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Laser ablation of dental enamel with subpicosecond laser pulses has been studied over the intensity range of (0.1-1.4) × 10[sup 14] W/cm² using 95 and 150 fs pulses at a pulse repetition rate of 1 kHz. The experimentally determined ablation threshold of 2.2+0.1 J/cm² was in good agreement with theoretical predictions based on an electrostatic ablation model. The ablation rate increased linearly with the laser fluence for up to 15 times the ablation threshold. The absence of collateral damage was observed using optical and scanning electron microscopy. Pulpal temperature measurements showed an increase of about 10°C during the 200 s course of ablation. However, air cooling at a rate of 5 1/min resulted in the intrapulpal temperature being maintained below the pulpal damage threshhold of 5.5°C. The material removal rates for subpicosecond precision laser ablation of dental enamel are compared with other techniques. [ABSTRACT FROM AUTHOR]

• Defined ablation thresholds of four major minerals of granite for fs-laser pulses. • Maximum ablation rate of 0.27 ± 0.03 mm3·(min·W)−1 at a fluence of 8.4 ± 1.0 J·cm−2. • Raman spectra show no structural damage to quartz and feldspar at high fluences. • Morphological damage at crater bed observed on biotite above its ablation threshold. • Safe fluence advised for cleaning/conservation of granite below 1.0 ± 0.1 J·cm−2. This paper investigates the ultrashort pulsed laser ablation of a high heritage value Australian granite comprising quartz, feldspar (plagioclase and minor potassium-feldspar), and Mg-Fe-bearing minerals (biotite and hornblende). This work is relevant to laser cleaning and texturing of heritage stone surfaces and the preparation and maintenance of granite in other applications. The experiments were undertaken in an ambient lab air environment (23 ± 2 °C and 20–40 % relative humidity) with nitrogen blow, on dry surfaces using 275 fs duration laser pulses at 1029 nm. The ablation behaviour of the different granite minerals was experimentally studied in a fluence range of 1– 17 J·cm−2 in the single shot per spot regime and the ablated surfaces were assessed with a multi-analytical approach including optical and scanning electron microscopy, Raman spectroscopy and optical profilometry. The ablation threshold was established for each different mineral in the stone. Biotite is the most readily ablated, with an ablation threshold of 1.1 ± 0.1 J·cm−2. It is followed by hornblende at 1.3 ± 0.1 J·cm−2, plagioclase at 1.4 ± 0.1 J·cm−2 and quartz, at 1.6 ± 0.1 J·cm−2, which is the most resistant component mineral. Following the electrostatic laser ablation model, the ablation thresholds were theoretically estimated for each mineral phase and demonstrated to be in reasonable agreement with the experimental results. The highest ablation efficiency of the granite as a whole was determined to occur at a fluence of 8.4 J·cm−2, giving an ablation efficiency of 0.27 ± 0.03 mm3·(min·W)−1 in terms of volume removal. From the experimental data, a maximum laser fluence for conservation cleaning was determined at 1.0 ± 0.1 J·cm−2, at which no discernible change or detrimental effect on the granite was observable. Above this fluence, morphological changes were induced in the stone and higher fluences should therefore be avoided for conservation cleaning. [ABSTRACT FROM AUTHOR]

Formation of carbon nanoclusters in a laser-pulse created plume expanding in vacuum and in a noble gas environment at various pressures was studied. Experiments were performed with carbon nanoclusters formed by laser ablation of graphite targets with 12-picosecond 532 nm laser pulses at MHz-range repetition rate in a broad range of ambient He, Ar, Kr, and Xe gas pressures from 2 × 10-2Torr to 1500 Torr. The experimental results confirmed our theoretical prediction that the average size of the nanoparticles depends weakly on the type of the ambient gas, and is determined exclusively by the single-laser pulse parameters. The most important finding relates to the fact that in vacuum the cluster size is mainly determined by hydrodynamic expansion of the plume, while in the ambient gas it is controlled by atomic diffusion in the gas. [ABSTRACT FROM AUTHOR]

This paper presents experimental and theoretical results on the temperature-dependent optical response of a single crystal of bismuth to excitation by femtosecond laser pulses. We demonstrate that the measured damping rate of the transient reflectivity oscillations relates to the lifetime of optical phonons. The lifetime is the inverse rate of the decay of optical phonons into two acoustic phonons. This lifetime also indicates the approach to the vibration instability (catastrophe) threshold that manifests the beginning of the disordering of a solid crystal and transition to a liquid state. We observe the red shift of phonon frequency, which increases with the rise of the initial lattice temperature. The red shift is different from the previously observed red shift proportional to the electron temperature, and thus to the excitation laser fluence. The coherent phonon excitation process imprinted into the initial change in the reflectivity and the following reflectivity oscillations allowed us to uncover the temporal phonon history preceding the structural transformation of solid Bi. [ABSTRACT FROM AUTHOR]

We have built a mode-locked Nd:YVO4 laser with a very long resonator which produces an average power of 50 W in 13-ps pulses at 1064 nm and was designed for applications in micro-machining, the deposition of optical thin films, and the growth of nano-clusters in the laser-ablated plumes. By operating the laser at very low mode-locking repetition rates (1.5 MHz, 2.6 MHz, and 4.1 MHz), high pulse power is available in a near diffraction limited beam, allowing focused intensities to exceed 1012 W/cm2 and permitting efficient evaporation of difficult materials such as Si. The high power also allows conversion into the second harmonic at 532 nm with an efficiency exceeding 80%. Measurements of the ablation mass in experiments with metals show a 30–100 times increase in the ablation rate compared to the conventional low-repetition-rate ns-range lasers. [ABSTRACT FROM AUTHOR]

The mechanism of ablation of solids by intense femtosecond laser pulses is described in an explicit analytical form. It is shown that at high intensities when the ionization of the target material is complete before the end of the pulse, the ablation mechanism is the same for both metals and dielectrics. The physics of this new ablation regime involves ion acceleration in the electrostatic field caused by charge separation created by energetic electrons escaping from the target. The formulas for ablation thresholds and ablation rates for metals and dielectrics, combining the laser and target parameters, are derived and compared to experimental data. The calculated dependence of the ablation thresholds on the pulse duration is in agreement with the experimental data in a femtosecond range, and it is linked to the dependence for nanosecond pulses. [ABSTRACT FROM AUTHOR]

Laser cleaning provides art and heritageconservators with an alternativemeans to restore objects when traditional chemical and mechanicalmethods are not viable. However, long (>nanosecond) laser pulsescancause unwanted damage from photothermal processes and provide limitedcontrol over ablation depth. Ultrashort (