Surface nanostructuring of borosilicate glass by femtosecond nJ energy pulses

Abstract

We report on a feature, that of hillock-shaped damage, formed on a glass surface by femtosecond pulses of t p ϭ180 fs ͓full width at half maximum ͑FWHM͒ value͔ duration produced by a recording beam focus with energy of 5 nJ/pulse at 800 nm wavelength ͑the corresponding irradiance of about 7.6 TW/cm 2 was evaluated for a 0.68 m FWHM spot size͒. Single hillocks of 40-50 nm height were recorded reproducibly in single-pulse irradiation. Surface nanopatterning over a large, curved area ͑over 200 m 2 ) can be achieved by implementing a confocal surface curvature tracking method that utilizes the reflection of a supplementary cw-laser beam. The ablation pattern achieved by this method is consistent with that of a Coulomb explosion.Femtosecond laser microfabrication is capable of recording waveguides, 1,2 three-dimensional ͑3D͒ optical memory, 3,4 and photonic crystal structures 5 in glass. In conjunction with wet etching, microchannels can be developed in glass 6 and polymers 7 that are first exposed to fs pulses. However, a large-scale ͑millimeter͒ fabrication with feature sizes of 100 nm or smaller ͑nanostructuring͒ by fs pulses has been elaborated neither on the surface nor in bulk. To further improve 3D optical memories, nanostructuring of materials is a key technology. The need for controllable nanorecording is anticipated particularly in light of the recent breakthrough in readout resolution by confocal microscopy, which reached /23Ӎ33 nm. 8 Here, we demonstrate nanostructuring of a glass surface with 100 nm resolution by single-pulse irradiation at 800 nm wavelength. Formation of hillock-shaped damage was observed. Such damage is consistent with the Coulomb explosion mechanism of dielectric ablation by ultrashort pulses. 9,10 The proposed type of nanostructuring can be used for onsurface and in-bulk recording in transparent materials.The experimental setup of fs fabrication ͑Tokyo Instru-ments͒ includes a femtosecond laser ͑Hurricane, Spectra Physics͒, upright-type optical microscope ͑Olympus MX40͒, and three-axis piezostage ͑Polytec PI͒. An electronic shutter allowed the fs shooting to be switched on and off at preprogramed moments according to the stage movement. Hereafter, the pulse energy is given at the focus, where it was controlled by attenuators built into the micromachining system. The pulse energy stability was about 3% ͑rms value͒. The laser emission at 800 nm wavelength was focused on the sample surface by a dry objective lens with a numerical aperture of NAϭ0.95 ͑Olympus͒. The focusing of the Gaussian beam would correspond to a DϭKM 2 /(2NA)Ӎ0.68 m diam spot ͓full width at half maximum ͑FWHM͒ value͔, 11 here, Kϭ1.07 is the beam's truncation factor for a truncation ratio Tϭ1.75, is the wavelength, and M 2 ϭ1.5 is the beam quality factor of femtosecond laser. The truncation ratio is defined as the Gaussian beam diameter at 1/e 2 divided by the diameter of limiting aperture, TϭD beam /D Ap . The waist length can be calculated as a doubled Rayleigh length L ϭ2 0 2 M 2 M tr 2 /Ӎ0.65 m, where 2 0 ϭ1.27 f /...

Atomic force microscopy and Raman scattering studies of femtosecond laser-induced nanohillocks on CR-39

“…To explore the role of Coulomb explosion for nanostructuring of material surfaces is the subject of considerable interest. Some good examples in which surface modification of different materials by femtosecond laser irradiation is explained on the basis of CE are: the hillock-shaped damage, formed on a glass surface by femtosecond pulses of 180 fs with energy of 5 nJ/pulse at 800 nm wavelength is reported by Vanagas et al [20]. The lateral crosssection of the hillocks observed was 115-155 nm with a height of 40-70 nm.…”

Silk patterns made by direct femtosecond laser writing

“…Surface bulging and ablation of a borosilicate glass (cover glass) under high repetition rate irradiation by fs-laser pulses takes place via interplay of strongly localized energy absorption, low heat conductance, ablation, and surface tension effects even at low pulse energy as used in this study. 24 Here, it was found that such structural modification of glass surface was required to obtain water insoluble crystalline silk. Since silk conformational transformation has a chemical origin, e.g., radiolysis of water and free radicals generation, establishing of crosslinks, and a thermal activation has only secondary effect, a possible explanation can be found in the onset of ablation at which silk printing occurred.…”

Three-Dimensional Micro- and Nano-Structuring of Materials by Tightly Focused Laser Radiation

“…The point-spread function (PSF) shown in Figure 5 is presented in normalized optical coordinates and the actual r, z dimensions of the focus can be found for the specific R, F values (the numerical aperture is NA ¼ n sinðÞ ' nR=F, here n is the refractive index). More precise estimations of the focal spot size would require accounting for the spatial profile of the laser beam, its clipping by the aperture of the objective lens, 44 vectorial nature of electromagnetic radiation 20,40 and spatio-temporal distortions of the laser radiation. 45,46 These factors typically reduce the resolution of DLW process.…”