K. K. Anoop

Femtosecond laser surface structuring of silicon using optical vortex beams generated by a q-plate

We report on laser surface structuring of silicon using Ti:Sa femtosecond laser ablation with optical vortex beams. A q-plate is used to generate an optical vortex beam with femtosecond pulse duration through spin-to-orbital conversion of the angular momentum of light. The variation of the produced surface structures is investigated as a function of the number of pulses, N, at laser fluence slightly above the ablation threshold value. At low N (≈10), only surface corrugation of the irradiated, ring-shaped area is observed. This is followed by a progressive formation of regular ripples at larger N (≈100–500), which eventually transform in smaller columnar structures for N ≈ 1000. Moreover, the central, non-ablated part is gradually decorated by nanoparticles produced during laser ablation, a process which eventually leads to the formation of a central turret of assembled nanoparticles. Our experimental findings suggest the importance of a feedback mechanism and a cumulative effect on the formation of ripples with interesting patterns not achievable by the more standard beams with a Gaussian intensity profile.

Multidiagnostic analysis of ion dynamics in ultrafast laser ablation of metals over a large fluence range

The dynamics of ions in ultrafast laser ablation of metals is studied over fluences ranging from the ablation threshold up to ≈75 J/cm2 by means of three well-established diagnostic techniques. Langmuir probe, Faraday cup, and spectrally resolved intensified charge coupled device imaging simultaneously monitored the ions produced during ultrafast laser ablation of a pure copper target with 800 nm, ≈50 fs, Ti: Sapphire laser pulses. The fluence dependence of ion yield is analyzed, resulting in the observance of three different regimes. The specific ion yield shows a maximum at about 4–5 J/cm2, followed by a gradual reduction and a transition to a high-fluence regime above ≈50 J/cm2. The fluence dependence of the copper ions angular distribution is also analyzed, observing a gradual increase in forward-peaking of Cu ions for fluences up to ≈10 J/cm2. A broader ion component is observed at larger angles for fluences larger than ≈10 J/cm2. Finally, an experimental characterization of the ionic angular distrib...

Ultrashort laser ablation of bulk copper targets: Dynamics and size distribution of the generated nanoparticles

We address the role of laser pulse fluence on expansion dynamics and size distribution of the nanoparticles produced by irradiating a metallic target with an ultrashort laser pulse in a vacuum, an issue for which contrasting indications are present in the literature. To this end, we have carried out a combined theoretical and experimental analysis of laser ablation of a bulk copper target with ≈50 fs, 800 nm pulses, in an interval of laser fluencies going from few to several times the ablation threshold. On one side, molecular dynamics simulations, with two-temperature model, describe the decomposition of the material through the analysis of the evolution of thermodynamic trajectories in the material phase diagram, and allow estimating the size distribution of the generated nano-aggregates. On the other side, atomic force microscopy of less than one layer nanoparticles deposits on witness plates, and fast imaging of the nanoparticles broadband optical emission provide the corresponding experimental charac...

Fast ion generation in femtosecond laser ablation of a metallic target at moderate laser intensity

The generation of ions during laser ablation of a metallic target (copper) with ≈50 fs Ti:Sa laser pulses of moderate intensity (≈1014 W cm−2) is studied by simultaneous fast-imaging and ion-probe techniques. The spatiotemporal distribution of excited ions and neutrals in the laser-produced plasma plume is analyzed by exploiting appropriate band-pass filters in the imaging set-up, while the ion flux angular distribution is characterized by angle-resolved ion probe measurements. An interesting feature of our results is the generation of a fast ion population separated from the neutral component of the atomic plasma plume and characterized by sub-keV kinetic energies, which is interpreted in the frame of a simple model of ambipolar diffusion.

Direct femtosecond laser ablation of copper with an optical vortex beam

Laser surface structuring of copper is induced by laser ablation with a femtosecond optical vortex beam generated via spin-to-orbital conversion of the angular momentum of light by using a q-plate. The variation of the produced surface structures is studied as a function of the number of pulses, N, and laser fluence, F. After the first laser pulse (N = 1), the irradiated surface presents an annular region characterized by a corrugated morphology made by a rather complex network of nanometer-scale ridges, wrinkles, pores, and cavities. Increasing the number of pulses (2   1000) and a deep crater is formed. The nanostructure variat...

Laser ablation of silicon induced by a femtosecond optical vortex beam.

We investigate laser ablation of crystalline silicon induced by a femtosecond optical vortex beam, addressing how beam properties can be obtained by analyzing the ablation crater. The morphology of the surface structures formed in the annular crater surface allows direct visualization of the beam polarization, while analysis of the crater size provides beam spot parameters. We also determine the diverse threshold fluences for the formation of various complex microstructures generated within the annular laser spot on the silicon sample. Our analysis indicates an incubation behavior of the threshold fluence as a function of the number of laser pulses, independent of the optical vortex polarization, in weak focusing conditions.

Direct femtosecond laser surface structuring of crystalline silicon at 400 nm

We have analyzed the effects of the laser pulse wavelength (400 nm) on femtosecond laser surface structuring of silicon. The features of the produced surface structures are investigated as a function of the number of pulses, N, and compared with the surface textures produced by more standard near-infrared (800 nm) laser pulses at a similar level of excitation. Our experimental findings highlight the importance of the light wavelength for the formation of the supra-wavelength grooves, and, for a large number of pulses (N ≈ 1000), the generation of other periodic structures (stripes) at 400 nm, which are not observed at 800 nm. These results provide interesting information on the generation of various surface textures, addressing the effect of the laser pulse wavelength on the generation of grooves and stripes.

Spectrally Resolved Imaging of Ultrashort Laser Produced Plasma

A spectrally resolved imaging technique is introduced to investigate the spatial and temporal evolution of different plasma plume components [ions, atoms and nanoparticles (NPs)] produced during ultrashort laser ablation of a pure copper target. The temporal evolution of neutral (Cu*) and ionic (Cu+) components of the atomic plasma are separately imaged by exploiting bandpass interference filters in front of a fast-gated intensified charge coupled device camera, whereas for the NPs plume, its broadband emission is imaged. The 2-D spectrally resolved images show a dominant neutral component in the atomic plasma plume emission and a faster, well-separated ionic component moving ahead of it. The more massive, slow NPs plume follows at much longer delay.