International audienc

We describe the optical, radiative, and laser-plasma physics of a new type of nanostructured surface especially promising as a very high absorption target for high-peak-power subpicosecond laser-matter interaction. This oriented-nanowire material, irradiated by 1 ps pulses at intensities up to 10(17) W cm(-2), produces picosecond soft x-ray pulses 50x more efficiently than do solid targets. We compare this to "smoke" or metallic clusters, and solid nanogroove-grating surfaces; the "metal-velvet" targets combine the high yield of smoke targets with the brief emission of grating surfaces.

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Ablation rates and etched-surface morphology of fused silica has been studied with i-ps Nd:giass laser pulses in a regime of near-diffraction-limited spot size. Shallow holes of 1.7-tim 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 N of laser pulses, scaling as N i.7 +80/F (fluence F in J/cm2). Below this limit -for accumulated etch depths less than -2 m -three-dimensional 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 -400 identical i-ps pulses, each separated by 7.5 ns, was applied. Very smooth and deep (-30-im) holes of 7-10-.tm diameter were excised at a total fluence of -400 kJ/cm2, establishing a new means for rapid and precise micromachining of fused silica and other brittle materials.

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Ultrafast laser pulses ( ≤ 1 ps) are qualitatively different in the nature of their interaction with materials, including biotissues, as compared to nanosecond or longer pulses. This can confer pronounced advantages in outcomes for tissue therapy or laser surgery. At the same time, there are distinct limitations of their strong-field mode of interaction. As an alternative, it is shown here that ultrafast laser pulses delivered in a pulse-train burst mode of radiant exposure can access new degrees of control of the interaction process and of the heat left behind in tissues. Using a laser system that delivers 1 ps pulses in 20 μ s pulse-train bursts at 133 MHz repetition rates, a range of heat and energy-transfer effects on hard and soft tissue have been studied. The ablation of tooth dentin and enamel under various conditions, to assess the ablation rate and characterize chemical changes that occur, are reported. This is compared to ablation in agar gels, useful live-cell-culture phantom of soft tissues, and presenting different mechanical strength. Study of aspects of the optical science of laser-tissue interaction promises to make qualitative improvements to medical treatments using lasers as cutting and ablative tools.Keywords: laser-ablation; ultrafast-laser; absorption; burst-mode.Zusammenfassung : Ultraschnelle Pulse ( ≤ 1 ps) unterscheiden sich von Nanosekunden-oder noch l ä ngeren Pulsen qualitativ in der Art ihrer Wechselwirkung mit Materialien, einschlie ß lich Biogewebe. Dies kann in der Gewebetherapie oder in der Laserchirurgie von Vorteil sein. Andererseits gibt es klare Einschr ä nkungen hinsichtlich ihrer Starkfeld-Effekte. Als Alternative wird in der vorliegenden Arbeit gezeigt, dass ultraschnelle Pulse, die im sogenannten Burst-Modus der Bestrahlung abgeben werden (d.h. in einer schnellen Folge sto ß weise ausgesendeter Impulse oder Pulsz ü ge) dazu beitragen k ö nnen, den Wechselwirkungsprozess und die dabei erzeugte W ä rme besser zu kontrollieren. Dazu wurden die W ä rme-und Energietransfereffekte an Hart-und Weichgewebe untersucht, die mittels eines Lasersystems erzeugt wurden, mit dem 1 ps-Pulse in 20 μ s-Impulsfolgen mit einer Wiederholrate von 133 MHz abgegeben wurden. Es wird ü ber Ablationsversuche an Dentin und Zahnschmelz unter verschiedenen Bedingungen berichtet, die mit dem Ziel durchgef ü hrt wurden, die Ablationsrate zu evaluieren und auftretende chemische Ver ä nderungen zu charakterisieren. Die Ergebnisse wurden mit der Ablation in Agargels verglichen, die gut als Weichgewebephantome geeignet sind und eine unterschiedliche mechanische Festigkeit aufweisen. Insgesamt verspricht die Untersuchung der optischen Aspekte der Laser-Gewebe-Wechselwirkung eine qualitative Verbesserung von medizinischen Laseranwendungen.Schl ü sselw ö rter: Laserablation; Absorption; Burst-Modus.

We have studied the characteristics of x-ray emission in the 12-16Å range from plasmas produced by the irradiation of a target of Xe clusters with picosecond laser pulses at intensities near 10 17 W cm −2 . These plasmas exhibit strong emission from Ni-like through to Mn-like Xe. We find that the strength and character of the x-ray spectra are very similar when either 1053 or 526 nm laser pulses are used to heat the clusters.

We report optimization of laser-driven proton acceleration, for a range of experimental parameters available from a single ultrafast Ti:sapphire laser system. We have characterized laser-generated protons produced at the rear and front target surfaces of thin solid targets (15 nm to 90 lm thicknesses) irradiated with an ultra-intense laser pulse (up to 10 20 W Á cm À2 , pulse duration 30 to 500 fs, and pulse energy 0.1 to 1.8 J). We find an almost symmetric behaviour for protons accelerated from rear and front sides, and a linear scaling of proton energy cutoff with increasing pulse energy. At constant laser intensity, we observe that the proton cutoff energy increases with increasing laser pulse duration, then roughly constant for pulses longer than 300 fs. Finally, we demonstrate that there is an optimum target thickness and pulse duration. V