V. Mizeikis

Structural changes in femtosecond laser modified regions inside fused silica

Structural characterization of photomodified microvolumes formed by tightly focused femtosecond laser pulses inside silica glass was carried out using synchrotron x-ray diffraction. The observed distinct separation between the O?O and Si?Si pair correlation peaks can be interpreted as a phase separation induced by microexplosions at the focal volume. The mechanisms of structural transitions induced by femtosecond laser pulses inside dielectrics are discussed.

Microactuation and sensing using reversible deformations of laser-written polymeric structures

We investigate reversible deformations of polymeric microstructures fabricated using direct laser writing three-dimensional lithography upon immersion in various solvents. Swelling and shrinkage of sub-micrometre size features are induced by interaction with surrounding solvent and such deformations can be exploited to create larger structures whose size, shape, and other structural parameters depend on the surroundings. We describe diffractive optical elements, micro-mechanical sensors and also hybrid deformable structures, that can be used to implement micro-actuation, micro-sensing, and other functionalities highly sought for micro-optical, micro-mechanical, and micro-fluidic systems.

Realisation of 3D metamaterial perfect absorber structures by direct laser writing

We report design, fabrication and optical properties of 3D electromagnetic metamaterial structures applicable as perfect absorbers (PA) at mid infra-red frequencies. PA architecture consisting of single-turn metallic helices arranged in a periodic two-dimensional array enables polarization-invariant perfect absorption within a considerable range of incidence angles. The absorber structure is all-metallic, and in principle does not require metallic ground plane, which permits optical transparency at frequencies away from the PA resonance. The samples were fabricated by preparing their dielectric templates using Direct Laser Write technique in photoresist, and metalisation by gold sputtering. Resonant absorption in excess of 90% was found at the resonant wavelength of 7.7 μm in accordance with numerical modelling. Similar PA structures may prove useful for harvesting and conversion of infrared energy as well as narrow-band thermal emission and detection.

Optical and thermal characterization on micro-optical elements made by femtosecond laser writing

Femtosecond laser polymerization of photonic crystals (PhCs) and diffractive micro-optical elements which can be easily integrated into complex 3D geometries of micro-fluidic chips is analysed in IR spectral domain. Thermal properties of such 3D optical elements and patterns were investigated by thermal imaging, IR spectroscopy and a heat-wave method using absorption-heating with visible light. Thermal imaging allows a simple in situ judgement on a 3D fabrication quality of photonic crystals and is simpler compared with scanning electron imaging. Photonic stop gaps at IR spectral range were clearly observed and IR mapping at the specific spectral wavelength reveals spatial uniformity of PhCs. Potential to use IR imaging with spectral IR plasmonic filters for sensor applications is discussed.

Collimation and imaging behind a woodpile photonic crystal

We investigate different linear effects that appear in the light beam propagation behind a three-dimensional woodpile photonic crystal. On one hand we report and analyse experimental observation of narrow and well collimated laser beam formation behind the photonic crystal. We show that this collimation depends on the input laser beam focusing conditions. On the other hand we also report the first theoretical and numerical observation double imaging formation behind the photonic crystal. That conclusion comes from the idea that the spatial dispersion possess multiple branches (Bloch branches), generally of different curvatures, for a fixed frequency.

3D glass-ceramic templates for micro-/nano-optics realized via laser nanolithography and pyrolysis

Ceramics play an important role in todays science and industry as it can withstand immense heat, mechanical and other hazards. For this reason, there is a constant drive to find new ways to acquire more free-form and sophisticated ceramic structures. Recently, stereolithographic 3D printing of hybrid organic-inorganic photopolymer and subsequent heat treatment was demonstrated to be capable of providing true 3D ceramic structures [1]. However, such approach was limited to millimeter scale, while one of the aims in the field of micro- and nano-optics is to acquire functional 3D ceramic structures in micro- or even nano-dimensions, respectively.

Nanostructures for highly efficient infrared detection

We propose a high sensitivity photodetection tool at near-infrared frequencies, based on a principle of slowed- and stopped-light in chirped photonic micro/nano-structures. The main goal is to substantially increase the efficiency of photodetection and provide chromatic resolution in infrared photodetection. In particular we concentrate on the design of the chirped photonic micro/nano-structures providing a maximum field enhancement, and frequency dependence of stopped light distribution.

Direct laser writing of metastable modifications in lithium niobate crystal with ultrashort laser pulses

This work demonstrates that direct laser writing (DLW) is an excellent technique for metastable modification formation in the bulk of pure and iron doped LiNbO3 crystals. Using laser pulses generated with Ti:sapphire and Yb:KGW femtosecond laser systems, it is possible to induce photorefractive modifications that appear only in the proximity of laser focus and allows employing all crystal volume for data or photonic device inscription. By correctly choosing laser parameters and writing algorithms simple or complex geometry regions with homogeneously modified refractive index can be formed. The magnitude of refractive index change can be controlled by varying total radiant exposure. We show that uneven charge distribution in the crystal can be rearranged multiple times with the same writing laser beam thus implementing possibility to locally erase or change previously recorded data.

Laser polymerization of Photonic Crystals for collimation of beams at visible wavelengths

Three-dimensional photonic crystals with woodpile structure are demonstrated to exhibit collimation of light beams behind the photonic structure. Woodpile structure photonic crystals with longitudinal modulation periods in a range of 7.4 – 7.8 µm, and transverse periods of 1 µm were fabricated using laser multi-photon polymerization technique in a negative-tone photoresist. As expected from theoretical predictions, the beams propagating along the axis of period variation collimate behind the crystal, which is a first experimental observation of the phenomenon.

Femtosecond laser interaction with photo-refractive crystals: High frequency field and internal field contributions to the refractive index changes

The interaction of single femtosecond laser pulses tightly focused in the bulk of LiNbO<inf>3</inf> doped with Fe (400 ppm doping) and LiTaO<inf>3</inf> crystals was recently studied [1,2]. Single shots of 150 fs, 800 nm laser pulses, 3–50 nJ per pulse, were focused inside a crystal to the depth of 50 microns to the focal spot with diameter of 1.8 μm; the intensity in the interaction region varied from 1.0 TW/cm² to 16.7 TW/cm². The threshold for optically detectable change of refractive index has been found to be 3.8±0.5 nJ in Fe: LiNbO<inf>3</inf> and 5.2±0.5 nJ in pure LiNbO<inf>3</inf> crystal (intensity TW/cm² and 1.37 TW/cm² respectively). The permanent modification of LiTaO<inf>3</inf> was observed at 32±5 nJ (∼10 TW/cm²) that might be considered as a result of breakdown. It has been found that the laser-induced material transformation is fully reversible at the energy per pulse of 14.5 nJ (3.8 TW/cm²).