The modification of transparent materials with femtosecond lasers has a lot of interest in data processing, waveguides and diffractive optical elements (DOE) development fields. In our research, we are focused on transparent materials processing with a deeply focused Gaussian beam. As well known, the internal modifications induced in fused silica with high numerical aperture objectives are affected by spherical aberration. In most cases, this phenomenon is unwanted and needs to be compensated to get the width to height ratio close to 1:1 for high-quality waveguides writing etc. However, when the focusing is quite deep (> 1 mm), due to the energy dissipation the modification can be formed only in the central part of the laser beam. Consequently, the radial size of modifications is reduced less than the diffraction limit. This property can be successfully used to record high-density volume DOE with the diffraction efficiency > 90%.
In this work, we develop the method to record DOE with multi-level binary refractive index modification distribution in bulk fused silica. In the beginning, the transverse modification length induced by single pulse elongation to the wider area is investigated to find the conditions where only type-I modification is induced. The two-level binary phase by a single modification depth > 50 µm can be achieved. In the next step, the desired multi-level binary phase distribution is simulated according to the required intensity distribution. Then the slicing of the multi-level binary phase to the two-level binary phase images is performed. This method involves recording the phase elements slice by slice with the resolution limited to the minimal induced phase change in one separate layer.
Several approaches exist to induce the internal modifications in fused silica by femtosecond laser irradiation depending on the dose: direct writing of refractive index change (type I modification), birefringence control by nanogratings for geometric phase elements and polarisation sensitive imaging (type II modification) and new phenomena arising from double pulse utilisation. In this presentation, we focus on two recent topics: enhancement of nanograting formation using a double pulse processing and fabrication of high-efficient diffractive optical elements (DOE) by I type modification in fused silica.
Most of the studies show that the orientation of the LIPSS is perpendicular to the first pulse polarisation. However, the intra-volume modifications with the induced nanogratings have the depth dimension where the double-pulse fabrication can provide more sophisticated morphology depending on the temporal delay and energy relation between two pulses. The nanogratings induced using the double-pulse irradiation with perpendicular polarisations demonstrates the grid-like structure at ~ 10 ps temporal delay, while the 45 degrees tilted gratings appear without delay between pulses. Variation of the nanograting period was observed in the case of parallel polarisation. Those new phenomena can be widely used for writing the two-dimensional diffraction gratings or the information coding applications and requires more deep investigations.
Most of the effects are observed at focusing with high numerical aperture objectives, which working range is limited by spherical aberration below 1 mm depth. However, that makes possible to restrict the radial size of modification below the diffraction limit and extend the longitudinal length of modification up to 50 µm during the single scan. Therefore such approach is capable of recording more compact volume diffractive optical elements with the total diffraction efficiency > 90%. By varying the refractive index within ~1x10-4 to 1x10-3, it is possible to get up to π/24 phase retardance resolution attractive to design the phase change optical elements for the low-loss Top-hat beam shaping and multi-beam splitters.