Generation and imaging of structural laser beams for optical trapping of airborne particles and light-matter interaction applications.

Abstract

Recently emerged non-diffracting optical Bessel-Gaussian beams have extended the `optical toolbox' in laser interaction with matter studies, providing many techniques that are not possible using conventional Gaussian beams. The research work performed under this thesis focuses on the design, generation, re-configuration, and exploration of the flexibility of BG beam for a broad range of studies of laser-matter interaction. The numerically modelled BG beam structures are experimentally tested and applied in two substantially different experimental environments. These include the generation and re-imaging of a dynamically varying BG beam, whose morphology is shaped using a Spatial Light Modulator for low-intensity applications and the generation of a static Bessel beam shaped by an axicon to be used with high-intensity femtosecond laser pulses.

The first set of experiments explores the construction of a slow-diverging optical funnel based on the generation of a higher-order BG beam using a continuous wave (cw) laser beam and a SLM. The use of hollow-core BG beams have already been demonstrated for pipeline-guiding of sub-micron particles and biological macromolecules in experiments of protein nano-crystallography using x-ray diffracting imaging. The particle dynamical interaction with the beam relies on two fundamental light-induced effects that can be applied on matter, namely, radiation pressure and photophoresis. Here, we generate a SLM-formed BG beam with variable topological charge in combination with different re-imaging systems that enable us to create flexible output beams. We also demonstrate the flexibility in controlling the beam shape, the diameter of the central dark core and the angle of divergence. As a result, the analysis of trajectories of the particles trapped inside the funnel and the modelled intensity distribution of the beam allowed us to evaluate the optical forces exerted on the sub-micron particles and uncover, for the first time in our knowledge, the temperature gradient across the particle surface, which is of paramount importance for laser guiding biological particles in airborne environments.

The second set of experiments investigates the construction and re-imaging of zero-order BG beam using axicons for high-intensity laser-induced cylindrical microexplosion in transparent dielectric media. A comparison between numerical modelling and the experimentally constructed BG beam using a commercially available axicons reveals that the origin of the observed undesirable intensity modulations along the beam propagation is due to the imperfections of the axicon shape. That knowledge was applied to manufacture an in-house perfectly shaped axicon and to construct the BG beam which near-ideal intensity distribution highly correlates with the numerical modelled beam. The application of the non-diffracting beam generation setup together with a reimaging system allowed us to produce high aspect-ratio elongated nanovoids inside a pristine sapphire crystal when high-intensity femtosecond laser pulses. Careful consideration of the numerically predicted intensity distribution of the beam inside the crystal and the resulting length and diameter of the nanochannels enable us to demonstrate higher energy concentration in the cylindrical geometry with BG pulses compared to the spherical geometry, common when Gaussian laser pulses were used. The close correlation between numerical and experimental generated beam allowed us, for the first time to our knowledge, to determine the nanochannel formation intensity threshold of 7.2 x 10^{13} W/cm^2 in sapphire.

The numerical modelling of the flexible zero- and higher-order BG beams performed in these studies and its application to very different experimental conditions of light-matter interactions is highly advantageous, as they allow tuning of laser parameters in terms of the beam structure and intensity distribution to match the required interaction conditions. Show more