R. X. Li

Alignment of nanoparticles formed on the surface of 6H-SiC crystals irradiated by two collinear femtosecond laser beams

Nanoripples with periods of 150 and 80 nm are formed on the surface of 6H-SiC crystals irradiated by the p-polarized 800 nm and the s-polarized 400 nm femtosecond lasers, respectively. When both of the two collinear laser beams focus simultaneously on the sample surface, nanoparticles are formed on the whole ablation area, and they array in parallel lines. We propose and confirm that the second harmonics in the sample surface excited by the incident lasers plays an important role in the formation of nanostructures.

Microscopic mechanisms of ablation and micromachining of dielectrics by using femtosecond lasers

We report measurements of damage threshold and ablation depth for SiO2 and CaF2 irradiated under lasers at wavelengths of 800 and 400 nm for duration of 45–800 fs. These results can be well understood by using a developed avalanche model. The model includes the production of conduction band electrons (CBEs), laser energy deposition, and CBE diffusion. The evolution of microexplosion is investigated based on this model.

ZnSe nanowires grown on the crystal surface by femtosecond laser ablation in air

Uniform ZnSe nanowires are observed on the ablation crater on ZnSe crystal surface irradiated by femtosecond lasers in air, while other parts of the sample surface are not polluted. The nanowire growth rate is about 5μm∕s, it is higher than that fabricated by chemical vapor deposition method by a factor of 104. The nanowire length and diameter can be controlled by varying laser pulse energy and pulse number. The formation mechanism is studied and found to be self-catalyzed vapor-liquid-solid process.

Tuned dielectric, pyroelectric and piezoelectric properties of ferroelectric P(VDF-TrFE) thin films by using mechanical loads

Using the phenomenological Landau thermodynamic theory, we have investigated the effect of external mechanical loads on the phase transition and physical properties of poly(vinylidene fluoride-trifluorethylene) [P(VDF-TrFE)] thin films. Quantitative calculation shows that the phase transition temperature, polarization, dielectric, pyroelectric, and piezoelectric properties are highly sensitive to external mechanical loads. External compressive stress increases the phase transition temperature and the out-of-plane polarization and decreases the out-of-plane dielectric constant below Tc, pyroelectric coefficient, and piezoelectric coefficient (absolute value) and vice versa. Compared with pervoskite-type ferroelectrics, the calculated results are opposite due to the different intrinsic parameters between pervoskite and polymer ferroelectrics, especially, electrostrictive constants. The ability to tailor the properties in ferroelectric P(VDF-TrFE) thin films with the mechanical load can offer a tremendously ...

Collisionless shocks driven by 800 nm laser pulses generate high-energy carbon ions

We present experimental studies on ion acceleration from diamond-like carbon (DLC) foils irradiated by 800 nm, linearly polarized laser pulses with peak intensity of 1.7 × 1019 W/cm2 to 3.5 × 1019 W/cm2 at oblique incidence. Diamond-like carbon foils are heated by the prepulse of a high-contrast laser pulse and expand to form plasmas of near-critical density caused by thermal effect before the arrival of the main pulse. It is demonstrated that carbon ions are accelerated by a collisionless shock wave in slightly overdense plasma excited by forward-moving hot electrons generated by the main pulse.

Generation of low-divergence megaelectronvolt ion beams from thin foil irradiated with an ultrahigh-contrast laser

Megaelectronvolt (MeV) ion beams with low divergence (10 Degree-Sign ) are experimentally generated from a thin foil irradiated by an ultrahigh-contrast laser at a peak intensity of {approx}10{sup 18} W/cm{sup 2}. The high-contrast ({approx}10{sup 11}) laser is obtained with a pulse cleaner based on noncollinear optical-parametric amplification and second-harmonic generation processes. The effects of the foil density, foil thickness, as well as the density gradients at the front and back sides of the foil are investigated with two-dimensional particle-in-cell simulations. The beam parameters of maximum energy and divergence strongly depend on the density gradients at the back side of the foil.

Effects of nanosecond-scale prepulse on generation of high-energy protons in target normal sheath acceleration

A pulse cleaner based on noncollinear optical-parametric amplification and second-harmonic generation processes is used to improve the contrast of a laser of peak intensity ∼2 × 1019 W/cm2 to ∼1011 at 100 ps before the peak of the main pulse. A 7 MeV proton beam is observed when a 2.5 μm-thick Al foil is irradiated by this high-contrast laser. The maximum proton energy decreases to 2.9 MeV when a low-contrast (∼108) laser is used. Two-dimensional particle-in-cell simulations combined with MULTI simulations show that the maximum proton energy sensitively relies on the detecting direction. The ns-time-scale prepulse can bend a thin target before the main pulse arrives, which reduces maximum proton energy in the target normal sheath acceleration.

Parasitic lasing suppression in large-aperture Ti:sapphire amplifiers by optimizing the seed-pump time delay

Theoretical and experimental investigations are carried out to determine the influence of the time delay between the input seed pulse and pump pulses on transverse parasitic lasing in a Ti:sapphire amplifier with a diameter of 80?mm, which is clad by a refractive index-matched liquid doped with an absorber. When the time delay is optimized, a maximum output energy of 50.8?J is achieved at a pump energy of 105?J, which corresponds to a conversion efficiency of 47.5%. Based on the existing compressor, the laser system achieves a peak power of 1.26?PW with a 29.0?fs pulse duration.

Threshold of ultra-short pulse laser-induced damage in dielectric materials

The one- and the two-photon absorption rates of conduction-band electrons (CBEs) in dielectric materials are calculated by second- and third-order perturbation theory, respectively. Here fused silica irradiated under 526 nm ultra-short pulse laser is used as an example. Compared with the case that only the one-photon absorption of CBE assisted by a phonon is considered, the rate of total energy gain of CBE from laser field will be enhanced by a factor of 3 when both of the one- and the two-photon absorption are included. The avalanche rate includes not only the term proportional to laser intensity, but also the terms proportional to the square of laser intensity and to the product of laser intensity and hole density. The damage threshold is calculated on the basis of the avalanche model, and find it is lowered by about 15% compared with the case for only the one-photon absorption of CBE assisted by a phonon is considered. The theoretical value agrees well with experimental results.

Carrier-envelope phase stabilized high temporal contrast femtosecond laser source at 1053 nm

We demonstrate a carrier-envelope phase (CEP) stabilized high temporal contrast generator at 1053 nm for the first time. The device relies on a collinear optical parametric amplifier (OPA) followed by a frequency-doubling crystal. It is driven by a femtosecond laser source centered at 800 nm and finally generates CEP passively stabilized pulses with an energy of 130 μJ and a FWHM for the spectrum of 40 nm with 46 fs pulse duration at 1053 nm. The temporal contrast reaches >1011 at a few picoseconds before the main pulse.