This PDF file contains the front matter associated with SPIE Proceedings Volume 6881, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

Ultrashort laser pulses recently found extensive application in micro- and nanostructuring, in refractive surgery of the eye, and in biophotonics. Due to the high laser intensity required to induce optical breakdown, nonlinear plasma formation is generally accompanied by a number of undesired nonlinear side-effects such as self-focusing, filamentation and plasma-defocusing, seriously limiting achievable precision and reproducibility. To reduce pulse energy, enhance precision, and limit nonlinear side effects, applications of ultrashort pulses have recently evolved towards tight focusing using high numerical aperture microscope objectives. However, from the theoretical and numerical point of view generation of optical breakdown at high numerical aperture focusing was barely studied. To simulate the interaction of ultrashort laser pulses with transparent materials, a comprehensive numerical model taking into account nonlinear propagation, plasma generation as well as the pulse's interaction with the generated plasma is introduced. By omitting the widely used scalar and paraxial approximations a novel nonlinear propagation equation is derived, especially suited to meet the conditions of high numerical aperture focusing. The multiple rate equation (MRE) model is used to simultaneously calculate the generation of free electrons. Nonparaxial and vectorial diffraction theory provides initial conditions. The theoretical model derived is applied to numerically study the generation of optical breakdown plasmas, concentrating on parameters usually found in experimental applications of cell surgery. Water is used as a model substance for biological soft tissue and cellular constituents. For focusing conditions of numerical aperture NA < 0.9 generation of optical breakdown is shown to be strongly influenced by plasma defocusing, resulting in spatially distorted breakdown plasmas of expanded size. For focusing conditions of numerical aperture NA ≥ 0.9 on the other hand generation of optical breakdown is found to be almost unaffected by distortive side-effects, perfectly suited for material manipulation of highest precision.

Due to nonlinear interaction with optical transparent and scattering samples the femtosecond technology is a very useful tool for high precision micro surgery on biological tissues. At the same time femtosecond lasers are ideal light sources for imaging methods such as optical coherence tomography (OCT) due to the broad spectrum of the laser, which is necessary for creating ultra short pulses. Using OCT structures within biological tissues can be imaged non invasive with a resolution within the low m-range. The combined use of an ultra short pulse laser for cutting of biological tissues as well as imaging via OCT is a very interesting tool. It opens up a wide range of new surgery techniques and improves many existing methods due to high precision and high flexibility of the cutting process. Therefore we combined a femtosecond cutting system and a fourier domain OCT. In this attempt the OCT is operated with an SLD and is used alternately to the cutting system. The OCT is integrated into the optical path which enables in situ imaging of the surgery area before and after treatment.

We report on the development and successful application of a femtosecond Yb:KGd(WO₄)₂ laser for multimodal imaging of various biological samples. Its operation at longer wavelengths, 1029 nm, provided efficient excitation and greatly reduced sample photobleaching. The laser produced ~300 fs pulses with up to 100 nJ of energy at 14.3 MHz repetition rate. This laser system enabled continuous imaging of various live samples for prolonged periods of time. The details of laser development and fluorescence imaging of isolated chloroplasts are reported.

Monte Carlo simulation experiments have shown that very high energy electrons (VHEE), 150-250 MeV, have potential advantages in prostate cancer treatment over currently available electrons, photon and proton beam treatment. Small diameter VHEE beamlets can be scanned, thereby producing a finer resolution intensity modulated treatment than photon beams. VHEE beams may be delivered with greater precision and accelerators may be constructed at significantly lower cost than proton beams. A VHEE accelerator may be optimally designed using laser-plasma technology. If the accelerator is constructed to additionally produce low energy photon beams along with VHEE, real time imaging, bioprobing, and dose enhancement may be performed simultaneously. This paper describes a Monte Carlo experiment, using the parameters of the electron beam from the UCLA laser-plasma wakefield accelerator, whereby dose distributions on a human prostate are generated. The resulting dose distributions of the very high energy electrons are shown to be comparable to photon beam dose distributions. This simple experiment illustrates that the nature of the dose distribution of electrons is comparable to that of photons. However, the main advantage of electrons over photons and protons lies in the delivery and manipulation of electrons, rather than the nature of the dose distribution. This paper describes the radiation dose delivery of electrons employing technologies currently in exploration and evaluates potential benefits as compared with currently available photon and protons beams in the treatment of prostate and other cancers, commonly treated with radiation.

Laser plasma accelerators provide electron beams with parameters of interest in many fields and in particular for radiotherapy. A short review of progress achieved recently will be presented. We have performed dose deposition simulations using a quasi-monoenergetic electron beam in the 200 MeV range as produced today with laser-plasma accelerators. The electron beam properties offer advantageous dosimetric characteristics compare to those calculated with conventional radiotherapy with high energy photons. The lateral penumbra of treatment fields for focused electron beams is smaller compared to 6 MeV photons at depths smaller than 10 cm. The depth dose curve shows a broad maximum at large depths (> 20 cm). These advantages result in an improvement of the quality of a clinically approved prostate treatment plan. While the target coverage is the same or even slightly better for 250 MeV electrons compared to photons the dose sparing of sensitive structures is improved. More precisely, the dose to the rectum is reduced by 19% for 250 MeV, focused electrons. These findings agree with previous results regarding very high energy electrons as a treatment modality.

This tutorial focuses on the new opportunities that may be harnessed when the operating wavelength of acceleration structures is reduced by more than 5 orders of magnitude from tens of centimeters to a fraction of a micro-meter. On the one hand, we show how a Bragg waveguide may be adapted to work as an acceleration structures and on the other hand, we examine particle acceleration by stimulated emission of radiation - a novel acceleration paradigm demonstrated recently. Implications on future medical accelerators are discussed.

We report the first experimental evidence for direct particle acceleration by stimulated emission of radiation (PASER) namely, energy stored in microscopic cavities such as molecules, that otherwise may be used to amplify radiation, may be directly utilized for acceleration of a train of electron micro-bunches. In the framework of this proof-of-principle experiment, conducted at the Brookhaven National Laboratory, a 45MeV electron macro-bunch was modulated by its interaction with a high-power CO₂ laser pulse, within an adequate wiggler, and then injected into an excited CO₂ gas mixture. The emerging micro-bunches experienced a 0.15% relative change in the kinetic energy, in a less than 40cm long interaction region. Both the fundamental frequency of the train of micro-bunches and the active medium main resonance frequency are matched. This proof-of-principle experiment demonstrates, for the first time ever, the feasibility of coherent collisions of the second kind i.e., a particle analog of the laser.

The average power and efficiency of processes that exhibit low interaction cross section and low optical loss can often be enhanced by recirculating the laser pulse in the cavity. Inverse Compton scattering of the photon pulse on an electron bunch, harmonic generation, and spectroscopy represent examples of such processes. Methods for laser recirculation that enhance the interaction efficiency have been proposed in the past, based on resonant cavity coupling, intracavity amplification, or electro-optical switching. Those methods exhibit limitations such as interferometric alignment accuracies, complexity, and nonlinear phase accumulation. A novel scheme for energetic short laser pulse recirculation, termed recirculation injection by nonlinear gating (RING), is described. RING is based on intracavity nonlinear frequency conversion for optical switching, does not exhibit interferometric alignment constraints, and is scalable to extreme peak power. Initial demonstration of the RING technique is presented at a 1-mJ level, with cavity enhancement factors exceeding 25 in a simple unstable resonator cavity. Applications of the RING technique in biomedical and other applications are outlined.

We report the generation of 63 fs pulses of 290 nJ energy and 4.6 MW peak power at 1050 nm based on the use of a single polarisation maintaining ytterbium-doped fiber parabolic amplification system. We demonstrate that the operation of the amplifier beyond the gain bandwidth limit plays a key role on the sufficient recompressibility of the pulses in a standard grating pair compressor. This results from the accumulated asymmetric nonlinear spectral phase and the good overall third-order dispersion compensation in the system.

We report on an industrial ultrafast laser system for internal marking and engraving of transparent materials. Current engraving techniques, using nanosecond infrared or UV laser, are either limited to surface marking, or create thermal stress which can lead to fracture of material. Using an infrared ultrafast laser, we describe an industrial equipment able to non-aggressively mark alphanumeric or datamatrices at a high speed, with a high readability. Anti-counterfeiting and normative applications will be presented, as well as integration on a production line.

We demonstrate an Ytterbium diode-pumped laser with >10 W average output power, pulse repetition rate up to 100 kHz, and pulse duration around one picosecond. This system does not use the chirped pulse amplification system, yet generates much shorter pulses than typical picosecond lasers. The amplified pulses are slightly chirped, and pulse duration after external compression was as short as 400 fs. The laser combines the advantages of a picosecond amplifier, i.e. no pulse stretcher and compressor, with ultra short pulse durations which makes the laser source extremely interesting for applications such as transparent or semiconductor materials processing.

We report on the generation of sub-500fs pulses of 3.5μJ energy at 6.49 MHz repetition rate and 1037 nm based on the amplification of an Ytterbium femtosecond oscillator, frequency locked to an external source in a double stage fiber chirped pulse amplification system. The system is operated in a burst mode where bunches of pulses of 900μs duration are amplified at 5Hz repetition rate. Such a system is dedicated to the measurement the spatial distribution of electron beam in future large scale ultra-high brightness particle collider.

We describe two techniques for measuring the complete spatio-temporal intensity and phase, E(x,y,z,t), of an ultrashort pulse. The first technique is an experimentally simple and high-spectral resolution version of spectral interferometry, which uses fiber optics to introduce the pulse that is to be characterized into the device. By scanning the fiber around the focus, this device can be used to measure the spatio-temporal field of a focusing ultrashort pulse. We illustrate this technique by measuring the spatio-temporal filed for several different focused pulses. The other technique measures the complete spatio-temporal field of a pulse using a very simple experimental setup. While this technique will not work at the focus, it is single shot and requires only a single camera frame to reconstruct the complete filed versus space and time. This technique involves measuring multiple holograms, each at a different wavelength, and all in a single camera frame. To test this technique we show that it can accurately measure the spectral phase. We also illustrate this technique by measuring, E(x,y,t) of a single laser pulse.

We demonstrate the use of two ultrafast fiber laser systems locked together at identical repetition rates of 100 MHz to achieve a timing resolution below 300 fs for pump-probe experiments. By sweeping the set-point of the locking electronics, we scan the time delay between the individual pulse trains by 800 ps. This scanning technique requires only sub-micrometer mechanical motion. Since the temporal scan range is determined electronically, the acquisition can be limited to regions where meaningful physical data is recorded. We discuss how our technique can approach asynchronous optical sampling based on GHz repetition rate lasers in terms of data collection efficiency while offering a number of practical advantages.

Femtosecond laser direct writing technique is a powerful tool to integrate micro-optics, compared to a conventional method of assembling bulk micro-optics. This technique has a great advantage to save the assembly time and to confirm effects of integration in short time. In this paper, we demonstrate the integration of a micro grating and a diffractive lens inside bulk silica glass by using the femtosecond laser direct writing technique. The grating can split an input beam into several numbers of beams and the diffractive lens can focus the beam at its focal plane. Therefore the integrated optics has a new function of the combination of the splitting and the focusing. Moreover we demonstrate the integration of two diffractive lenses inside silica glass and a pin hole on its surface. The integrated optics can realize collimating and focusing simultaneously. From the industrial point of view, we propose two potential applications: a high efficiency photo detector and a miniature spectrometer. Device concepts and the evaluation results are described.

When ultrafast laser pulses are focused inside the interface between a couple of transparent materials, the optical intensity in the focal area can usually become high enough to initiate the filamentary propagation of optical pulses and almost simultaneously the nonlinear absorption occurs in the filamentary area. Due to this absorption of optical energy, both of the materials can locally be melted and the interface is joined after resolidification. The laser microwelding technique based on the nonlinear phenomena has several unique features: (i) the insertion of any intermediate layers is no need for the microwelding, (ii) it's possible to weld the materials with different thermal expansion coefficients, (iii) the joint area can also be arbitrarily extended by scanning the filamentary area. We call this powerful technique "ultrafast laser microwelding". In this paper, we present the results on "ultrafast laser microwelding" for transparent materials such as the silica and borosilicate glass, and heterogeneous materials such as glass and metals.

In this work, spot-size dependence of surface femtosecond laser-induced damage threshold in fused silica is put in evidence when the damage reach the micrometer scale. Measurements of the threshold with various numerical apertures and different techniques are performed, revealing a noticeable threshold increase while decreasing the laser beam-focus size below ~10 μm. This unexpected result could be explained by the presence of micrometer-sized defects pre-existing in the SiO₂ sample.

Low-energy femtosecond laser pulses (typically in the tenth of nJ per pulse regimes at 250kHz) focused into fused silica substrates induce various modifications in the material properties of the base material, including a localized increase of the refractive index. Related sub-microns periodic structures found in the laser-exposed regions have also been recently described by several authors. The characterization of the laser-affected zones is particularly challenging due to their small sizes - typically micron or sub-micron. Experimental methods previously reported have either limited spatial resolution or require additional material processing to reveal the zone of interest, leaving open questions related to the influence of the processing itself. Using an Atomic Force Microscope equipped with a thermal probe, we recently published that low-energy femtosecond laser pulses leave thermal conductivity change footprints. The thermal footprints match very well the zone where a higher refractive index is observed. This novel analytical method does not require any processing of the surface prior to the observation and yields high-quality, sub-micron resolution, maps of the laser affected zones. Furthermore, it also opens new interesting and fundamental questions on the effect of femtosecond laser irradiation on fused silica. In this paper, we report on systematic observations made on fused silica specimens exposed to various pulse energies under different polarization conditions. We analyze and discuss the effect of the laser exposure on the thermal properties of the fused silica substrate.

Flexible hollow-glass fibers with inner coating of silver and polymer are employed for delivery of femtosceond laser pulses. In the experiment, ultrashort pulses with a pulse width of 196 fsec, energy of 0.7 mJ, and repetition rate of 1 kHz are transmitted with no damage on the hollow optical fiber. The transmission loss is 0.13 dB in the 1.0-mm bore fiber and pulse broadening after transmission in 1-m long fiber is 17 fsec. Theoretical calculation shows that the numbers of transmitted modes in the fiber is estimated from the pulse broadening.

A flexible method for fabricating shallow optical waveguides by using femtosecond laser writing of patterns on a metal coated glass substrate followed by ion-exchange is described. This overcomes the drawbacks of low index contrast and high induced stress in waveguides directly written using low-repetition rate ultrafast laser systems. When compared to conventional lithography, the technique is simpler and has advantages in terms of flexibility in the types of structures which can be fabricated.

We have systematically studied femtosecond-laser fabrication of optical waveguides in an Er-Yb doped phosphate glass. Waveguides were written using the IMRA America FCPA μJewel D-400 femtosecond fiber laser system with pulse repetition rates ranging from 250 kHz to 2.2 MHz. At every pulse repetition rate a series of waveguides was written while varying scan speeds from 50 μm/s to 100 mm/s and pulse energies from 80 nJ to 320 nJ. The optical quality of the waveguides was evaluated by measuring the waveguide mode profile as well as the optical loss. Laser-induced defects and structural changes in the glass were characterized using confocal fluorescence and Raman microscopy.

The authors report on the fabrication of buried waveguides in both lithium niobate and periodically poled lithium niobate. First a low insertion loss waveguide is fabricated in z-cut lithium niobate using femtosecond laser waveguide inscription. To fabricate a waveguide exhibiting both low propagation and coupling losses, we used the multiscan fabrication technique to control the size of the waveguide cross section. We measured coupling losses of 1.1 dB/facet and propagation losses as low as 0.6 dBcm^-1. Optical waveguides have been also inscribed in periodically poled lithium niobate by femtosecond laser pulses with the same multiscan technique. Second harmonic generation experiments from a fundamental wavelength of 1567 nm demonstrate that the nonlinear optical coefficient in the waveguides is preserved, yielding a conversion efficiency of 18% W^-1.

We present some of our results on the femtosecond laser direct writing and characterization of micro-structures in silicate, Foturan^TM, and tellurite glasses. Structures with different sizes were fabricated with varying input energy and spatially modified pulse using a slit. Various characterization techniques including fluorescence spectroscopy, micro- Raman spectroscopy, and laser confocal microscopy were employed to analyze the structural and physical modifications at focal volume resulting in the change of refractive index (RI). The RI change due to material modification was estimated using diffraction from a continuous wave laser beam and is presented in this work. The results obtained are analyzed vis-a-vis the recent work in similar glasses and the applications of such structures in the fields of photonics.

We present some of our results on the femtosecond laser direct writing and characterization of micro-gratings in Baccarat glass. Gratings were inscribed with amplified 800 nm, ~100 femtosecond pulses at 1 kHz repetition rate. The change in refractive index of the modified region was estimated from grating efficiency measurements and was found to be ~10^-3. Micro-Raman studies demonstrated an increase in the intensity of the band near 596 cm^-1 in the laser irradiated region clearly indicating an increase in the refractive index. Micro-Raman mapping of the grating showed a periodic variation of the band intensity further confirming the formation of grating. Structures with sub wavelength dimensions (<800 nm) were achieved with shaping of the input pulses using a rectangular slit. Waveguides were inscribed by optimizing parameters like slit width, focusing conditions, translation speed etc. We shall present our results on the physical, spectroscopic and optical characterization of these structures.

Thin film solar cells have shown a big potential to decrease cost of manufacturing for photovoltaic power generation. Despite of all research attempts to optimize materials and efficiency the mass production of thin film solar cells is still employing some mechanical steps of structuring, where thin films with a thickness of approximately 1 μm are selectively separated to be galvanic isolated. Here we show the structuring of CIS (CuInSe₂) thin films solar cells by picosecond laser ablation. We show a selective removal of single layers on sample substrates and on real solar cells. We utilized high repetition rate lasers in order to maximize process speed enabling an application of ultrafast laser structuring for mass production.

We have prepared absorbing structures for photovoltaic cells with different nano-texturization, obtained by means of a femtosecond laser, without the use of corrosive gas (i.e. under vacuum). To take in account the 3D structured front surface, the emitter doping has been realized by using Plasma Immersion Ion Implantation (so-called PULSION). The results show a photocurrent increase up to 60 % in the laser texturized zones.

Picosecond pulses are ideal for processing of materials where thermal influence must be minimized. Furthermore, for ps pulse durations, the technical approach to generating the pulses can be greatly simplified, allowing average output powers of 50 W at the fundamental wavelength of 1030 nm. We report on the latest results for efficient micro machining. Fundamental investigations show the measured ablation thresholds for the most convenient materials (i.e. Fe, SS, Al, Cu, Brass, Si, NiTi, etc.) and the measured transition from so called optical ablation to thermal ablation. By using these results we optimized micro machining processes, like cutting, drilling and thin film ablation with ps pulses.

Laser breakdown in liquid induced by ultrafast high repetition rate laser pulses tightly focused close to a flat or curved surface of a liquid-gas boundary is investigated. It is shown, that in case of focusing a laser beam close to the liquid - gas boundary a low divergent jet appears consisting of liquid and bubbles of micron size and less. The direction of the jet coincides with the perpendicular to the boundary surface, passing through the objective focal point. The length of the jet depends on the kind of liquid, laser pulse parameters and the distance between the beam focus point and the liquid - gas boundary and may reach several centimeters.

Lasers are able to saw complex diamond crystals with improved yields and provide better accuracy, greater speed, and lower breakage rates even for sensitive diamonds and lower weight loss for difficult stones. In the present investigation, four different types of nanosecond Q-switched Nd:YAG lasers namely an arc lamp pumped Nd:YAG laser operating at 1064 nm, arc lamp pumped Nd:YAG laser operating at second harmonically generated 532 nm, diode pumped Nd:YVO₄ laser operating at 1064 nm, a diode pumped Nd:YAG laser operating at 1064 nm, 532nm picosecond Diode pumped Nd:YAG laser and 510 nm Cu vapor laser have been employed for the processing of single crystal gem quality natural diamond to study the effects of pulse width and wavelength on various aspects of processing and the relative merits and demerits. The overall weight loss of the diamond and formation of micro cracking during processing have been studied for the above four cases. The characteristics of graphite formed during processing, elemental analysis, surface morphology of cut face and process dynamics have been studied using Micro Raman spectroscopic technique and Scanning Electron Microscopy (SEM). The Micro Raman and SEM analysis show that the surface quality is obtained superior by using diode pumped Nd:YVO₄ laser due to its extremely high peak power. The maximum graphite content is observed while processing using lamp pumped Nd:YAG laser at 532 nm. Shorter laser pulses result in higher cutting rate of natural diamonds due to better localization of absorbed laser energy.