Colloidal particle lens arrays-assisted nano-patterning by harmonics of a femtosecond laser

Abstract

We consider nanopatterning of dielectric substrates by harmonics of single powerful femtosecond pulses from a Ti:Sapphire laser. The nanopatterning is mediated by closely packed monolayers of polystyrene microspheres that act as microlenses at the surface. Observed modification of the material proceeds via ionization. By our theory, the second harmonic is more effective in multi-photon ionization and is better focused than the fundamental frequency which is effective in multiplying of the amount of free electrons via impact ionization. Experiments show that conversion of a part of the pulse energy into the second harmonic decreases the modification threshold and improves the localization of the structures. Optimization of the time offset between the harmonics could further improve the efficiency and quality of nanostructuring.

Two-color beam improvement of the colloidal particle lens array assisted surface nanostructuring

“…In the previously studied case of 1-lm diameter, when the fluence of the laser pulse is increased above a certain threshold level (the threshold intensities are about 5 Â 10 11 W=cm 2 for SH þ FF and about 10 12 W=cm 2 for FF; the corresponding threshold fluences are 25 mJ=cm 2 and 50 mJ=cm 2 ), the spheres are eliminated from the substrate within the irradiated area. 18,19 This process is similar to the cleaning phenomenon. 26,27 When increasing the fluence up to 15% above the cleaning threshold, welldefined structures are obtained.…”

“…Previously, 18,19 we have studied the surface nanostructuring of polymers and glasses by single femtosecond laser pulses using colloidal arrays of 1-lm diameter polystyrene spheres. It is demonstrated that conversion of only 5% of the laser pulse energy into the second harmonic (SH), i.e., irradiation by bichromatic pulses, reduces the laser nanostructuring threshold of polymers and decreases the transverse size of elementary structures, e.g., ablation craters.…”

“…The experimental setup is described elsewhere. 18 The pulse duration was 50 fs, the pulse energy was 1.7 mJ, the centre wavelength was 800 nm, and the beam diameter was 7 mm. The beam was focused by a flat-convex lens with a focal length of 15 cm.…”

Multiscale Visualization of Colloidal Particle Lens Array Mediated Plasma Dynamics for Dielectric Nanoparticle Enhanced Femtosecond Laser-Induced Breakdown Spectroscopy

“…These resonances can provide hot spots in the near field around the particle for nanomanufacturing. For example, silica micro-/nanospheres-assisted laser fabrication has been widely investigated in the formation of sub-wavelength features. − It has been reported that the use of CPLA can enhance the local light intensity to enhance the laser ablation . A monolayer of micro-/nanospheres on the sample surface acts as an array of near-field lenses that focus the incident light into the multitude of distinct spots .…”

“…For example, silica micro-/nanospheres-assisted laser fabrication has been widely investigated in the formation of sub-wavelength features. − It has been reported that the use of CPLA can enhance the local light intensity to enhance the laser ablation . A monolayer of micro-/nanospheres on the sample surface acts as an array of near-field lenses that focus the incident light into the multitude of distinct spots . Previous extensive studies focused on the nanostructures fabrication on the substrate using low laser pulse energy; however, the strong ablation of the CPLA-deposited substrate using high pulse energy has not been investigated.…”