Rodger Evans

Pump-probe imaging of nanosecond laser-induced bubbles in agar gel

In this paper we show results of Nd:YAG laser-induced bubbles formed in a one millimeter thick agar gel slab. The nine nanosecond duration pulse with a wave length of 532 nm was tightly focused inside the bulk of the gel sample. We present for the first time a pump-probe laser-flash shadowgraphy system that uses two electronically delayed Nd:YAG lasers to image the the bubble formation and shock wave fronts with nanosecond temporal resolution and up to nine seconds of temporal range. The shock waves generated by the laser are shown to begin at an earlier times within the laser pulse as the pulse energy increases. The shock wave velocity is used to infer a shocked to unshocked material pressure difference of up to 500 MPa. The bubble created settles to a quasi-stable size that has a linear relation to the maximum bubble size. The energy stored in the bubble is shown to increase nonlinearly with applied laser energy, and corresponds in form to the energy transmission in the agar gel. We show that the interaction is highly nonlinear, and most likely is plasma-mediated.

Pump-probe imaging of nanosecond laser-induced bubbles in distilled water solutions: Observations of laser-produced-plasma

This article presents the analysis of the laser-produced-plasma (LPP) formed by the focusing of a 9 ns laser pulse, λ=532 nm, with a NA=0.6 aspherical lens using energies between 100–1500 μJ, into distilled water with varying solutions of table salt. Observations of the filamentation plasma were made, which are explained by self-focusing of the laser pulse by the LPP through ponderomotive cavitation of the electron plasma in the center of the beam. The filamentation of the beam through a low density plasma wave guide explains why the transmission of the pump laser through the interaction region was notably higher on previous experiments that we performed [R. Evans et al., Opt. Express 16, 7481 (2008)], than a very similar set of experiments performed by Noack and Vogel [IEEE J. Quantum Electron. 35, 1156 (1999)].

Laser-induced cavitation phenomenon studied using three different optically-based approaches – An initial overview of results

Abstract This report presents a study of shock wave and cavitation bubble dynamics induced by nanosecond laser pulses in pressurized water. Three methods were used to obtain data from the irradiated sample: (1) pump-probe laser flash shadowgraphy, (2) pressure wave sensing by means of a fiber optic interferometer hydrophone, and (3) a novel technique based on the modulation of spatial transmittance by the cavitation bubble. The medium used in these experiments was distilled water in a chamber under different pressure conditions which included values found in human intraocular liquid. It could be shown that while external pressure does not affect either the shock wave propagation or the initial bubble growth rate, it does affect the first collapse time of the bubble and its maximum diameter. Zusammenfassung Die vorliegende Arbeit untersucht die Dynamik von Schockwellen und Kavitationsblasen, die mittels Nanosekunden-Laserpulsen in unter Druck gesetztem Wasser erzeugt wurden. Dabei wurden drei unterschiedliche Methoden verwendet: (1) Laserflash-Verfahren, (2) Druckwellenmessung mittels faseroptischem Interferometer und (3) eine neuartige Technik basierend auf der Modulation der räumlichen Transmission durch die Kavitationsblase. Die Untersuchungen wurden an destilliertem Wasser unter unterschiedlichen Drücken – u.a. auch physiologische Werte, wie sie intraokular vorkommen – vorgenommen, die mittels einer speziellen Druckkammer erzeugt wurden. Es konnte gezeigt werden, dass weder die Ausbreitung der Schockwelle noch das anfängliche Blasenwachstum durch den äußeren Druck beeinflusst wird, die Kollapszeit der Blase und ihr maximaler Durchmesser jedoch sehr wohl.

Ultrabroadband photon pair preparation by spontaneous four-wave mixing in a dispersion-engineered optical fiber

We present a study of the spectral properties of photon pairs generated through the process of spontaneous four wave mixing (SFWM) in single mode fiber. Our analysis assumes narrowband pumps, which are allowed to be frequency-degenerate or non-degenerate. Based on this analysis, we derive conditions on the pump frequencies and on the fiber dispersion parameters which guarantee the generation of ultra-broadband photon pairs. Such photon pairs are characterized by: i) a very large degree of entanglement, and ii) a very high degree of temporal synchronization between the signal and idler photons. Through a numerical exercise, we find that the use of photonic crystal fiber (PCF) facilitates the fulfilment of the conditions for ultra-broadband photon pair generation; in particular, the spectral region in which emission occurs can be adjusted to particular needs through an appropriate choice of the PCF parameters. In addition, we present a novel quantum interference effect, resulting from indistinguishable pathways to the same outcome, which can occur when pumping a SFWM source with multiple spectral lines.

Short and ultrashort laser pulse induced bubbles on transparent and scattering tissue models.

Bubble formation is a well identified phenomenon within short (ns) and ultrashort (fs) laser pulses-aqueous media interactions. Bubble formation might be produced by three different mechanisms: (1) optical breakdown, (2) rarefraction wave and (3) overheating of the material. Experiments where transparent and scattering tissue models that mimic biological tissue were irradiated with a Q-switched, 532 nm, 5 nanosecond, Nd:YAG and Ti:sapphire femtosecond laser systems. The type of bubble (transient or permanent) and initial bubble diameter were characterized as a function of time as well as the number of pulses and repetition rate at which they were delivered. Threshold fluence for bubble formation in scattering tissue model was also studied. Two types of bubbles were identified depending on the number of pulses and the repetition rate at which they were delivered: transient (type 1) and permanent (type 2) bubbles. There is an insignificant difference in the fluence required to form a bubble in transparent tissue models regardless of the depth at which the beam was focused; in contrast, for scattering tissue models, the fluence required to form a bubble in deep positions is significantly higher than that of more superficial beam focus positions.

The effects of degraded spatial coherence on ultrafast-laser channel etching.

When laser-etching channels through solid targets, the etch-rate is known to decrease with increasing depth, partly because of absorption at the sides of the channel. For ultrafast-laser pulses at repetition rates >100 MHz, we show that the etch-rate is also affected by optical properties of the beam: the channel acts as a waveguide, and so the pulses will decompose into dispersive normal modes. Additionally, plasma on the inner surface of the channel will cause scattering of the beam. These effects will cause a loss of spatial coherence in the pulse, which will affect the propagated intensity distribution and ultimately the etch-rate. We have characterized this effect for various foil thicknesses to determine the evolution of the beam while drilling through metal.

Processing of Metallic Thin Films Using Nd:YAG Laser Pulses

Nd:YAG lasers are possibly the more widely used lasers either for basic research or for industrial and technological applications (Dubey, A. K. & Yadava, V. 2008). These lasers are also excellent pump sources for laser development, for instance Ti:sapphire ultrashort pulse lasers are based on CW Nd:YAG pumping. In particular, Nd-YAG lasers have been applied to study laser-induced oxidation in metals as titanium and chromium; semiconductors as silicon (Aygun, G. et al., 2006). (Perez del Pino, A. et al., 2004) demonstrated that the rutile phase of TiO2 is obtained by laser oxidation in air of titanium films. Nd:YAG laser pulses have been used to laser-induce a phase transformation from W3O thin films to WO3 (Evans R., et al., 2007); laser ablation for micromachining of bulk metals as copper, bronze and aluminum has also been done using Nd:YAG nanosecond pulses (Maisterrena-Epstein R., et al., 2007); laser-induced oxidation and novel LIPSS formation in titanium thin films deposited on silicon substrates was demonstrated by using a single laser beam from a frequency doubled Nd:YAG nanosecond pulsed laser (Camacho-Lopez S., et al., 2008). Some works about pulsed laser oxidation have been reported (Dong, Q. et al., 2002). (Pereira, A. et al. 2004) have investigated the laser treatment in steel irradiating at various wavelengths by using different laser sources. In Table 1, we cited some works on the oxidation induced by pulsed laser irradiation in various metals. Recently, we have published results on fs-laser

Cell damage extent due to irradiation with nanosecond laser pulses under cell culturing medium and dry environment

Cell mono-layers were irradiated with nanosecond laser pulses under two distinct scenarios: (a) with culturing medium positioning the beam waist at different stand-off distances γ and (b) without cell culturing medium, positioning the beam waist directly on top of the cell mono-layer. Damaged cells were marked with Trypan Blue, a vital cell marker. Three different zones of damage were identified: (1) a zone of complete cell clearance, surrounded by (2) a ring of dead cells marked with Trypan Blue and (3) the rest of the cell culture where the cells remain alive and viable. Different hydrodynamic mechanisms damage cells as it was shown by high speed video for γ=0 and comparison with time resolved imaging. The cell damage mechanism has its origin on the optical breakdown plasma formation. For the case with culturing medium, a combination of plasma formation and shear stresses are responsible for cell damage; wheras for the case without cell culturing medium, the plasma formation is the only mechanism of interaction between laser pulses and cells. The rapidly expanding plasma generates shock waves whose pressure is most likely responsible for the cell detachment observed.

TIME-RESOLVED STUDY OF LASER-INDUCED BUBBLES AND SHOCKWAVES IN AGAR GEL TISSUE PHANTOMS

Laser-tissue interactions have been extensively used in a number of biomedical treatments. However, the high optical absorption in tissue and the use of relatively long laser pulses or, in many cases, cw laser exposure, frequently results in excessive laser-heating producing undesirable collateral damage. Short pulsed lasers are one of the most precise tools for delivering energy and can allow for the greatest finesse [1]. Laser pulses with duration of only a few nanoseconds, and as short as a few hundreds of femtoseconds, seem to be a good alternative to minimize or even suppress laser-heating undesirable effects [2].Copyright

Mechanical response of agar gel irradiated with Nd:YAG nanosecond laser pulses

Nanosecond long laser pulses are used in medical applications where precise tissue ablation with minimal thermal and mechanical collateral damage is required. When a laser pulse is incident on a material, optical energy will be absorbed by a combination of linear and nonlinear absorption according to both: laser light intensity and material properties. In the case of water or gels, the first results in heat generation and thermoelastic expansion; while the second results in an expanding plasma formation that launches a shock wave and a cavitation/boiling bubble. Plasma formation due to nonlinear absorption of nanosecond laser pulses is originated by a combination of multiphoton ionization and thermionic emission of free electrons, which is enhanced when the material has high linear absorption coefficient. In this work, we present measurements of pressure transients originated when 6 ns laser pulses are incident on agar gels with varying linear absorption coefficient, mechanical properties and irradiation geometry using laser radiant exposures above threshold for bubble formation. The underlying hypothesis is that pressure transients are composed of the superposition of both: shock wave originated by hot expanding plasma resulting from nonlinear absorption of optical energy and, thermoelastic expansion originated by heat generation due to linear absorption of optical energy. The objective of this work is to evaluate the relative contribution of each absorption mechanism to mechanical effects in agar gel. Real time pressure transients are recorded with PVDF piezoelectric sensors and time-resilved imaging from 50 μm to 10 mm away from focal point.