| Abstract |
It is the past few years that a new type of non-diffracting beams, finite-energy Airy beams, was first introduced. The Airy wavepacket features the unique property to freely accelerate, following a parabolic trajectory, while keeping a constant central lobe diameter over propagation.
Radially symmetric Airy beams or the so-called ring-Airy beams, were recently shown. These beams can abruptly autofocus, along their propagation, delivering high intensity contrast at the focus position. These non-diffractive beams have attracted a great deal of interest due to their remarkable characteristics. It is quite interesting the fact that such beams present long working distances, tight focusing and small focal volumes, while nonlinear propagation effects can be very exciting.
In the following Master Thesis, we demonstrate experimentally generated ring-Airy beams using a Fourier Transform (FT) approach and a simple method to spatially control their autofocus position. The radially symmetric Airy distribution can be generated using the FT of a properly modulated input wave. In particular, a Gaussian beam is modulated; using a phase reflecting only spatial light modulator (SLM), and its FT will give the ring-Airy distribution. By modulating the characteristics of such beams at the plane of generation, we prove that we control the working distance of these beams and the focal voxel dimensions and shape. We show that the generated ring Airy beams have a high aspect ratio focal voxel that can be positioned at different working distances keeping almost invariant its dimensions and shape. Moreover, we report on parametric results of such ring Airy beams and demonstrate that the working distance control of Bessel beam is also possible.
These notable abilities of ring Airy beams make them ideal candidates for direct laser writing by Multi-photon polymerization. Multi-photon polymerization (MPP) of photosensitive materials allows one to fabricate complicated three-dimensional (3D) microstructures. When a fs infrared laser is tightly focused into the volume of a photosensitive resin, femtosecond laser pulses can cause MPP and produce structures with sub-micrometers resolution and lower. The controllable, long working distance and the high aspect ratio focal volume, surpass the restrictions set to the overall height of a 3D structure when using Gaussian beams and small working distance, high NA objective lenses; and long-range MPP can be achieved. Therefore, we present 3D structures made using ring-Airy beams that were set to autofocus inside the volume of a photoresist while the sample was moved only on the x-y plane.
In conclusion, tunable ring Airy beams are employed for direct laser writing using Multi-photon polymerization. These beams present high aspect ratio focal voxels which can be shifted at different working distances while remains almost invariant, an attribute that makes these beams ideal candidates for long range Multi-photon polymerization. Examples of fabricated 3D microstructures are presented as well as theoretical calculations and comparison with Bessel beams.
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Laser beam shaping for multi-photon polymerization
