for bulk micromachining of axisymmetric features in energetic material samples with three-dimensional cylindrical geometry. One hundred twenty femtosecond pulses from an 800-nm Ti:sapphire laser were utilized to machine hexanitrostilbene (HNS) rods with diameters less than 200 micrometers and greater than 5:1 aspect ratio without ignition and subsequent bulk combustion or detonation. To date, this work represents the smallest energetic material rod structures fabricated by this technology. Results indicate that surface roughness is dependent upon rotation speed and feed rate. Valuable explosive nano-particles were discovered, collected, and analyzed as a byproduct of fabrication.

When a femtosecond laser pulse is focused at the interface of two transparent substrates, localized melting and quenching of the two substrates occur around the focal volume due to nonlinear absorption. The substrates can then be joined by resolidification of the materials. We demonstrate the joining of similar and dissimilar glass substrates using a 1-kHz 800-nm Ti:sapphire amplifier. We investigated the laser parameter to join transparent substrates and characterized the joint strength and the transmittance through joint volumes.

The micro-welding technique based on the nonlinear absorption via focused femtosecond laser pulses is useful for welding transparent materials without introducing a light-absorbing intermediate layer. In fact, it has been successful to weld a wide variety of glass materials using 800-nm or 1045-nm pulses. In this paper, we show that this technique can be extended to semiconductor materials, which are opaque in the above wavelength regions, by demonstrating the welding of silicon and borosilicate glass. The key is the use of long-wavelength pulses. We used 1558-nm, 947-fs, 500-kHz pulses from an amplified femtosecond Er-fiber laser. We used a 20× objective lens with a numerical aperture of 0.40 to focus the pulses at the interface of silicon and borosilicate glass, which were mounted on a two-dimensional translation stage. By translating the stage perpendicular to the optical axis in the two-dimensional plane, we produced a 3 × 3 array that consists of welding areas of 100 &mgr;m × 100 &mgr;m. After welding, we performed a simple tensile test. The joint strength was found to be 3.74 MPa, which was on the same order as that between borosilicate glasses (9.87 MPa). Although the welding between silicon substrates is currently hindered by the difficulty of observing focal point with visible light, our result is an important step toward the welding of semiconductor materials, which may have various applications such as three-dimensional stack of electronic devices and the fabrication of micro-electro-mechanical systems.

For various applications it is interesting to directly visualize the propagation of light in waveguides. For this purpose, we used special fused silica glasses with a high content of OH. This leads to the formation of color centers when waveguides are written with fs laser pulses. When light is launched into the waveguides the color centers are excited and the fluorescence can be directly observed. This is especially interesting in waveguide arrays for the visualization of the evanescent coupling, since the discrete light evolution exhibits many features which are in strong contrast to propagation in common isotropic media. As an example for the visualization we will discuss here the possibility to excite a completely incoherent propagation within the waveguide array although the sources are fully coherent. When multiple waveguides are excited, the light evolution in the array can be described as a superposition of the single propagating amplitudes. The formula for the resulting intensity contains an interference term. One can explicitly show that this interference term vanishes for certain excitation patterns. When for instance two adjacent waveguides are excited the light propagates as there was no interference term, which is equivalent to the simple sum of the two intensities of the single amplitudes. This suggests the term "quasi-incoherent" for this new kind of propagation effect. In contrast a coherent superposition including the interference term is obtained for an excitation of two waveguides when there is one waveguide located between the two excited ones.

Pump-probe experiments using a delay line are one important approach in the investigation of molecular dynamics on the femtosecond to picosecond time scale. As the pulse energies for femtosecond pulses are usually small, the measured signal has to be obtained from the overlap region of two focused laser beams. Due to the low density in the gas phase high sensitivity experiments are essential, particularly if one or both laser pulses are in the infrared (IR) or near-IR range with much smaller absorption cross-section as compared to the visible or ultraviolet. To increase the interaction volume between pump- and probe-pulse, the two laser beams can be focused into a hollow waveguide with an inner diameter d_ID = 100 to 500 &mgr;m. We have calculated the focusing condition for a near-IR pump- and an UV probe-beam to excite nearly exclusively the lowest HE₁₁-mode within the waveguide. For molecular samples with a low absorption coefficient (alpha < 0.1 m^-1 for the probe beam in the ultraviolet) an enhancement of the measured probe signal of a factor of 9-10, relative to a confocal arrangement in a cell, is calculated from the intensity distribution within a hollow waveguide with an inner diameter d_ID = 250 &mgr;m and length L_wg = 500 mm. The theoretical calculations were confirmed in pump-probe experiments of intramolecular vibrational energy redistribution (IVR) in CH₃I vapour. In the experiments the first overtone of the CH-stretching vibration is excited with a near-IR pump-pulse and the redistribution of the vibrational energy to other vibrational degrees of freedom, especially to the CI-stretching vibration, is detected through a change of the UV-spectrum by a probe pulse around 310 nm.