Egidijus Vanagas

Surface nanostructuring of borosilicate glass by femtosecond nJ energy pulses

We report on a feature, that of hillock-shaped damage, formed on a glass surface by femtosecond pulses of tp=180 fs [full width at half maximum (FWHM) value] 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/cm2 was evaluated for a 0.68 μm FWHM spot size). Single hillocks of 40–50 nm height were recorded reproducibly in single-pulse irradiation. Surface nanopatterning over a large, curved area (over 200 μm2) 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.

Discrete damage traces from filamentation of Gauss-Bessel pulses

Filamentation of Bessel-Gauss pulses propagating in borosilicate glass is found to produce damage lines extending over hundreds of micrometers and consisting of discrete, equidistant damage spots. These discrete damage traces are explained by self-regeneration of Gauss-Bessel beams during propagation and are potentially applicable in laser microfabrication of transparent materials.

Glass cutting by femtosecond pulsed irradiation

We report on quartz and glass cutting by a lateral scanning of femtosecond pulses (150 fs at 1 kHz repetition rate) of 800 nm wavelength at room and low pressure (5 Torr) air ambience. Pulses were focused by a low numerical aperture (NA0.1) objective lens. Optimization of fabrication conditions: pulse energy and scanning speed were carried out to achieve large-scale (millimeter-to-centimeter) cutting free of microcracks of submicron dimensions along the edges and walls of the cut. Cutting through out the samples of 0.1-0.5 mm thickness was successfully achieved without apparent heat affected zone. At low air pressure (5 Torr) ambience, redeposition of ablated material was considerably reduced. It is demonstrated that the damage on the rear surface was induced by the stress waves, which originated from the plasma ablation pressure pulse. The mechanism of femtosecond-laser cutting of transparent materials at high irradiance and the influence of stress waves generated by plasma plume are discussed.

Micro-Character Printing on a Diamond Plate by Femtosecond Infrared Optical Pulses

Processing of less than 400 nm has been performed on the surface of a diamond plate by means of a femtosecond infrared pulse laser. Various characters with a size of about 1 µm were drawn by the femtosecond pulse laser system in conjunction with a microscope equipped with a precisely controlled piezo-stage. The tightly focused laser light on the flat surface of the diamond made it possible to minimize the light-induced graphitization. The surface of the diamond plate after laser machining was analyzed by micro-Raman measurements to estimate the graphitization effect induced by laser irradiation. The obtained results indicate that graphitization increased with the number of irradiated laser pulses.

Studies of femtosecond pulse filamentation in glasses

Self-focusing and filamentation of nearly-Gaussian femtosecond laser pulses propagating in photosensitive silicate glass was investigated. The filamentation was visualized by the precipitation of NaF nano-crystallites along the beam pass after post-exposure treatment of photosensitive glass, which provides a direct proof that ionization of silicate matrix takes place along the filament propagation lines. Theoretical model of the multi-filamentation based on the propagation of a Gaussian beam with elliptical transverse intensity profile modulated by a spatial noise in a medium with multi-photon absorption is proposed. Self-action of the femtosecond Gaussian-Bessel pulses in borosilicate glass was observed at high fluence. This model reproduces qualitatively the dotted damage lines observed after the beam propagation in borosilicate glass.

Thermal effects in three-dimensional recording by femto/nano-second pulses

Thermal effects are unavoidable in laser material processing and are present, to some extent, even in the case when ultra-short (sub-picosecond) pulsed irradiation is used. We discuss here the matters of high-precision energy delivery into micrometer-sized volumes for three-dimensional (3D) laser microfabrication. Precise account of the absorbed energy, pulse duration, and focal spot size allows to optimize laser processing parameters. As an example, a 3D micro-structuring of silica with better than 15 μm resolution is demonstrated by pulses of 11 ns duration and 266 nm wavelength (for a focusing by a low numerical aperture NA = 0.029 lens). The two photon absorption coefficient of silica, β ≃ 60 ± 10 cm/GW, at 266 nm has been determined. The thermal black-body type emission of non-equilibrated electrons is discussed as a possible light source for 3D modification and structuring of photo-sensitive and photo-polymerizable materials. It is also demonstrated that optical properties of ionized dielectrics can be used to determine the temperature.

Studies of femtosecond pulse filamentation in borosilicate glass

Light filament formation in borosilicate glass under femtosecond pulse irradiation was investigated. The main observations were successfully reproduced by numerical modeling.

Micrometer and sub-micrometer structures fabrication and analysis with femtosecond laser micro-nanomachining system

A concept of modular laser processing system, which can be flexibly optimized for processing by femtosecond laser pulses, is presented. The system capabilities and fabricated structures are demonstrated.

Wide band-gap materials microprocessing by femtosecond laser pulses

The experimental results of wide band-gap materials treatment by femtosecond laser pulses are presented and discussed. Borosilicate glass drilling in air and vacuum, the surface and in-bulk three-dimensional laser processing with sub-micrometer resolution are subject of investigatios. Technical issues relevant for achieving high spatial resolution in order to meet requirements of nanotechnology at feature size smaller than 10 nm along with the issues of fabrication efficiency are outlined. Also, we show a concept of modular laser processing system, which can be flexibly optimized for processing by femtosecond laser pulses.

Direct femtosecond laser writing system for sub-micron and micron scale patterning

Commercial femtosecond micromachining system (FMS) has been developed that capable to process the material in sub-micron (< 200 nm) and micron scale. Core of the system are: optical unit, controller unit and software. The other parts: fs-laser system; focusing unit; stage unit can be varied (exchangeable). Two different fs-laser systems already are compatible with core of FMS: Mira/RegA (Coherent) and Hurricane (Spectra-Physics). FMS controller unit allows to control every single fs-pulse delivery on the target. Three possible types of focusing unit are available: microscope type unit, long focal distance lens unit, and axicon lens based unit. Standard stage unit options are: three-axis piezostage, and two-axis air bearing stage combined with Z-axis piezostage. Repeatability for all dimensions is within ±5 nm. Also, step motor stages are available. The system allows 3D scan with confocal laser-microscope (resolution δr=200nm, δz=540nm) build in optical unit. Software controls all basic functions of the system performance and writing any pattern (including 3D) on or into specimen. The results obtained by direct fs-laser writing method are presented and discussed: bits in the range of 100 - 200 nm sizes, 6 TB/cm3 density optical storage matrix, waveguides fabrication inside transparent materials, high aspect ratio (1:125) patterning of dielectric materials with Gauss-Bessel beam.