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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.

Abstract:Spectra from xenon ions have been recorded at the NIST EBIT and the emission into a 2% bandwidth at 13.5 nm arising from 4d_5p transitions compared with that from 4d_4f and 4p_4d transitions in Xe XI and also with that obtained from the unresolved transition array (UTA) observed to peak just below 11 nm. It was found that an improvement of a factor of five could be gained in photon yield using the UTA rather than the 4d_5p emission. The results are compared with atomic structure calculations and imply that a significant gain in efficiency should be obtained using tin, in which the emission at 13.5 nm comes from a similar UTA, rather than xenon as an EUVL source material. BACKGROUND:The search for the optimum radiation source at 13.5 nm is one of the main challenges in EUV physics today. Currently the quest is to find a source with a conversion efficiency (CE) of 3% into 2% bandwidth as defined by the needs of the microelectronic industry [1]. The earliest work was performed with laser produced plasmas generated on solid targets. Kauffman et al. [2] attained a CE close to 1% into 3% bandwidth using 7.5 ns, 300 mJ frequency doubled Nd:YAG pulses focussed to a power density of 2×10 . In their work they performed an extensive survey of the emission from a large range of elements and found that the emission at the required wavelength peaked near tin. At higher power densities they noted that the intensity decreased. Shevelko et al. [4] also undertook an extensive study of the spectra of a large number of elements from laser produced plasmas with a KrF excimer laser focussed to a power density of 10 12 Wcm -2 onto planar targets including tin and found that the maximum intensity was in fact obtained for Ge and Re under these conditions. So early work already pointed to the sensitivity of the emission of specific elements to laser power density.In order to avoid the debris problems associated with solid targets other target materials were sought with strong emission in the 13-14 nm region. Jin and Richardson [5] used mass limited water ice targets, where the emission is from the O VI lines near 2 13 nm, to limit the debris. However, all of their early work with mass-limited targets produced significant levels of debris both in the form of ions and particulates of various sizes. The need to reduce particulate emission led to the choice of xenon. There is a line group between 13-14 nm in the spectrum of Xe which has been shown by a number of researchers to arise from 4d 8 _ 4d 7 5p transitions in Xe XI [6]. These lines have recently been identified by Churilov et al. [7] by comparison of new very high resolution data with atomic structure calculations using the suite of codes developed by Cowan [8]. Considerable work has been expended on exploring the feasibility of using laser produced plasmas of xenon clusters produced by supersonic jets or gas puffs from nozzles or solid xenon targets [9]. The highest conversion efficiencies (1.2% into 2% bandwidth) have been achieved using solid xenon by Shields et al. [1...

The time-independent ion distributions of variable composition laser-produced Sn plasmas are studied for a wide range of electron temperatures and atomic number densities, the purpose of which is to elucidate the effect that varying the number density of Sn within a mixed species plasma has upon the steady state populations of Sn and its ions. Particular emphasis will be placed on binary mixtures of Sn with Li, C, O or Sm and more specifically the charge states Sn8+ to Sn13+ within these mixed plasmas, where it will be assumed that the plasma is optically thin. It is found that using these composites has relatively little effect upon the Sn ion population distributions for plasma atomic number densities of less than approximately 1019.5 cm−3. However, for greater values of number densities the Sn ion populations can be shifted by as much as 10–15 eV for Li mixtures. These results are of particular relevance to current research being carried out on extreme ultraviolet lithographic technologies for the optimization of XUV sources in the 13.5 nm wavelength region, which include composite target investigations.

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