Burst-mode ultrafast laser treatments in biological tissues or in materials-processing use high-repetition-rate (⪆MHz) delivery of femtosecond laser pulses. This takes advantage of characteristically tiny residual heat left in a substrate through individual femtosecond-laser-matter interaction. At the same time, the approach opens the door to manipulating the accumulation of that same tiny heat during rapid pulse-repetition. This mode of fluence-delivery may, for instance, be able to denature the protein in the walls of a laser-cut wound and possibly improve infection rates in ultrashort-pulse laser surgery in certain contexts. Isolated intense sub-picosecond laser pulses typically do not rely on intrinsic chromophores for absorption, instead they first create a limited plasma via nonlinear ionization, then increase that plasma through collisional ionization. Used in burst-mode, plasma-mediated ablation can exploit some residual ionization which persists for a few nanoseconds, meaning that subsequent pulses need not re-initiate dielectric breakdown. In effect, the plasma is ‘simmered’ continuously throughout a burst, controlling the mode and amount of absorption and opening the door to particularly gentle laser cutting of tissues and dielectric materials. We describe pulse-by-pulse studies of the persistence of the plasma state within a burst of approximately 60 pulses, each of 300 fs duration, arriving with an intra-burst repetition rate of 200 MHz (5 ns separation). We also present the impact of these burst-mode treatments on cellular necrosis in a phantom of rat-glioma cells suspended in hydrogels and in porcine cartilage samples.
Conventional solid-density laser-plasma targets quickly ionize to make a plasma mirror, which largely reflects ultra-intense laser pulses. This Fresnel reflection at the plane boundary largely wastes our e
orts at ultra-intense laser/solid interaction, and limits target heating to nonlinear generation of high-energy electrons which penetrate inward. One way around this dual problem is to create a material with an anisotropic dielectric function, for instance by nanostructuring a material in such a way that it cannot support the material responses which generate a specularly reflected beam. We present linear theory for metallic and plasma nanowires, particle-incell simulations of the interaction of ultra-intense femtosecond pulses with nickel nanowires, showing penetration of laser light far deeper than a nickel skin-depth, helping to uniformly heat near-solid material to conditions of high energy-densities, and XFEL experiments giving insight into their ionization and excitation.
S. Fourmaux, S. Buffechoux, S. Gnedyuk, B. Albertazzi, D. Capelli, L. Lecherbourg, A. Lévy, P. Audebert, D. Houde, R. Marjoribanks, F. Martin, H. Pépin, J. Fuchs, J. C. Kieffer
Focusing a high intensity laser pulse, onto a thin foil target generates a plasma and energetic proton and ion beams from
the target rear and front sides, propagating along the target normal. Such laser produced collimated and energetic protons
beams are of high interest because of the wide range of applications: ion based fast ignitor schemes, probing of
electromagnetic fields in plasma, isotopes production or hadron therapy. The 100 TW class laser system at the Advanced
Laser Light Source facility, is used with an intensity close to 1019 W/cm2, to study protons acceleration with
femtosecond laser pulses, ultra thin foil target and high contrast laser pulse ratio. To characterize the plasma expansion,
we monitor it with an imaging technique using a femtosecond laser probe. In this configuration we were able to reach a
proton critical energy of 12 MeV and to work with target foil thickness as small as 15 nm.
S. Fourmaux, S. Buffechoux, B. Albertazzi, S. Gnedyuk, L. Lecherbourg, S. Payeur, P. Audebert, D. Houde, R. Marjoribanks, F. Martin, H. Pépin, J. Fuchs, J. C. Kieffer
Collimated beams of energetic protons are produced by the interaction of short duration high intensity laser pulses with
solid foils. This field has been the subject of many studies in the last decade. This interest is motivated by the wide range
of application of such beams: ion based fast ignitor schemes, probing of electromagnetic fields in plasma, isotope
production or hadron therapy. The recently commissioned 200 TW laser system (5 J, 25 fs, 1010 laser pulse contrast, 10
Hz repetition rate at 800 nm) at the Advanced Laser Light Source (ALLS) facility has been used to study proton
acceleration with femtosecond laser pulses. The proton spectrum was characterized using a time of flight detector. Due
to the high contrast of the laser pulse, foil targets as thin as 30 nm could be studied.
Pulsetrain-burst machining has been shown to have advantages over single-pulse laser processing of materials
and biological tissues. Ultrafast lasers are often able to drill holes in brittle and other difficult materials without
cracking or swelling the target material, as is sometimes the case for nanosecond-pulse ablation; further,
pulsetrain-bursts of ultrafast pulses are able to recondition the material during processing for instance, making
brittle materials more ductile and striking advantages can result. In the work we report, we have investigated
hole-drilling characteristics in metal and glass, using a Nd:glass pulsetrain-burst laser (1054 nm) delivering 1-10
ps pulses at 133 MHz, with trains 3-15 μs long. We show that as the beam propagates down the channel being
drilled, the beam loses transverse coherence, and that this affects the etch-rate and characteristics of channel shape:
as the original Gaussian beam travels into the channel, new boundary conditions are imposed on the
propagating beam principally the boundary conditions of a cylindrical channel, and also the effects of plasma
generated at the walls as the aluminum is ablated. As a result, the beam will decompose over the dispersive
waveguide modes, and this will affect the transverse coherence of the beam as it propagates, ultimately limiting
the maximum depth that laser-etching can reach.
To measure transverse beam coherence, we use a Youngs two-slit interference setup. By measuring the fringe
visibility for various slit separations, we can extract the transverse coherence as a function of displacement across
the beam. However, this requires many data runs for different slit separations. Our solution to this problem
is a novel approach to transverse coherence measurements: a modified Michelson interferometer. Flipping the
beam left-right on one arm, we can interfere the beam with its own mirror-image and characterise the transverse
coherence across the beam in a single shot.
Energy absorption and heat transfer are important factors for regulating the effects of ablation of biological
tissues. Heat transfer to surrounding material may be desirable when ablating hard tissue, such as teeth or bone,
since melting can produce helpful material modifications. However, when ablating soft tissue it is important to
minimize heat transfer to avoid damage to healthy tissue - for example, in eye refractive surgery (e.g., Lasik),
nanosecond pulses produce gross absorption and heating in tissue, leading to shockwaves, which kill and thin the
non-replicating epithelial cells on the inside of the cornea; ultrafast pulses are recognized to reduce this effect.
Using a laser system that delivers 1ps pulses in 10μs pulsetrains at 133MHz we have studied a range of heat- and
energy-transfer effects on hard and soft tissue. We describe the ablation of tooth dentin and enamel under
various conditions to determine the ablation rate and chemical changes that occur. Furthermore, we characterize
the impact of pulsetrain-burst treatment of collagen-based tissue to determine more efficient methods of energy
transfer to soft tissues. By studying the optical science of laser tissue interaction we hope to be able to make
qualitative improvements to medical treatments using lasers.
Ultrafast-laser micromachining has promise as an approach to trimming and 'healing' small laser-produced damage sites in laser-system optics--a common experience in state-of-the-art high-power laser systems. More-conventional approaches currently include mechanical micromachining, chemical modification, and treatment using cw and long-pulse lasers. Laser-optics materials of interest include fused silica, multilayer dielectric stacks for anti-reflection coatings or high-reflectivity mirrors, and inorganic crystals such as KD*P, used for Pockels cells and frequency-doubling. We report on novel efforts using ultrafast-laser pulsetrain-burst processing (microsecond bursts at 133 MHz) to mitigate damage in fused silica, dielectric coatings, and KD*P crystals. We have established the characteristics of pulsetrain-burst micromachining in fused silica, multilayer mirrors, and KD*P, and determined the etch rates and morphology under different conditions of fluence-delivery. From all of these, we have begun to identify new means to optimize the laser-repair of optics defects and damage.
Photonics Research Ontario (PRO) is an Ontario Provincial Center of Excellence supporting a broad range of laser- processing activities within its photonics program. These activities are centered at the University of Toronto, and split between an industrial-user facility and the individual research programs of principal investors. The combined effort furnishes forefront laser system and advanced optical tools to explore novel processing applications in photonic, biomedical, and microelectronic areas. Facilities include laser micromachining stations, excimer-based mask-projection stations, extremely short wavelength lasers such as the molecular fluorine laser, and ultrafast laser systems. The latter two advanced laser offer interesting advantages and contrast in processing 'difficult' materials through linear and nonlinear absorption processes, respectively. These laser systems provide fine precision and strong interaction with a wide range of materials, including 'transparent' glasses, and also ceramics and metals. Applications fall broadly into several areas: wafer-level circuit trimming, high-resolution ultrasonic transducers, and the shaping of optical waveguides and Bragg-gratings for photonic components. This paper summarizes the laser-processing infrastructure and research activities at PRO.
Ablation rates and etched-surface morphology of fused silica has been studied with 1-ps Nd:glass laser pulses in a regime of near-diffraction-limited spot size. Shallow holes of 1.7- micrometers diameter were too small for the formation of laser- induced periodic-surface structures. Atomic-force and scanning-electron microscopy showed that reproducible etch depth and moderately smooth surfaces are attainable for low fluences of 5.5 - 45 J/cm2--the `gentle' ablation regime. Etch depth progressed linearly with the number of laser pulses until the onset of surface swelling and shock- induced microcracks after a critical number Nc of laser pulses, scaling as Nc equals 1.7 + 80/F (fluence F in J/cm2). Below this limit--for accumulated etch depths less than approximately 2 micrometers --3D surface structuring with sub-micron precision is possible with picosecond-laser pulses. In the strong ablation regime (F > 45 J/cm2), surface morphology was poor and microcracking developed within 2 - 4 pulses. These shock-induced microcracking effects were eliminated when a mode-locked train of approximately 400 identical 1-ps pulses, each separated by 7.5 ns, was applied. Very smooth and deep (approximately 30- micrometers ) holes of 7 - 10-micrometers diameter were excised at a total fluence of approximately 100 kJ/cm2, establishing a new means for rapid and precise micromachining of fused silica and other brittle materials.
A self-healing target has been demonstrated for synchronous photo-pumping of extreme- ultraviolet lasers. The threaded, mercury-wetted copper rod, rotating in a pool of mercury, offers similar photoionization efficiency as gold targets, but without target-surface degradation. A gain coefficient of 1.4 cm-1 was obtained with mercury for the 1089 angstrom Xe2+ laser at a pump-laser intensity of 140 GW(DOT)cm-2, comparing favorably with results from gold targets.
The ratio of isoelectronic lines was used to measure the electron temperature of subnanosecond and picosecond plasmas of a range of two-element materials: NaF, Mg-Al alloy, and KCl. Modelling shows that although populations may be far from steady-state in picosecond plasmas, the ratio of isoelectronic lines may be nearly steady-state, simplifying interpretation. Contour plots of the ratios of a number of isoelectronic helium-like line-pairs, suitable for steady-state electron-temperature interpretation, are provided.
A fiberless 1-TW all-Nd:glass chirped-pulse amplification laser system is described in this paper. Starting from high-contrast 1-ps pulses produced directly from a Nd:glass feedback- controlled oscillator, this system employs a fiberless, gratings-only expansion/compression scheme, and produces clean (5 X 107 prepulse contrast) 1-J, 1.2-ps recompressed pulses without added pulse-cleaning. The same system can also be configured to produce up to 5-J uncompressed 410-ps pulses. A novel subpicosecond cross-correlation technique is also described.
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