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• 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]

Two-photon ionization by focused femtosecond laser pulses initiates the development of micrometer-scale plasmas in the bulk of silicon. Using pump-and-probe transmission microscopy with infrared light, we investigate the space-time characteristics of these plasmas for laser intensities up to 101 2W/cm2 . The measurements reveal a self-limitation of the excitation at a maximum free-carrier density of ffi1019cm~3, which is more than one order of magnitude below the threshold for permanent modification. The plasmas remain unchanged in the --100 ps timescale revealing slow carrier kinetics. The results underline the limits in local control of silicon dielectric permittivity, which are inherent to the use of single near-infrared ultrashort Gaussian pulses. [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]

We study experimentally the formation of refractive index voxels (volume elements) in photorefractive LiNbO3 and LiTaO3 crystals illuminated with high irradiance femtosecond laser pulses. We used 150 fs pulses at 800 nm wavelength (energy 6–50 nJ) tightly focused inside the crystals in a single shot regime. This resulted in a formation of a micrometer size region of elevated refractive index, which may be used as memory bits in information storage/retrieval application. The maximum refractive index change of 5×10−4 was recorded in undoped LiNbO3 at an average light intensity of ∼TW/cm2 that is close to the breakdown threshold. A simple setup for photorefractive recording and in situ monitoring of the refractive index changes has been proposed. [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]

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 (

We study experimentally the formation of refractive index voxels (volume elements) in photorefractive LiNbO3 and LiTaO3 crystals illuminated with high irradiance femtosecond laser pulses. We used 150 fs pulses at 800 nm wavelength (energy 6–50 nJ) tightly focused inside the crystals in a single shot regime. This resulted in a formation of a micrometer size region of elevated refractive index, which may be used as memory bits in information storage/retrieval application. The maximum refractive index change of 5×10−4 was recorded in undoped LiNbO3 at an average light intensity of ∼TW/cm2 that is close to the breakdown threshold. A simple setup for photorefractive recording and in situ monitoring of the refractive index changes has been proposed. [ABSTRACT FROM AUTHOR]