Glass and crystals are attractive materials in many application such as optics, photonics, or micromechanics. Glass and crystals have good mechanical properties, it is transparent for visible and near-infrared (IR) radiation, and is chemically inert in organic solvents. However, the fabrication of high aspect ratio structures in glass and crystals is a challenge. A femtosecond laser is a good option for such structure fabrication.

In this work, we demonstrate various femtosecond structure formation techniques which allow forming high aspect ratio structures in glass and crystalline materials. Due to high femtosecond pulses intensity and its caused nonlinear material-light interaction, it is possible to form modifications inside of a bulk of glass. There are a few possible techniques in which structures can be formed: by changing refractive index[1], inducing nanograting and in combination with etching removing affected material, this technique is called selective laser etching[2]. Also, it is possible to ablate the surface of the sample[3]. Here we combine these techniques with advanced beam shaping strategies such as Bessel beams[4] and interference pattern formation[5]. By using Bessel beams we demonstrate high aspect ratio channels in the bulk of glass. Meanwhile, by using interference we demonstrate low periodicity grating, which period could be a part used wavelength. The optical setup for beam interference formation and ablation result with interference pattern is shown in Figure 1. An interference pattern is formed by splitting a 1030nm wavelength laser beam into 4 separate beams by using diffractive optical element (DOE), collimating it with an axicon, and focussing with 20x 0,4 NA objective (Figure 1 part (a)). As a result, we get dot matric which period is ~2 um, which shows that modification is near to the focusing limit for 1030 nm radiation (Figure 1 part (b)). In this manner, it is possible to fabricate low periodicity photonic crystals which can be applied in various optical systems such as sensors or lasers resonators.