Vytautas Purlys

Diffusion-Assisted High-Resolution Direct Femtosecond Laser Writing

We present a new method for increasing the resolution of direct femtosecond laser writing by multiphoton polymerization, based on quencher diffusion. This method relies on the combination of a mobile quenching molecule with a slow laser scanning speed, allowing the diffusion of the quencher in the scanned area and the depletion of the multiphoton-generated radicals. The material we use is an organic-inorganic hybrid, while the quencher is a photopolymerizable amine-based monomer which is bound on the polymer backbone upon fabrication of the structures. We use this method to fabricate woodpile structures with a 400 nm intralayer period. This is comparable to the results produced by direct laser writing based on stimulated-emission-depletion microscopy, the method considered today as state-of-the-art in 3D structure fabrication. We optically characterize these woodpiles to show that they exhibit well-ordered diffraction patterns and stopgaps down to near-infrared wavelengths. Finally, we model the quencher diffusion, and we show that radical inhibition is responsible for the increased resolution.

Three-Dimensional Metallic Photonic Crystals with Optical Bandgaps

The fabrication of fully three-dimensional photonic crystals with a bandgap at optical wavelengths is demonstrated by way of direct femtosecond laser writing of an organic-inorganic hybrid material with metal-binding moieties, and selective silver coating using electroless plating. The crystals have 600-nm intralayer periodicity and sub-100 nm features, and they exhibit well-defined diffraction patterns.

Flat lensing in the visible frequency range by woodpile photonic crystals

We experimentally demonstrate full two-dimensional focalization of light beams at visible frequencies by a three-dimensional woodpile photonic crystal. The focalization (the flat lensing) with focal distances of the order of 50-70 μm is experimentally demonstrated. Experimental results are compared with numerical calculations and interpreted by harmonic expansion studies.

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.

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.

Spatial filtering by axisymmetric photonic microstructures.

We propose and show experimentally axisymmetric spatial (angular) filtering of two-dimensional light beams by axisymmetric photonic microstructures. Such three-dimensional microstructures (similar to photonic crystals), in gapless configuration, were recorded in bulk of glass, where the refractive index has been point-by-point modulated using tightly focused femtosecond laser pulses. Axisymmetric angular filtering of approximately 25 mrad is demonstrated experimentally.

Femtosecond laser-induced two-photon photopolymerization for structuring of micro-optical and photonic devices

Light initiated liquid polymer quasi-instant solidification is attractive for its ultra precise spatial and temporal control of the reaction. Here we present femtosecond laser induced two-photon photopolymerization for structuring of microoptical and sample photonic devices. Due to nonlinear phenomena the fabrication resolution is not restricted to diffraction limit for the applied laser excitation wavelength but determined by the exposure dose. Furthermore, pinpoint structuring enables one to produce 3D structures of any form out of photopolymer. The smallest structural elements voxels of 200 nm lateral dimensions can be achieved reproducibly by using high numerical aperture optics. Axial resolution which is fundamentally few times worse than lateral can be controlled in few hundred nanometers precision by forming polymeric pad as an additional structure. In our work we applied commercially available and widely used hybrid zirconium-silicon based hybrid sol-gel material ORMOSIL (SZ2080) and an acrylate based AKRE37 photopolymer. Arrays of custom parameters spherical microlenses for microscopy applications have been fabricated. Their surface roughness, focal distance and imaging quality were tested. 3D custom form woodpile structures with submicron period and chain-mail structure were made as a sample photonic bandgap structures. Finally, we show some structures produced out of fluorescent dyes rhodamine 6G doped photopolymer.

Photonic Crystal Microchip Laser

Microchip lasers are very efficient, compact and convenient sources of coherent light. However they have a serious disadvantage that, especially in high power operation regimes, the emitted beam structure is relatively bad (with large angular divergence). We show that the intracavity photonic crystals with spatial filtering functionality, can strongly improve the spatial characteristics of the microchip laser. The angular divergence of the emitted beam is strongly reduced, and the radiation brightness is significantly increased due to the intracavity photonic crystal filter.

Femtosecond laser fabrication of hybrid micro-optical elements and their integration on the fiber tip

Femtosecond laser photo-polymerization of zirconium-silicon based sol-gel photopolymer SZ2080 is used to fabricate micro-optical elements with a single and hybrid optical functions. We demonstrate photo-polymerization of the solid immersion and Fresnel lenses. Gratings can be added onto the surface of lenses. The effective refractive index of polymerized structures can be controlled via the volume fraction of polymer. We used woodpile structure with volume fraction of 0.65-0.8. Tailoring of dispersion properties of micro-optical elements by changing filling ratio of polymer are discussed. Direct write approach is used to form such structures on a cover glass and on the tip of an optical fiber. Close matching of refractive indices between the polymer and substrate in visible and near infra red spectral regions (nSZ2080 = 1.504, nglass = 1.52) is favorable for such integration. The surface roughness of laser-polymerized resits was ~30 nm (min-max value), which is acceptable for optical applications in the visible range. For the bulk micro-optical elements the efficiency of 3D laser polymerization is increased by a factor ~ (2 - 4) × 102 times (depends on the design) by the shell-formation polymerization: (i) contour scanning for definition of shell-surface, (ii) development for removal of nonfunctional resist, and (iii) UV exposure for the final volumetric polymerization of an enclosed volume.

Chirped axisymmetric photonic microstructures for spatial filtering

Abstract. We explore, theoretically and experimentally, the spatial (angular) filtering of two-dimensional light beams by longitudinally chirped axisymmetric photonic microstructures. The structures comprise a set of planes of concentric rings with the separation between the plates smoothly varying along the propagation direction. Axisymmetric structures were recorded in a bulk of glass, in which the refractive index has been modulated using tightly focused femtosecond laser pulses. We show that the spatial filtering recently shown in nonchirped axisymmetric structures can be substantially improved by the chirp: the angular range of filtering was increased approximately two times. The numerical study reveals that the filtering efficiency can be strongly increased using the longer and larger index contrast axisymmetric photonic structures.