Vygandas Jarutis

Feature-size reduction of photopolymerized structures by femtosecond optical curing of SU-8

The universal scaling of a width of photopolymerized line on the exposure dose was observed in polymerization by direct laser writing using tightly focused femtosecond pulses in SU-8 resist. This scaling can be explained as the photopolymerization by a blackbody-type emission. A spectrally broad thermal emission of electrons heated up to temperatures approximately Te∼103K coincides with an IR-absorption band of SU-8 centered at the 2.9μm and had caused polymerization by a cumulative direct absorption. Three-dimensional photonic crystal templates with approximately twice reduced feature size were fabricated with stop band at the fiber communication wavelength of 1.3μm.

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.

Optical third harmonic generation during femtosecond pulse diffraction in a Bragg grating

We report on a novel phenomenon occurring when ultrashort laser pulses are diffracted in a Bragg grating recorded in a photo-thermo-refractive glass: two intense beams at the third harmonic of the incident laser frequency appear between the direct and the diffracted laser beams. A high conversion efficiency of about 0.04% at an incident intensity of 5 × 1011 W cm−2 in a 1 mm thick grating is attributed to self-phase matching for the third harmonic generation via Cherenkov radiation from the interfering third order polarization of the incident and diffracted beams. It is assumed that this is a general phenomenon for diffraction in a volume grating.

Nanofabrication by direct laser writing and holography (Invited Paper)

The physical principles underlying a three-dimensional (3D) laser microstructuring technique are outlined, its applications for the fabrication of 3D (nano)micro-structured materials are presented. The direct laser writing and holographic recording in SU-8 photoresist are described. The limits of the lateral and axial light localization of a Gaussian pulse/beam at the focus are derived for the multi-photon absorption taking into account the threshold of photomodification. Prospective holographic patterns formed by interference of circularly and linearly polarized beams are discussed.

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.

Ultrabright femtosecond source of biphotons based on a spatial mode inverter

A method of enhancing the efficiency of entangled biphoton sources based on a type II femtosecond spontaneous parametric downconversion (SPDC) process is proposed and implemented experimentally. Enhancement is obtained by mode inversion of one of the SPDC output beams, which allows the beams to overlap completely, thus maximizing the number of SPDC photon pairs with optimum spatiotemporal overlap. By use of this method, biphoton count rates as high as 16 kHz from a single 0.5-mm-long beta-barium borate crystal pumped by second-harmonic radiation from a Ti:sapphire laser were obtained.

Femtosecond laser microfabrication of 3D photonic structures

The physical principles underlying the three-dimensional (3D) laser microstructuring technique are outlined, its applications for the fabrication of 3D photonic crystals are described. Structures recorded by direct laser writing and holographic recording techniques in SU-8 photoresist are presented along with their structural and optical characterization.

Extension of supercontinuum spectrum, generated in photonic crystal fiber, by using chirped femtosecond pulses

We present results of experimental and numerical investigation of supercontinuum generation in polarization maintaining photonic crystal fiber (PCF) using chirped femtosecond pulses. The initial unchirped pump pulse source was a mode-locked Yb:KGW laser generating 52 nJ energy 110 fs duration pulses at 1030 nm with 76 MHz repetition rate. The nonlinear medium was a 32 cm long polarization maintaining PCF manufactured by NKT Photonics A/S. We demonstrated the influence of pump pulse chirp on spectral characteristics of supercontinuum. We showed that by chirping pump pulses positively or negatively one can obtain broader supercontinuum spectrum than in case of unchirped pump pulses at the same peak power. Moreover, the extension can be controlled by changing the amount of pump pulse chirp. In our case the supercontinuum spectrum width was extended by up to 115 nm (at maximum chirp value of +10500 fs2 that we could achieve in our setup) compared to the case of unchirped pump at the same peak power.

Photonic crystal templates obtained by two-photon laser lithography in photoresist SU-8

High quality templates of three-dimensional photonic crystals were fabricated in photoresist SU-8 using femtosecond laser lithography. The templates have woodpile and spiral architectures, which are widely expected to yield spectrally wide and robust photonic band gaps. Their optical characterization reveals fundamental and higher-order photonic stop-gaps in the infrared wavelength range of 2.0–8.0 μm. These templates are mechanically stable, nearly free of shrinkage-related deformations, and can be used for subsequent infiltration by optically active or high refractive index materials.