Fiber laser systems offer unique properties for the amplification of ultrashort pulses to high powers. Two approaches are discussed, the amplification of linearly chirped parabolic pulses and a fiber based chirped pulse amplification system. Using the first method, we succeeded to generate 17-W average power of linearly chirped parabolic pulses at 75 MHz repetition rate and diffraction-limited beam quality in a large-mode-area ytterbium-doped fiber amplifier. The recompression of these pulses with an efficiency of 60% resulted in 80-fs pulses with a peak power of 1.7 MW. Furthermore, we report on a diode-pumped ytterbium-doped double-clad fiber based chirped pulse amplification system delivering 350-fs pulses, at 1060 nm wavelength, 75 MHz repetition rate and up to 60 W average power, corresponding to a peak power of 2.3 MW. Key element is a diffraction grating compressor consisting of highly efficient transmission gratings in fused silica allowing the recompression at this high power. Power scaling to the >100 W level is discussed.

Novel optical components enable the generation of the shortest pulses and broadest spectra from Kerr-Lens mode-locked laser oscillators without extracavity spectral broadening, namely 20-fs-pulses from Cr⁴⁺:YAG around 1.5μm, 14-fs-pulses from Cr:forsterite around 1.3μm, 5-fs-pulses from Ti:sapphire around 0.8μm, and 10-fs-Pulses from Cr³⁺:LiCAF around 0.8μm. Key components are well adapted phase correcting mirrors ("double-chirped mirrors") which allow for high reflectivity and dispersion compensation in bandwidths up to one octave. In parallel to the development of new broadband light sources based on femtosecond technology micron resolution imaging with Optical Coherence Tomography using theses sources has been achieved. The availability of the high resolution OCT technology for future clinical applications will depend on the development of low cost, compact sources of ultrabroad bandwidth light. Especially Cr³⁺:LiCAF is a very promising material for femtosecond laser sources as compact replacements for Ti:sapphire oscillators because of its low quantum defect, a broadband emission range around 800 nm, and an absorption band in a spectral range where high-brightness laser diodes are available.

Future commercial applications of fs-lasers require compact, reliable beam sources with low effort of operating expense. We report for the first time to our knowledge on a real all solid state master oscillator power amplifier system delivering pulse energies in the range of regenerative Ti:Sapphire amplifier systems (> 100 μJ) with sub 100 fs pulselength. The use of colquiriite crystals requires a pump source of high luminosity in the red spectral range. We realized such a pump source with red diode laser bars. Direct diode pumping and actively controlled thermoelectric cooling of diode bars and laser crystals ensure stable operation of oscillator and regenerative amplifier. For simplicity of the system we investigate stretcher-free CPA exploiting intracavity dispersion of the amplifier resonator. Due to low amplification of colquiriite crystals many roundtrips are required resulting in a stretched seed pulse. For pulse-compression a 6-prism-compressor was designed minimizing third order dispersion. Remaining dispersion is pre-compensated with chirped resonator mirrors. The presented laser system has been used for ablation of ear bones.

By integration of a semiconductor mirror into a chirped mirror based Ti:Sapphire oscillator a very compact pulsed Terahertz source is demonstrated. Terahertz radiation is generated by a transient photocurrent in a LT-GaAs layer grown on a semiconductor saturable absorber mirror. This technique allows the manufacturing of ultra-stable, small-size (600 x 200 mm) and self-starting THz systems pushing forward the usability and availability for commercial pulsed Terahertz sources. The Terahertz spectrum goes up to 3 THz and average output power of about 7 μW is achieved.

We describe several concepts for real time shaping and detection of femtosecond laser pulses using optical nonlinearities. Cascaded second order wave mixing is used for real-time conversion of spatial-domain images to ultrafast time-domain optical waveforms. We experimentally demonstrate a cascaded nonlinearity arrangement allowing generation of complex amplitude femtosecond waveforms with high fidelity and good conversion efficiency. Single-shot, phase-sensitive detection of femtosecond pulses is demonstrated using both nonlinear wave-mixing and 2-photon absorption in semiconductor detector arrays. Using commercial silicon charge-coupled device (CCD), the latter approach allows detection of broadband ultrashort signals in the important wavelength range around 1.5 microns without phase-matching limitations. Finally we describe an approach to characterization of the multimode fiber using ultrashort pulse interferometry.

We have measured the laser-induced breakdown (LIB) thresholds in water using an artificial eye for chirped and non-chirped laser pulses at 44 fs and 810 nm. We compare these measured thresholds to calculated values for a range of pulse widths from 20 fs to 120 fs and for various focal point diameters. The LIB threshold using a flat phase pulse, i.e., no chirped compensation for propagation through the water was measured to be 0.285 (0.280 - 0.290) μJ. Using a pre-chirp on the laser pulse, the LIB threshold dropped by one-third to 0.192 (0.191 - 0.194) μJ.

Ultrafast high-intensity laser pulses incident upon condensed matter targets can generate high-density plasmas that emit x-ray pulses with sub-picosecond temporal structure, significant spatial coherence, and high brightness at kilohertz repetition rates. Such laser-driven plasma x-ray sources based on solid and liquid metal targets have been developed in our laboratory. Essential performance features are discussed along with a feasibility evaluation for future routine application in chemical research. Laser-driven x-ray sources are usable for ultrafast x-ray diffraction and ultrafast x-ray absorption spectroscopy. X-ray absorption near-edge spectra of solvated transition metal complexes are presented. Ultrafast molecular dynamics depends on the structure of the solvated molecule before photo-excitation. This solvation structure, in turn, depends on the solute's interaction with the solvent molecules. Furthermore, the solute's vibrational modes and its structure are correlated, solvent dependent, and can be measured by mid-infrared and x-ray absorption spectroscopy. Such measured spectra are presented and correlated with quantum calculations in order to elucidate the solvation environment of various transition metal coordination complexes.

Recent progress in the development of a compact, high-repetition rate, ultrashort-pulse laser-driven hard-x-ray source based on the combination of a femtosecond laser system with an x-ray diode is reported. The x-ray source is characterized in terms of spectral and spatial properties. Hard-x-ray fluxes exceeding 2x10¹⁰ photons/s (emitted in 4π sr) are realized at a repetition rate of 250 kHz. A comparison with available laser-plasma hard-x-ray sources is presented. Numerical modeling is performed which proves that picosecond and sub-picosecond hard-x-ray pulses can be produced with this source. Further prospects and possible applications of the femtosecond laser-driven x-ray diode are outlined.

The use of ultrafast laser pulses to generate very high brightness, ultrashort (10^-14 to 10^-12 s) pulses of x-rays is a topic of great interest to the x-ray user community. In principle, femtosecond-scale pump-probe experiments can be used to temporally resolve structural dynamics of materials on the time scale of atomic motion. However, further development of this field is severely hindered by the absence of a suitably intense x-ray source that would drive the development of improved experimental techniques and establish a broader range of applicability. We report on a project at the Lawrence Livermore National Laboratory to produce a novel x-ray source and essential experimental techniques that will enable unprecedented dynamic measurements in matter. Based on scattering of a sub-50-fs, multi-terawatt, multi-beam laser from a co-synchronous and highly focused relativistic electron bunch, PLEIADES (Picosecond Laser Electron Interaction for Dynamic Evaluation of Structures) will produce tunable, ultrafast, hard x-ray (10- 200 keV) probes that greatly exceed existing 3rd generation synchrotron sources in speed (100 fs - 1 ps), peak brightness (10²⁰ ph/mm²s mrad² 0.1% BW, and >10⁹ ph/pulse), and simplicity (100-fold smaller). Such bright, ultrafast high energy x-rays will enable pump-probe experiments using radiography, dynamic diffraction, and spectroscopy to address the equation of state and dynamics of phase transitions and structure in laser heated and compressed heavy dense metals of interest for materials science.

Photon echo spectroscopy has been used to resolve the amplitudes and time scales of reorganization resulting from electronic excitation of the chromophore in three fluorescein-binding antibodies. The spectral density of nuclear motions derived by fitting the data serves as a characterization of protein flexibility. The three antibodies show motions that range in time scale from tens of femtoseconds to nanoseconds. Relative to the others, one antibody, 4-4-20, possesses a rigid binding site, that likely results from a short and inflexible HCDR3 loop and residue TyrL32 acting as a 'molecular splint,' to rigidify the Ag across its most flexible degree of freedom. The remaining two antibodies possess binding sites that are considerably more flexible, possibly due to the increased length of the HCDR3 loops. These variations in binding site flexibility may result in differing mechanisms of antigen recognition, including lock-and-key, induced-fit, and conformational selection.

We have demonstrated that intracellular Ca²⁺ waves in a living HeLa cell can be induced by femtosecond near-infrared laser pulses. In this paper, we present the results of investigation on the process of the Ca²⁺ wave generation using pharmacological methods to determine generation mechanisms. A mode-locked Ti:Sapphire laser (780 nm, 80 fs, 82 MHz) was used as a wave-triggering light source. The laser beam was focused into HeLa cells by using a water immersion objective lens (NA 0.9). Ca²⁺ waves were visualized by using a fluorescent Ca²⁺ indicator (Fluo-4) and monitored by a fluorescence microscope. Three mechanisms for the Ca²⁺ wave generation were considered; (1) Ca²⁺ flow into cells by destruction of the cell membrane, (2) mechanical stress by shock waves associated with the laser absorption, and (3) the leaking of Ca²⁺ through the destruction of intracellular Ca²⁺ stores. To investigate the mechanisms, we have performed experiments to determine the dependence of the probability of Ca²⁺ wave generation with two kinds of extracellular solutions; (a) a Ca²⁺ free extracellular solution (by use of EGTA), and (b) a solution containing U-73122 to inhibit the response to shockwave-based mechanical effects. From these experimental results, we can conclude the main mechanism of Ca²⁺ wave generation by laser irradiation is due to the leaking of Ca²⁺ through the destruction of intracellular Ca²⁺ stores.

Ultra-short pulsed-laser radiation has been shown to be an effective tool for controlled material processing and surface nano/micro-modification because of minimal thermal and mechanical damage. Nanostructuring of a variety of materials is gaining widespread importance owing to ever-increasing applications of nanostructures in numerous fields. This study demonstrates that controllable surface nanostructuring can be achieved by effectively utilizing the local field enhancement in the near field of a SPM probe tip irradiated with femtosecond laser pulses. Results of nanostructuring of various metallic and semiconductor thin film samples utilizing an 800nm femtosecond laser system in conjunction with a commercial SPM in ambient air are presented. Additionally, results from a companion micro-ablation study on gold thin films and numerical Finite Difference Time Domain (FDTD) simulation results for the spatial distribution of the laser field intensity beneath the tip are presented in an effort to achieve better understanding of the laser-material interaction. Flexibility in the choice of the substrate material and the processing environment, high spatial resolution (~10-12nm) and possibility of high processing rates by massive integration of the tips make this method an effective nanostructuring tool. Potential applications of this method include nanolithography, mask repair, nanodeposition, high-density data storage, as well as various nano-biotechnology related applications.

A new micromachining technique using user-defined trains of amplified femtosecond laser pulses is described. In this method, a 2-fold Michelson interferometer is used to split each output pulse of an amplified femtosecond laser system operating at 1 kHz into four different pulses at desired seperations ranging from 1 ps to 1 ns. These quadruple pulses are then focused on metal, semiconductor and dielectric samples and the material removal characteristics are noted. The experimental results show that there is a distinct effect of the pulse separation on the machining characteristics. It is observed that, in some cases, use of the quadruple pulses separated by 1 ns provides better material removal than the original pulses separated by 1 ms. The femtosecond laser-material interaction is also modeled for the case of metal samples using the two-temperature model. Numerical simulations that were carried out show that irradiation with quadruple pulses lead to a reduction in the predicted melting threshold fluence, which agrees with the experimental observation.

Localized structural and refractive index modifications can be generated inside transparent solids by using focused ultrashort laser pulses, which allows for example the fabrication of optical waveguides. In this paper we present the fabrication of true three-dimensional integrated optical devices. The optical properties of the produced 3D structures as well as processing details and requirements on the positioning accuracy will be discussed. The experimental results will be compared with beam propagation simulations and limitations of this technique will be evaluated.

Permanent refractive-index change can be induced by a self-trapped filament of infrared femtosecond laser pulses in silica glass. We present the fabrication of waveguides, couplers, and gratings by use of the self-trapped filament. The self-trapped filament with 30 μm long was found to bend from the direction of the incident laser pulses by two-dimensional translation. This technique leads to a formation of a permanent curved waveguide. We fabricate a 2-mm long directional coupler with the core diameter of 2 μm to split the coupled beam into 1:1. We also show the fabrication of gratings. Finally, we investigate the dependence of refractive-index change on polarization of incident laser pulses.

Laser processing of glass components is of significant commercial interest for the optoelectronics and telecommunications industries. Several fundamentally different interactions are employed to produce active components: after generating optical waveguides and gratings inside glass, external features must be machined in the modules to allow light to couple into the functional regions. In this paper, we present laser processing techniques using microsecond, nanosecond, and femtosecond lasers for surface and sub-surface glass modification. A regeneratively amplified Ti-Sapphire laser operating at a near-IR wavelength with femtosecond pulses and a 250 kHz repetition rate is used to generate 3-D optical waveguides and Bragg gratings in glass and silica substrates. Surface structures, mainly groove geometries, are generated with a diode-pumped solid-state nanosecond pulsed UV laser operating at 266 nm, a Q-switched CO₂ laser operating at 9.25 μm, a CO₂ laser operating at 10.6 μm and the femtosecond pulsed laser operating at 800 nm. The material interactions are examined with respect to the differences in time scale and the appropriateness of each laser type for particular processes.

A microfabrication system with the use of a femtosecond laser was designed for 3D processing of industrially important materials. The system includes a 120 fs, 1 kHz laser; beam delivery and focusing system, systems for automated 3D target motion and real-time imaging of the sample placed in a vacuum chamber. The first tests of the system on the processing of stainless steel and silicon are presented. We established thresholds and regimes of ablation for both materials. It was found that at relatively low laser fluences I < 3-5 J/cm² the regime of “gentle” ablation takes place, which is characterized by exceptional quality of the ablated surface, but slow ablation rate (< 25 nm/pulse). This regime is especially efficient for the patterning of markers on steel or silicon surfaces. The “fast” ablation regime at I > 10 J/cm² provides much higher ablation rate of 30-100 nm/pulse, giving an opportunity of fast high-quality processing of materials. This regime is well suited for drilling of through holes and fast cutting of materials. However, it was found that fast ablation regime imposes additional requirements on the quality of delivery and focusing of the laser beam because of the presence of parasitic ablation around the main spot on the tail of the radiation intensity distribution. As industrial machining examples, we demonstrate heat-affected-zone free drilling of through holes in a 50 μm thick stainless steel foil and cutting of a 50 μm thick Si wafer with a net cutting speed of 8 μm/sec.

The availability of ultra short (ps and sub-ps) pulsed lasers has stimulated a growing interest in exploiting the enhanced flexibility of femtosecond and/or picosecond laser technology for micro-machining. The high peak powers available at relatively low single pulse energies potentially allow for a precise localization of photon energy, either on the surface or inside (transparent) materials. Three dimensional micro structuring of bulk transparent media without any sign of mechanical cracking has been demonstrated. In this study, the potential of ultra short laser processing was used to modify the cladding-core interface in normal fused silica wave guides. The idea behind this technique is to enforce a local mismatch for total reflection at the interface at minimal mechanic stress. The laser-induced modifications were studied in dependence of pulse width, focal alignment, single pulse energy and pulse overlap. Micro traces with a thickness between 3 and 8 μm were generated with a spacing of 10 μm in the sub-surface region using sub-ps and ps laser pulses at a wavelength of 800 nm. The optical leakage enforced by a micro spiral pattern is significant and can be utilized for medical applications or potentially also for telecommunications and fiber laser technology.

We report on the experimental and analytical study of fiber delivery of femtosecond laser pulse through a step index (SI) and graded index (GI) multimode fiber. The 1.1 ps pulses of 8.7 μJ at 800 nm were delivered through a 10 cm GI multimode fiber with a 0.6 dB transmission loss. In the meantime, the output beam profile is nearly Gaussian and is independent of bending of the fiber. It is also shown that the prospect that the frequency pre-chirped pulse is usable for the utlrashort laser pulse delivery through GI multimode fibers by the numerical analysis.

Refractive surgery in the pursuit of perfect vision (e.g. 20/10) requires firstly an exact measurement of abberations induced by the eye and then a sophisticated surgical approach. A recent extension of wavefront measurement techniques and adaptive optics to ophthalmology has quantitatively characterized the quality of the human eye. The next milestone towards perfect vision is developing a more efficient and precise laser scalpel and evaluating minimal-invasive laser surgery strategies. Femtosecond all-solid-state MOPA lasers based on passive modelocking and chirped pulse amplification are excellent candidates for eye surgery due to their stability, ultra-high intensity and compact tabletop size. Furthermore, taking into account the peak emission in the near IR and diffraction limited focusing abilities, surgical laser systems performing precise intrastromal incisions for corneal flap resection and intrastromal corneal reshaping promise significant improvement over today's Photorefractive Keratectomy (PRK) and Laser Assisted In Situ Keratomileusis (LASIK) techniques which utilize UV excimer lasers. Through dispersion control and optimized regenerative amplification, a compact femtosecond all-solid-state laser with pulsed energy well above LIOB threshold and kHz repetition rate is constructed. After applying a pulse sequence to the eye, the modified corneal morphology is investigated by high resolution microscopy (Multi Photon/SHG Confocal Microscope).

In recent years, femtosecond laser processing of human hard/soft tissues has been studied. Here, we have demonstrated ablation etching of hydroxyapatite. Hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) is a key component of human tooth and human bone. The human bone is mainly made of hydroxyapatite oriented along the collagen. The micromachining of hydroxyapatite is highly required for orthopedics and dentistry. The important issue is to preserve the chemical property of the ablated surface. If chemical properties of hydroxyapatite change once, the human bone or tooth cannot grow again after laser processing. As for nanosecond laser ablation (for example excimer laser ablation), the relative content of calcium and phosphorus in (Ca₁₀(PO₄)₆(OH)₂) is found to change after laser ablation. We used here pulsewidth tunable output from 50 fs through 2 ps at 820 nm and 1 kpps. We measured calcium spectrum and phosphorus spectrum of the ablated surface of hydroxyapatite by XPS. As a result, the chemical content of calcium and phosphorus is kept unchanged before and after 50-fs - 2-ps laser ablation. We also demonstrated ablation processing of human tooth with Ti:sapphire laser, and precise ablation processing and microstructure fabrication are realized.

The measurement of femtosecond pulse laser induced refractive index changes inside transparent solids is challenging since only small phase differences of buried structures on a micrometer scale have to be resolved. Here we report on a new technique that allows to precisely determine the laser-induced refractive index changes in situ. It is based on the modification of an integrated optical Mach-Zehnder interferometer.

Photothrombotic microstrokes are produced in rat cortex by 532-nm single-photon optical excitation of an intravenously injected photosensitizer, rose bengal. The dynamics of blood flow and clot formation in the cortical vasculature are observed using two-photon laser scanning microscopy of an intravenously injected fluorescent dye. Flowing and clotted vessels are clearly distinguishable in both large and small vessels, down to individual capillaries, using this technique. We find that by tightly focusing the laser light used to excite the photosensitizer, clots can be formed in individual blood vessels without affecting neighboring vessels tens of micrometers away. We observe many changes in blood flow as a result of localized clot formation, including upstream vascular dilation, clot clearing, i.e. recanalization, and complete reversal of blood flow direction downstream.

Rare-earth doped fiber lasers provide a versatile technology platform for ultrafast laser systems, providing new flexibility in wavelength and timing capabilities.

The development of lasers and of femtosecond laser pulses provides many examples, how science and technology mutually reinforce each other. A brief historical overview is presented of this synergy between curiosity-driven and goal-oriented research in this field of optics, which supports the current activity in pulsed laser-materials interactions.