Photopolymerization, the process of using ultraviolet light to activate polymerization within resins, is a powerful approach to create arbitrary, transparent micro-objects with a resolution below the diffraction limit. Importantly, to date all photopolymerization studies have been performed with incident light fields with planar wavefronts and have solely exploited the intensity profile of the incident beam. We investigate photopolymerization with light fields possessing orbital angular momentum (OAM), characterized by the topological charge “l”. We show that, as a consequence of nonlinear self-focusing of the optical field, photopolymerization creates an annular-shaped vortex-soliton and an associated optical fibre, which exhibits a helical trajectory, with a chirality determined by the sign of “l”. In particular, due to a transverse modulation instability in the nonlinear self-focusing photopolymer, the vortex beam breaks up into the “l” solitons or microfibers, each of which exhibit helical trajectories and together form a bundle of helical microfibers. Our numerical simulations, based on the nonlinear paraxial wave equation for the photopolymer, captures all the experimental observations for a variety of optical vortices characterized by “l”. This therefore represents a new physical manifestation of the use of OAM light fields. This research opens up a new application for light fields with OAM, and our generated microfibers may have applications in optical communications and micromanipulation. In a broader context, our work adds a new facet to the emergent field of helical fibres that have themselves recently come to the fore in the photonic crystal community as a route to generating fields with OAM.
Photopolymerization, the process of using ultraviolet light to activate polymerization within resins, is a powerful approach to create arbitrary, transparent micro-objects with a resolution below the diffraction limit. Such microstructures have been optimized for optical manipulation and are finding application elsewhere, including micro-optics, mechanical microstructures and polymer crystallography. Furthermore, due to self-focusing, photopolymerization can form a waveguide, which develops into an optical fibre as long as submillimeters. Importantly, to date virtually all photopolymerization studies have been performed with incident light fields possessing planar wavefronts and simply exploit the beam intensity profile. Here we investigate photopolymerization of ultraviolet curing resins with a light field possessing orbital angular momentum (OAM). We show that the annular vortex beam breaks up via modulation instability into the m-microfibers, depending on the azimuthal index m of an incident optical vortex. These microfibers exhibit helical structures with chirality determined by the sign of m and mirror the helical nature of the incident vortex beam wavefront. We have developed a numerical model based on the Beam Propagation Method that captures the key experimental observations for a variety of optical vortices characterized by their azimuthal index m. This research opens up a range of new vistas and has broad consequences for the fields of structured light, new approaches to writing novel mesoscopic structures and applications such as in detecting or sorting the OAM mode (e.g. photonic lanterns) in areas including optical communications and manipulation.
We have identified major paradigm shifts relative to near-IR filamentation when high power multiple terawatt laser pulses are propagated at mid-IR and long-IR wavelengths within key atmospheric transmission windows. Individual filaments at near-IR (800 nm) wavelengths typically persist only over tens of centimeters, despite the whole beam supporting them being sustained over about a Rayleigh range. In the important mid-IR atmospheric window (3.2 - 4 μm) optical carrier wave self-steepening (carrier shocks) tend to dominate and modify the onset of long range filaments. These shocks generate bursts of higher harmonic dispersive waves that constrain the intensity growth of the filament to well below the traditional ionization limit, making long range low loss propagation possible. For long wavelength pulses in the 8-12 μm atmospheric transmission window, many-electron dephasing collisions from separate gas species act to dynamically suppress the traditional Kerr self-focusing lens and leads to a new type of whole beam self-trapping over multiple Rayleigh ranges. This prediction is key, since strong linear diffraction at these wavelengths are the major limitation and normally requires large launch beam apertures. We will present simulation results that predict multiple Rayleigh range propagation paths for whole beam self-trapping and will also discuss some recent efforts to extend the HITRAN linear atmospheric transmission/refractive index database to include nonlinear responses of important atmospheric molecular constituents.
There is a strong push worldwide to develop multi-Joule femtosecond duration laser pulses at wavelengths around 3.5-4 and 9-11μm within important atmospheric transmission windows. We have shown that pulses with a 4 μm central wavelength are capable of delivering multi-TW powers at km range. This is in stark contrast to pulses at near-IR wavelengths which break up into hundreds of filaments with each carrying around 5 GW of power per filament over meter distances. We will show that nonlinear envelope propagators fail to capture the true physics. Instead a new optical carrier shock singularity emerges that can act to limit peak intensities below the ionization threshold leading to low loss long range propagation. At LWIR wavelengths many-body correlations of weakly-ionized electrons further suppress the Kerr focusing nonlinearity around 10μm and enable whole beam self-trapping without filaments.
We investigate the nonlinear propagation of intense Bessel and Airy beams forming filaments in transparent media. We identify two propagation regimes separated by the relative importance of multiphoton absorption and self-focusing of the main Bessel or Airy lobe, due to the Kerr effect. We show that intense Bessel or Airy beams are reshaped into stationary nonlinear beams whose propagation is sustained by a continuous energy flux to the main lobe from its neighbors. The stationary propagation regime is obtained for Bessel cone angles exceeding a certain threshold; by focusing a Gaussian beam of sufficient power with an axicon. With respect to linear Bessel beams, stationary nonlinear Bessel beams exhibit ring compression and attenuation of contrast. For small cone angles, the nonlinear Bessel beams become unstable leading to an unsteady propagation regime. We demonstrate similar physics for intense Airy beam freely propagating in a Kerr medium: stationary nonlinear Airy beams are demonstrated in a planar geometry. These beams preserve the intensity profile and the transverse acceleration of the Airy peak. For powers in the main Airy lobe exceeding a certain threshold, this stationary propagation regime becomes unstable. In the 2-dimensional case, Airy beams with high powers in the main lobe reshape into a multifilamentary pattern induced by Kerr and multiphoton nonlinearities. The nucleation of new filaments and their interaction, affects the acceleration of the main Airy lobes.
We investigate the nonlinear propagation of intense Airy beams forming filaments in transparent media. We
demonstrate the existence of stationary nonlinear Airy beams in a planar geometry. These beams preserve
the intensity profile and the transverse acceleration of the Airy peak. We show that stationary propagation is
sustained by a continuous energy flux to the main Airy lobe from its neighbors. For powers in the main Airy lobe
exceeding a certain threshold, this stationary propagation regime becomes unstable. We extend our results to
the 2-dimensional case: Airy beams with high powers in the main lobe reshape into a multifilamentary pattern
induced by Kerr and multiphoton nonlinearities. The nucleation of new filaments and their interaction, affects
the acceleration of the main Airy lobes. Experiments performed in water corroborate the existence of these two