Tamper performance for confined laser drive applications.

The shock imparted by a laser beam striking a metal surface can be increased by the presence of an optically transparent tamper plate bonded to the surface. We explore the shock produced in an aluminum slab, for a selection of tamper materials and drive conditions. The experiments are conducted with a single-pulse laser of maximum fluence up to 100 J/cm.

Controlling interdependent meso-nanosecond dynamics and defect generation in metal 3D printing.

State-of-the-art metal 3D printers promise to revolutionize manufacturing, yet they have not reached optimal operational reliability. The challenge is to control complex laser-powder-melt pool interdependency (dependent upon each other) dynamics. We used high-fidelity simulations, coupled with synchrotron experiments, to capture fast multitransient dynamics at the meso-nanosecond scale and discovered new spatter-induced defect formation mechanisms that depend on the scan strategy and a competition between laser shadowing and expulsion.

Plasma mirror focal spot quality for glass and aluminum mirrors for laser pulses up to 20 ps.

High-intensity short-pulse lasers are being pushed further as applications continue to demand higher laser intensities. Uses such as radiography and laser-driven particle acceleration require these higher intensities to produce the necessary x-ray and particle fluxes. Achieving these intensities, however, is limited by the damage threshold of costly optics and the complexity of target chambers.

Enhancement of laser material drilling using high-impulse multi-laser melt ejection.

Laser drilling and cutting of materials is well established commercially, although its throughput and efficiency limit applications. This work describes a novel approach to improve laser drilling rates and reduce laser system energy demands by using a gated continuous wave (CW) laser to create a shallow melt pool and a UV ps-pulsed laser to impulsively expel the melt efficiency and effectively. Here, we provide a broad parametric study of this approach applied to common metals, describing the role of fluence, power, spot size, pulse-length, sample thickness, and material properties.

Resonance excitation of surface capillary waves to enhance material removal for laser material processing.

The results of detailed experiments and high fidelity modeling of melt pool dynamics, droplet ejections and hole drilling produced by periodic modulation of laser intensity are presented. Ultra-high speed imaging revealed that melt pool oscillations can drive large removal of material when excited at the natural oscillation frequency. The physics of capillary surface wave excitation is discussed and simulation is provided to elucidate the experimental results.

Metal vapor micro-jet controls material redistribution in laser powder bed fusion additive manufacturing.

The results of detailed experiments and finite element modeling of metal micro-droplet motion associated with metal additive manufacturing (AM) processes are presented. Ultra high speed imaging of melt pool dynamics reveals that the dominant mechanism leading to micro-droplet ejection in a laser powder bed fusion AM is not from laser induced recoil pressure as is widely believed and found in laser welding processes, but rather from vapor driven entrainment of micro-particles by an ambient gas flow. The physics of droplet ejection under strong evaporative flow is described using simulations of the laser powder bed interactions to elucidate the experimental results.

Interference effects in laser-induced plasma emission from surface-bound metal micro-particles.

The light-matter interaction of an optical beam and metal micro-particulates at the vicinity of an optical substrate surface is critical to the many fields of applied optics. Examples of impacted fields are laser-induced damage in high power laser systems, sub-wavelength laser machining of transmissive materials, and laser-target interaction in directed energy applications. We present a full-wave-based model that predicts the laser-induced plasma pressure exerted on a substrate surface as a result of light absorption in surface-bound micron-scale metal particles.

Light self-focusing in the atmosphere: thin window model.

Ultra-high power (exceeding the self-focusing threshold by more than three orders of magnitude) light beams from ground-based laser systems may find applications in space-debris cleaning. The propagation of such powerful laser beams through the atmosphere reveals many novel interesting features compared to traditional light self-focusing. It is demonstrated here that for the relevant laser parameters, when the thickness of the atmosphere is much shorter than the focusing length (that is, of the orbit scale), the beam transit through the atmosphere in lowest order produces phase distortion only.

Mechanisms governing the interaction of metallic particles with nanosecond laser pulses.

The interaction of nanosecond laser pulses at 1064- and 355-nm with micro-scale, nominally spherical metallic particles is investigated in order to elucidate the governing interaction mechanisms as a function of material and laser parameters. The experimental model used involves the irradiation of metal particles located on the surface of transparent plates combined with time-resolved imaging capable of capturing the dynamics of particle ejection, plume formation and expansion along with the kinetics of the dispersed material from the liquefied layer of the particle. The mechanisms investigated in this work are informative and relevant across a multitude of materials and irradiation geometries suitable for the description of a wide range of specific applications.

Damage on fused silica optics caused by laser ablation of surface-bound microparticles.

High peak power laser systems are vulnerable to performance degradation due to particulate contamination on optical surfaces. In this work, we show using model contaminant particles that their optical properties decisively determine the nature of the optical damage. Borosilicate particles with low intrinsic optical absorption undergo ablation initiating in their sub-surface, leading to brittle fragmentation, distributed plasma formation, material dispersal and ultimately can lead to micro-fractures in the substrate optical surface.

Characterization of laser-induced plasmas associated with energetic laser cleaning of metal particles on fused silica surfaces.

Time-resolved plasma emission spectroscopy was used to characterize the energy coupling and temperature rise associated with single, 10-ns pulsed laser ablation of metallic particles bound to transparent substrates. Plasma associated with Fe(I) emission lines originating from steel microspheres was observed to cool from >24,000 to ~15,000 K over ~220 ns as τ(-0.28), consistent with radiative losses and adiabatic gas expansion of a relatively free plasma.

Modeling of laser interactions with composite materials.

We develop models of laser interactions with composite materials consisting of fibers embedded within a matrix. A ray-trace model is shown to determine the absorptivity, absorption depth, and optical power enhancement within the material, as well as the angular distribution of the reflected light. We also develop a macroscopic model, which provides physical insight and overall results.

Change of self-focusing behavior of phosphate glass resulting from exposure to ultraviolet nanosecond laser pulses.

The self-focusing characteristic of 355 nm, 3.3 ns pulses propagating through phosphate glass samples is found to significantly change during repeated exposure. The results indicate this change is related to the formation of color centers in the material as well as the generation of a transient defect population during exposure to the laser pulses.

On the theory of the modulation instability in optical fiber amplifiers.

The modulation instability (MI) in optical fiber amplifiers and lasers with anomalous dispersion leads to cw radiation breakup. This can be both a detrimental effect limiting the performance of amplifiers and an underlying physical mechanism in the operation of MI-based devices. Here we revisit the analytical theory of MI in fiber optical amplifiers.

Modulation instability in high power laser amplifiers.

The modulation instability (MI) is one of the main factors responsible for the degradation of beam quality in high-power laser systems. The so-called B-integral restriction is commonly used as the criteria for MI control in passive optics devices. For amplifiers the adiabatic model, assuming locally the Bespalov-Talanov expression for MI growth, is commonly used to estimate the destructive impact of the instability.

Evaluation of the contribution of the renal capsule and cortex to kidney autofluorescence intensity under ultraviolet excitation.

The use of reduced nicotinamide adenine dinucleotide (NADH) fluorescence to gain metabolic information on kidneys in response to an alteration in oxygen availability has previously been experimentally demonstrated, but signal quantification has not, to date, been addressed. In this work the relative contribution to rat kidney autofluorescence of the capsule versus cortex under ultraviolet excitation is determined from experimental results obtained using autofluorescence microscopy and a suitable mathematical model. The results allow for a quantitative assessment of the relative contribution of the signal originating in the metabolically active cortex as a function of capsule thickness for different wavelengths.

Sub-critical regime of femtosecond inscription.

We apply well known nonlinear diffraction theory governing focusing of a powerful light beam of arbitrary shape in medium with Kerr nonlinearity to the analysis of femtosecond (fs) laser processing of dielectric in sub-critical (input power less than the critical power of self-focusing) regime. Simple analytical expressions are derived for the input beam power and spatial focusing parameter (numerical aperture) that are required for achieving an inscription threshold. Application of non-Gaussian laser beams for better controlled fs inscription at higher powers is also discussed.

Laser-induced damage in deuterated potassium dihydrogen phosphate.

Laser-induced pinpoint bulk damage of deuterated potassium dihydrogen phosphate at 351 nm is shown to depend on the propagation direction relative to the crystallographic axes and on growth temperature in addition to the previously reported dependence on continuous filtration. Pulse-length scaling is also consistent with earlier reports. The leading hypothesis for the cause of pinpoint damage is absorbing nanoparticle impurities, and our results are consistent with but not conclusive for that model.

Quantitative second-harmonic generation microscopy in collagen.

The second-harmonic signal in collagen, even in highly organized samples such as rat tail tendon fascicles, varies significantly with position. Previous studies suggest that this variability may be due to the parallel and antiparallel orientation of neighboring collagen fibrils. We applied high-resolution second-harmonic generation microscopy to confirm this hypothesis.

Polarization-modulated second harmonic generation in collagen.

Collagen possesses a strong second-order nonlinear susceptibility, a nonlinear optical property characterized by second harmonic generation in the presence of intense laser beams. We present a new technique involving polarization modulation of an ultra-short pulse laser beam that can simultaneously determine collagen fiber orientation and a parameter related to the second-order nonlinear susceptibility. We demonstrate the ability to discriminate among different patterns of fibrillar orientation, as exemplified by tendon, fascia, cornea, and successive lamellar rings in an intervertebral disc.

Polarization-dependent optical second-harmonic imaging of a rat-tail tendon.

Using scanning confocal microscopy, we measure the backscattered second harmonic signal generated by a 100 fs laser in rat-tail tendon collagen. Damage to the sample is avoided by using a continuous scanning technique, rather than measuring the signal at discrete points. The second harmonic signal varies by about a factor of 2 across a single cross section of the rat-tail tendon fascicle.

Ultrashort pulse laser ossicular ablation and stapedotomy in cadaveric bone.

Background And Objective: The purpose of this study was to evaluate the ablation of ossicular tissue using a 1,053 nm Ti:Sapphire chirped pulse amplifier laser system configured to deliver ultrashort pulses of 350 femtoseconds (fs) (3.5x10(-13) seconds) in cadaver temporal bone.

Study Design/materials And Methods: Ablation of the formalin-fixed incus and stapes was performed using an ultrashort pulse laser (USPL) (0.