Roaldas Gadonas

Mechanisms of three-dimensional structuring of photo-polymers by tightly focussed femtosecond laser pulses

Three-dimensional (3D) micro/nano-structuring of photo-resists is systematically studied at the close-to-dielectric- breakdown irradiance. It is demonstrated that avalanche absorption is playing a major part in free electron generation and chemical bond breaking at these conditions. The steps of photo-initiation and chemical bond breaking in propagation of polymerization are altered as compared with photo-polymerization at low-irradiance and one-photon stereo-lithography. The avalanche dominates radical generation and promotion of polymerization at tight focusing and a high approximately TW/cm(2) irradiance. The rates of electron generation by two-photon absorption and avalanche are calculated for the experimental conditions. Simulation results are corroborated by 3D polymerization in three resists with different photo-initiators at two different wavelengths and pulse durations. The smallest feature sizes of 3D polymerized logpile structures are consistent with spectral dependencies of the two photon nonlinearities. Implications of these findings for achieving sub-100 nm resolution in 3D structuring of photo-polymers are presented.

Micro-structured polymer scaffolds fabricated by direct laser writing for tissue engineering

This work presents the latest results on direct laser writing of polymeric materials for tissue engineering applications. A femtosecond Yb:KGW laser (300 fs, 200 kHz, 515 nm) was used as a light source for non-linear lithography. Fabrication was implemented in various photosensitive polymeric materials, such as: hybrid organic-inorganic sol-gel based on silicon-zirconium oxides, commercial ORMOCER® class photoresins. These materials were structured via multi-photon polymerization technique with submicron resolution. Porous three-dimensional scaffolds for artificial tissue engineering were fabricated with constructed system and were up to several millimeters in overall size with 10 to 100 μm internal pores. Biocompatibility of the used materials was tested in primary rabbit muscle-derived stem cell culture in vitro and using laboratory rats in vivo. This interdisciplinary study suggests that proposed technique and materials are suitable for tissue engineering applications.

A femtosecond laser-induced two-photon photopolymerization technique for structuring microlenses

Light-initiated quasi-instant solidification of a liquid polymer is attractive for its ultra-precise spatial and temporal control of the photochemical reaction. In this paper we present microlenses structured by femtosecond laser-induced photopolymerization. Due to nonlinear phenomena the fabrication resolution is not restricted to the diffraction limit for the applied laser excitation wavelength but is determined by the intensity of a focused beam. Furthermore, pin-point structuring enables one to produce three-dimensional structures of any form from the photopolymer. The smallest structural elements of 200 nm lateral dimensions can be achieved reproducibly by using high numerical aperture oil immersion focusing optics (NA = 1.4). Axial resolution (which is fundamentally a few times worse than lateral resolution due to the distribution of light intensity in the focal region) can be controlled to a precision of a few hundred nanometers by decreasing the scanning step. In our work we applied the commercially available and widely used zirconium–silicon based hybrid sol–gel photopolymer (Ormosil, SZ2080). Arrays of custom-parameter spherical microlenses for microscopy applications have been fabricated. Their surface roughness, focal distance and imaging quality were tested. The obtained results show potential for fast and flexible fabrication of custom-parameter microlenses by the proposed technique.

Fabrication of micro-tube arrays in photopolymer SZ2080 by using three different methods of a direct laser polymerization technique

In this paper we demonstrate femtosecond laser fabrication of micro-tubes with a height of several tens of micrometers in the photopolymer SZ2080 by three different methods: direct laser writing, using the optical vortex beam and holographic lithography. The flexibility of direct laser writing and dramatic increase of production efficiency by applying the vortex-shaped beam and four-beam interference approaches are presented. Sample arrays of micro-tubes were successfully manufactured applying all three methods and the fabrication quality as well as efficiency of the methods is compared. The processing time of a single micro-tube with 60 ?m height and 3 ?m inner radius is reduced 400 times for the holographic lithography technique and 500 times for the optical vortex method compared with the direct laser writing technique. The processing time of a micro-tube array containing 400?micro-tubes is the shortest for the holographic lithography method but not for the optical vortex method as in the case of a single micro-tube, because the holographic lithography method does not require time for sample translation. Additionally, the holographic lithography enables manufacturing of the whole micro-tube array by a single exposure. Although point-by-point photo-structuring ensures unmatched complexity of manufactured microstructures, employing nowadays high repetition rate amplified femtosecond lasers combined with beam shaping or several beam interference can envisage industrial applications for practical demands.

Angular structure formation in single-pass optical parametric generators pumped by intersecting beams

Investigation results of angular structure formation in a picosecond traveling-wave optical parametric generator (OPG) based on a KDP crystal (type II phase matching) pumped by intersecting beams are presented. It is demonstrated that the angular structure of the OPG output radiation is controlled by pump geometry. Frequency-tuned diffraction-limited output even in a single-pass OPG was observed.

Self-action of Bessel beam in nonlinear medium

The modifications of the angular spectrum of an intense Bessel beam caused by its self-action in nonlinear medium are revealed. The appearance of a central spot and outer ring in its angular spectrum was observed. Experimental results obtained for benzene are in good qualitative agreement with the theoretical predictions.

Closely packed hexagonal conical microlens array fabricated by direct laser photopolymerization

We apply femtosecond laser direct writing in photopolymers for manufacturing of conical microlenses and closely packed arrays thereof. We demonstrate the fabrication of high optical quality axicons of 15 µm in radius, having 150°, 160°, and 170° cone angles. Their optical properties and performance are modeled using the finite-difference time-domain method and compared with experimentally measured data. Additionally, optimization of the laser direct writing parameters regarding these types of micro-objects is presented. Possible applications of closely packed arrays of axicon microlenses are discussed, having potential attractivity in the fields of modern microscopy, light-based material processing, particle manipulation in microfluidic, and optofluidic applications.

Two-photon polymerization for fabrication of three-dimensional micro- and nanostructures over a large area

Two-photon polymerization has emerged as a technology for rapid fabrication of three-dimensional micro-structures with nanoscale resolution. Commonly, a Ti:Sapphire femtosecond laser (operating at 780-800 nm wavelength) working at MHz pulse repetition rate is applied as an irradiation source to photomodify the resin. We present a system for pinpoint two-photon polymerization which utilizes second harmonic (515 nm) of amplified Yb:KGW femtosecond laser working at 312.5 kHz pulse repetition rate. Shorter irradiation wavelength enables one to focus laser beam to a smaller spot. High repetition rate and high average power capacitates rapid fabrication of three-dimensional structures over a large area. Results obtained prove the highest resolution of fabrication to be up to ~100 nm, and reproducible resolution 200 nm. Some micro-structures fabricated rapidly over millimeter area and revealing the specific problems arising at high speed fabrication over large lateral dimensions are presented in this report. Results obtained show the system and technologies applied to be well suitable for future routine 3D structuring over the large area and application of such structures in photonics, micro-optics, micromechanics, microelectronics and cell growth for tissue engineering.

Annihilation enhanced four-wave mixing in molecular aggregates

The influence of exciton annihilation on the efficiency of four-wave mixing process was investigated theoretically. The study was performed in the framework of the full semiclassical description of light and matter interaction to account for the interplay of nonlinear effects for high pump intensities. Significant enhancement of the four-wave mixing efficiency in the region of saturation due to the annihilation processes of excitons was found. The contribution of annihilation to intensity and wavelength dependence of DFWM signal was analysed and found to be in good qualitative agreement with recent experimental findings of DFWM at the excitonic resonance in J-aggregates of pseudoisocyanine chloride.

Wavelength and intensity-dependent transient degenerate four-wave mixing in pseudoisocyanine J-aggregates

The results of transient degenerate four-wave mixing (DFWM) and “pump-probe” spectroscopy in aggregates of 1,1′-diethyl-2,2′-cyanine (pseudoisocyanine, PIC) chloride at 300 K are reported. Spectral dispersion of DFWM efficiency within the J-band and near exciton resonance has been measured. Time response of both transient absorption changes and DFWM signal is found to be strongly dependent on pump photon fluence and wavelength. This behavior is qualitatively explained within a physical model of nonlinear optical dynamics in aggregate domains which accounts for exciton annihilation, and the effect of nonthermal phonons produced as a result of decay of two-exciton states. Intensity-dependent evolution of excess dynamic disorder due to nonthermal phonons manifests itself in pump-probe experiments as dispersion-type differential spectrum appearing at high pump intensity. The third-order nonlinear susceptibility of PIC aggregates has been calculated supposing highest density packing. The nonlinear figure of me...