Simple method for the characterization of intense Laguerre-Gauss vector vortex beams

We report on a method for the characterization of intense, structured optical fields through the analysis of the size and surface structures formed inside the annular ablation crater created on the target surface. In particular, we apply the technique to laser ablation of crystalline silicon induced by femtosecond vector vortex beams. We show that a rapid direct estimate of the beam waist parameter is obtained through a measure of the crater radii. The variation of the internal and external radii of the annular crater as a function of the laser pulse energy, at fixed number of pulses, provides another way to evaluate the beam spot size through numerical fitting of the obtained experimental data points. A reliable estimate of the spot size is of paramount importance to investigate pulsed laser-induced effects on the target material. Our experimental findings offer a facile way to characterize focused, high intensity complex optical vector beams which are more and more applied in laser-matter interaction experiments

Direct femtosecond laser surface structuring with complex light beams generated by q-plates

Direct femtosecond (fs) laser surface structuring became a versatile way to generate surface structures on solid targets demonstrating a high degree of flexibility and controllability in creating different types of structures for many applications. This approach demonstrated an alteration in various properties of the surface, such as optical properties, wetting response, etc. This paper focuses on direct fs laser surface structuring using complex light beams with spatially variant distribution of the polarization and fluence, with emphasis on the results obtained by the authors by exploiting q-plate beam converters. Although striking scientific findings were achieved so far, direct fs laser processing with complex light fields is still a novel research field, and new exciting findings are likely to appear on its horizon

Femtosecond laser surface structuring of silicon in dynamic irradiation conditions

The process of surface structuring of silicon in dynamic irradiation conditions is investigated by analyzing the features of the surface structures generated on a target moving at different scanning velocities under the irradiation by a 1030 nm femtosecond laser beam. Scanning electron microscopy analysis of the shallow linear craters produced in different experimental conditions allows characterizing the dependence of threshold fluence for the formation of surface structures (ripples and grooves) as well as their spatial periods on the effective number of laser pulses. Moreover, the effect of the accumulated laser fluence dose has been also addressed. Our findings evidence clear trends of the morphological features of the surface structure on the effective number of laser pulses as well as the existence of interesting differences in their characteristics when processing occurs at the same value of the accumulated laser fluence dose achieved by appropriate selection of laser pulse energy and scanning speed

Laser ablation and structuring of CdZnTe with femtosecond laser pulses

We report an experimental investigation on laser ablation and associated surface structuring of CdZnTe by femtosecond Ti:Sa laser pulses (laser wavelength λ≈800 nm, ≈35 fs, 10 Hz), in air. By exploiting different static irradiation conditions, the fluence threshold and the incubation effect in CdZnTe are estimated. Interestingly, surface treatment with a low laser fluence (laser pulse energy E≈5-10 μJ) and number of shots (5≤ N ≤50) show the formation of well-defined cracks in the central part of the shallow crater, which is likely associated to a different thermal expansion coefficients of Te inclusions and matrix during the sample heating and cooling processes ensuing femtosecond laser irradiation. Irradiation with a larger number of pulses (N≈500, 1000) with higher pulse energies (E≈30-50 μJ) results in the formation of well-defined laser-induced periodic surface structures (LIPSS) in the outskirts of the main crater, where the local fluence is well below the material ablation threshold. Both low spatial frequency and high spatial frequency LIPSS perpendicular to the laser polarization are found together and separately depending on the irradiation condition. These are ascribed to a process of progressive aggregation of randomly distributed nanoparticles produced during laser ablation of the deep crater in the region of the target irradiated by a fluence below the ablation threshold with many laser pulses

Optical emission and dynamics of aluminum plasmas produced by ultrashort and short laser pulses

Optical emission and dynamics of ionized and neutral species present in plasmas generated by irradiating an aluminum target with ultrashort (100 fs) and short (7 ns) laser pulses is investigated, in the background pressure range of 10−2 to 760 Torr. Emission spectra exhibit lines from neutrals (Al I) as well as singly and doubly ionized ions (Al II, Al III). Maximum emission is seen in the pressure range of 10 to 20 Torr, around a distance of 1 mm from the target surface. While thermal ablation and laser–plasma interaction are prevalent in short pulse laser ablation (SLA), Coulomb explosion is prominent in ultrashort pulse laser ablation (ULA) because of the high optical intensity of the fs pulse. Ion population is an order of magnitude higher for SLA compared to ULA. Time of flight (TOF) studies reveal both fast and slow moving species in SLA, while this distinction is not quite obvious in ULA. Fast neutrals observed in SLA are formed by the recombination of fast ions with electrons. Space-charge effect in the expanding plasma and the associated Coulomb force results in the acceleration of charged particles, with ions reaching peak velocities up to 30 km s−1 in SLA. Velocities decrease at higher background pressures because of plume confinement. TOF data of Al neutrals fitted to shifted Maxwell–Boltzmann distributions indicate a general thermal nature, while the fast species in SLA appear to show a non-thermal behavior. Time-resolved ICCD images of the plasma depict a relatively rapid initial expansion for the SLA plume. These investigations provide valuable insights into the composition and expansion of Al plasmas produced by ultrashort and short laser pulses respectively, which might be of importance for a number of applications including EUV and X-ray generation, pulsed laser deposition, cluster production, and nanoparticle formation and growth

Broadband MSM photodetector based on S-doped black silicon fabricated by femtosecond laser

S-doped black silicon (Si) characterized by high absorption over a wide spectral band, from visible to infrared, is directly processed in ambient air by femtosecond laser irradiation. The S-doped black Si extends the absorption towards infrared range below its bandgap; approximately over 85 % absorptance is achieved in the interval of 1.1 ~ 2.5 μm even after thermal annealing. Obvious increase in valent state ratio of Si4+ to Si0 is observed by XPS due to the formation of SiO2. Thanks to the passivation effect of SiO2, the background free carrier concentration in the S-doped black Si and the complex structural defects caused by the laser irradiation are effectively reduced, which is beneficial to extend the lifetime of photo-generated carriers as well as improve the thermostability. The metal–semiconductor-metal (MSM) infrared photodetector fabricated by the S-doped black Si shows excellent photosensitivity characteristics: under 1030 nm laser illumination, a responsivity of 367 mA/W@10 V and rising and falling times of 53.82 and 64.51 ms, respectively, are obtained, remarkably outweighing the performance of the unprocessed Si (47 mA/W@10 V) under the same conditions

On the formation and features of the supra-wavelength grooves generated during femtosecond laser surface structuring of silicon

We have investigated the formation process of supra-wavelength quasi-periodic surface structures, commonly termed as grooves, produced during femtosecond laser surface structuring of a silicon target, in air. The experiments have been carried out at two wavelengths, namely 513 nm and 1026 nm, and in multi-pulse irradiation conditions. Both static, i.e., irradiating the same target location, and dynamic, namely scanning the beam along a line, configurations have been explored. We have also carried out simulations of the process by using a multiscale numerical model, based on the two-temperature model coupled to a fluid dynamics approach. The comparison of experimental and numerical results has allowed us to clearly establish the role of hydrothermal waves in the grooves formation process

Influence of ambient pressure on surface structures generated by ultrashort laser pulse irradiation

We report an experimental investigation on the surface structures induced by linearly polarized ≈ 900 fs laser pulses, at λ = 1055 nm, on silicon at different values of the ambient pressure, from ~ 10−4 mbar to one atmosphere. Our experimental findings address interesting influences of the surrounding pressure on: (1) the spatial period of ripples; (2) the formation of micro-grooves; (3) the shape of the structured area. Moreover, the effects of various states of polarization in vacuum as well as of circularly polarized pulses in air vs vacuum are also addressed. We identify as one possible key element of such experimental observations: the fact that as the pressure raises the ablated nanoparticles produced during the femtosecond ablation process of the target get deposited more and more on the sample surface covering the irradiated spot area and influencing the structuring process