Femtosecond pulse laser cleaning of spray paint from heritage stone surfaces.

We explore the use of femtosecond laser pulses to clean a variety of colors of spray paint from the Moruya granite, a stone with high heritage value that is widely used for monuments and sculptures in Sydney and New South Wales (Australia). The efficiency of the cleaning treatment and the effects on the stone substrate are evaluated using optical microscopy, optical profilometry, Raman spectroscopy, energy-dispersive X-ray spectroscopy, and colorimetry. We demonstrate that femtosecond laser cleans granite without damaging it and without discoloration when the laser fluence is set below the damage threshold of the stone.

Using Diffuse Scattering to Observe X-Ray-Driven Nonthermal Melting.

We present results from the SPring-8 Angstrom Compact free electron LAser facility, where we used a high intensity (∼10^{20} W/cm^{2}) x-ray pump x-ray probe scheme to observe changes in the ionic structure of silicon induced by x-ray heating of the electrons. By avoiding Laue spots in the scattering signal from a single crystalline sample, we observe a rapid rise in diffuse scattering and a transition to a disordered, liquidlike state with a structure significantly different from liquid silicon. The disordering occurs within 100 fs of irradiation, a timescale that agrees well with first principles simulations, and is faster than that predicted by purely inertial behavior, suggesting that both the phase change and disordered state reached are dominated by Coulomb forces. This method is capable of observing liquid scattering without masking signal from the ambient solid, allowing the liquid structure to be measured throughout and beyond the phase change.

Photoluminescence from voids created by femtosecond-laser pulses inside cubic-BN.

Photoluminescence (PL) from femtosecond-laser-modified regions inside cubic-boron nitride (c-BN) was measured under UV and visible light excitation. Bright PL at the red spectral range was observed, with a typical excited state lifetime of ∼4 ns. Sharp emission lines are consistent with PL of intrinsic vibronic defects linked to the nitrogen vacancy formation (via Frenkel pair) observed earlier in high-energy electron-irradiated and ion-implanted c-BN. These, formerly known as the radiation centers, RC1, RC2, and RC3, have been identified at the locus of the voids formed by a single femtosecond-laser pulse. The method is promising to engineer color centers in c-BN for photonic applications.

Experimental evidence of new tetragonal polymorphs of silicon formed through ultrafast laser-induced confined microexplosion.

Ordinary materials can transform into novel phases at extraordinary high pressure and temperature. The recently developed method of ultrashort laser-induced confined microexplosions initiates a non-equilibrium disordered plasma state. Ultra-high quenching rates overcome kinetic barriers to the formation of new metastable phases, which are preserved in the surrounding pristine crystal for subsequent exploitation. Here we demonstrate that confined microexplosions in silicon produce several metastable end phases. Comparison with an ab initio random structure search reveals six energetically competitive potential phases, four tetragonal and two monoclinic structures. We show the presence of bt8 and st12, which have been predicted theoretically previously, but have not been observed in nature or in laboratory experiments. In addition, the presence of the as yet unidentified silicon phase, Si-VIII and two of our other predicted tetragonal phases are highly likely within laser-affected zones. These findings may pave the way for new materials with novel and exotic properties.

Simple convergent-nozzle aerosol injector for single-particle diffractive imaging with X-ray free-electron lasers.

A major challenge in high-resolution x-ray free-electron laser-based coherent diffractive imaging is the development of aerosol injectors that can efficiently deliver particles to the peak intensity of the focused X-ray beam. Here, we consider the use of a simple convergent-orifice nozzle for producing tightly focused beams of particles. Through optical imaging we show that 0.5 µm particles can be focused to a full-width at half maximum diameter of 4.2 µm, and we demonstrate the use of such a nozzle for injecting viruses into a micro-focused soft-X-ray FEL beam.

Small atomic displacements recorded in bismuth by the optical reflectivity of femtosecond laser-pulse excitations.

Subtle atomic motion in a Bi crystal excited by a 35 fs-laser pulse has been recovered from the transient reflectivity of an optical probe measured with an accuracy of 10(-5). Analysis shows that a novel effect reported here-an initial negative drop in reflectivity-relates to a delicate coherent displacement of atoms by the polarization force during the pulse. We also show that reflectivity oscillations with a frequency coinciding with that of cold Bi are related to optical phonons excited by the electron temperature gradient through electron-phonon coupling.

Long, low loss etched As(2)S(3) chalcogenide waveguides for all-optical signal regeneration.

We report on the fabrication and optical properties of etched highly nonlinear As(2)S(3) chalcogenide planar rib waveguides with lengths up to 22.5 cm and optical losses as low as 0.05 dB/cm at 1550 nm - the lowest ever reported. We demonstrate strong spectral broadening of 1.2 ps pulses, in good agreement with simulations, and find that the ratio of nonlinearity and dispersion linearizes the pulse chirp, reducing the spectral oscillations caused by self-phase modulation alone. When combined with a spectrally offset band-pass filter, this gives rise to a nonlinear transfer function suitable for all-optical regeneration of high data rate signals.

Compact high-power optical source for resonant infrared pulsed laser ablation and deposition of polymer materials.

We propose a novel tuneable table-top optical source as an alternative to the free electron laser currently used for resonant infrared pulsed laser deposition of polymers. It is based on two-stage pulsed optical parametric amplification using MgO doped periodically poled lithium niobate crystals. Gain in excess of 10(6) in the first stage and pump depletion of 58% in the second stage were achieved when the system was pumped by a high-power Nd:YVO(4) picosecond laser source at 1064 nm and seeded by a CW tuneable diode laser at 1530 nm. An average power of 2 W was generated at 3.5 microm corresponding to 1.3 microJ pulse energy.

Passive mode locking of a Nd:YVO4 laser with an extra-long optical resonator.

We describe a mode-locked, diode-pumped Nd:YVO4 laser with a very long optical cavity operating at 1064 nm. High-modulation, InGaAs quantum-well, semiconductor saturable-absorber mirrors were used for passive mode locking, providing a stable train of 13-ps pulses. A novel zero-q-transformation multipass cell provided a variable-length optical cavity as much as 100 m long. The output beam had M2 < 1.1 at average powers of 4.1, 3.9, and 3.5 W at repetition rates of 4.1, 2.6, and 1.5 MHz, respectively. To the best of our knowledge the last of these is the lowest repetition rate ever generated directly from a mode-locked nonfiber solid-state laser without cavity dumping.

Precision ablation of dental enamel using a subpicosecond pulsed laser.

In this study we report the use of ultra-short-pulsed near-infrared lasers for precision laser ablation of freshly extracted human teeth. The laser wavelength was approximately 800nm, with pulsewidths of 95 and 150fs, and pulse repetition rates of 1kHz. The laser beam was focused to an approximate diameter of 50microm and was scanned over the tooth surface. The rise in the intrapulpal temperature was monitored by embedded thermocouples, and was shown to remain below 5 degrees C when the tooth was air-cooled during laser treatment. The surface preparation of the ablated teeth, observed by optical and electron microscopy, showed no apparent cracking or heat effects, and the hardness and Raman spectra of the laser-treated enamel were not distinguishable from those of native enamel. This study indicates the potential for ultra-short-pulsed lasers to effect precision ablation of dental enamel.

Dynamics of light-induced reflectivity switching in gallium films deposited on silica by pulsed laser ablation.

We present what is to our knowledge the first experimental study of light-induced reflectivity changes at an alpha-Ga/Si interface irradiated by femtosecond and picosecond laser pulses. After exposure, the reflectivity can increase from R?0.55 , which is typical for alpha-Ga , to R?0.8 , which is close to that of liquid Ga. The initial step in the reflectivity change of 2-4 ps is resolved with 150-fs laser pulses. The light-induced reflectivity change relaxes during 100ns-10 mus , depending strongly on the background temperature of the Ga mirror and the laser fluence.

X-ray focusing with lobster-eye optics: a comparison of theory with experiment.

We report an experimental investigation and comparison with simulation of the x-ray focusing of a flat, square profile microchannel plate. We use x rays with an energy of ~1.5 keV from a laser-produced plasma. The images were recorded with x-ray film. We find the focal structure to be consistent with theoretical expectations. The angular resolution of the focus is 0.96 mrad, which is a major improvement over previous results. The measured peak intensity gain is 27 ± 4, which is ~33% of that for a perfect optic.

Geometric optics of arrays of reflective surfaces.

We present an analysis of the geometric optics of spherically curved arrays of reflective surfaces. In particular we consider optical devices in which reflective surfaces are arranged on a spherical interface and every ray reflects once from a reflector. The orientation of the reflective surfaces is not necessarily related in any way to the orientation of the interface. The analysis can be applied to any radiation that may specularly reflect from the reflectors. This may be reflection from stacks of mirrors or diffraction from the atomic planes. The principles are applied to x-ray optical systems such as capillary arrays and curved crystals. The calculations are used to find optimum configurations of reflective arrays for applications such as x-ray condensers and telescopes, to find the tolerances to which reflective arrays must be constructed, and to find the conditions in which primary aberrations are eliminated.

Measurement of x-ray spectral line wavelengths by using two bragg reflections.

A novel method of measuring absolute x-ray wavelengths using simultaneous Bragg reflections at two parallel crystal plates of equal atomic spacing is presented. The accuracy of wavelength determination Δλ/λ for this method is discussed and a value of 5 × 10-5is achieved. Absolute wavelengths of L-shell laser-produced spectra of Cu, Ge, As, and Se and M-shell spectra of Gd and Tb have been measured in the wavelength region 7.5-8.5 Å using a quartz (10T0) quasimonolithic crystal blank.