All-Optical Bessel Beam Generation in Carbon Nanotube Suspension for a Formation of Biocompatible Organic Nanomaterials

Generation of laser radiation with different spatial shapes is attractive for various fields such as optical switching, optical tweezers, two-photon polymerization, ect. This paper presents all-optical conversion of Gaussian beam to Bessel beam by means of spatial cross-phase modulation in the suspension of carbon nanotubes. The probe light from femtosecond laser (1030 nm) was modified using pump radiation from quasi-continuous laser (690 nm) that is focused into the suspension. The manifestation of nonlinear optical refraction in a suspension of carbon nanotubes leads to the formation of a phase shift, which affects the change in the laser beam shape at the exit from the medium. The influence of the pump beam of on the probe beam diameter is shown. Increasing the power above 20 mW leads to the generation of dark-hollow beam. Subsequent focusing of dark-hollow beam using a double-convex lens leads to Bessel beam generation near the focal plane. It is shown that the central peak of the Bessel beam has a much smaller divergence when moving away from the lens focus compared to the Gaussian beam. The diameter of the central peak increased from <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$50 \mu \mathrm{m}$</tex> to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$63 \mu \mathrm{m}$</tex> when moving the observation plane along the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$Z$</tex> axis from 0 to 10 mm. The laser beam with high Rayleigh length and femtosecond pulse duration opens the possibility of laser formation of filamentous biocompatible nanomaterials with high aspect ratio by twophoton polymerization.