Birth and evolution of an optical vortex

Propagation dynamics and crosstalk of orbital angular momentum beams influenced by a supersonic wind-induced environmental disturbance

Near field airflow induced by wind is an important factor influencing vortex beams propagation under airborne optical communication, and the cross-talk among different orbital angular momentum (OAM) modes occurs in OAM-based optical communication. In this paper, the propagation of vortex beams through a supersonic wind-induced random environment is investigated. The wind-induced phase model is firstly validated by wind tunnel experiment, with the phase model, vortex beams propagation under supersonic wind condition is analyzed, and the spiral spectrum distortion is discussed in detail. It is demonstrated that the larger wind velocity and boundary-layer thickness leads to the enhanced distortion and spiral spectrum expansion of OAM beams. The behavior of multiplexed vortex beams influenced by supersonic wind is also studied, and the effect of the topological charge interval is given. Our results may provide a powerful tool to estimate the effect of a random airflow environment on OAM-based communication performance under airborne condition.

Quantifying the quality of optical vortices by evaluating their intensity distributions

Optical vortices are widely used in optics and photonics, impacting the measurements and conclusions derived from their use. Thus, it is crucial to evaluate optical vortices efficiently. This work aims to establish metrics for evaluating optical vortex quality to support the implementation procedure and, hence, provide a tool supporting research purposes and technological developments. We propose to assess vortex quality using the following intensity parameters: eccentricity, cross-sectional peak-to-valley, cross-sectional peak difference, and the doughnut ratio. This methodology provides a low-cost, robust, and quantitative approach to evaluating optical vortices for each specific optical technology.

Structural stability of spiral vortex beams to sector perturbations

Conditions of breaking down the structural stability of a spiral vortex beam subject to sector perturbations were considered. Employing methods of computer simulation and processing experimental results, we have shown that the spiral vortex beam has a caustic surface, the intersection of which sharply changes a shape of the Poynting vector streamlines and critical points of the spiral beam. Nevertheless, the beam propagation (scaling and rotation) does not change the perturbed streamline's shape and phase pattern. We also revealed that strong beam perturbations can cause the conversion of the circulation direction of streamlines in the perturbation region, which entails the appearance of a network of optical vortices with negative topological charges. However, the beam's orbital angular momentum remains unchanging, despite increasing the information entropy (growing a number of vortex modes), so that the perturbed beam keeps new stable states.

Generation of Supra-Wavelength Grooves in Femtosecond Laser Surface Structuring of Silicon

Extensive research work has been carried out on the generation and application of laser-induced periodic surface structures (LIPSS). LIPSS with a sub-wavelength period generated by femtosecond laser irradiation, generally indicated as ripples, have been extensively investigated. Instead, the other ordered surface structures characterized by a supra-wavelength period, indicated as grooves, have been much less studied. Grooves typically form at larger irradiance levels or for higher number of laser pulses. Here, we report a comprehensive overview of recent investigations on the supra-wavelength grooves formed on crystalline silicon irradiated by femtosecond laser pulses. The authors’ recent experimental work is mainly addressed giving an explicit picture of the grooves generation process, namely illustrating the influence of the various experimental parameters, including, e.g., polarization, wavelength, fluence and repetition rate of the laser beam as well as number of laser pulses hitting the surface of the material. The effect of irradiation of a static or moving target and of the environmental conditions (e.g., vacuum or air ambient) will also be discussed. Finally, possible mechanisms envisaged to explain grooves formation and still open issues are briefly discussed.

Machine Learning-Based Classification of Vector Vortex Beams

Structured light is attracting significant attention for its diverse applications in both classical and quantum optics. The so-called vector vortex beams display peculiar properties in both contexts due to the nontrivial correlations between optical polarization and orbital angular momentum. Here we demonstrate a new, flexible experimental approach to the classification of vortex vector beams. We first describe a platform for generating arbitrary complex vector vortex beams inspired to photonic quantum walks. We then exploit recent machine learning methods-namely, convolutional neural networks and principal component analysis-to recognize and classify specific polarization patterns. Our study demonstrates the significant advantages resulting from the use of machine learning-based protocols for the construction and characterization of high-dimensional resources for quantum protocols.

Dynamic multilevel spiral phase plate generator

The design and characterisation of a reconfigurable multi-level spiral phase plate is shown. The device is based on a pie-shape liquid-crystal structure with 24 slices driven by custom electronics that allow independent excitation control of each electrode. The electrooptical cell was manufactured using maskless laser ablation lithography and has shown an unprecedented high fill factor. The topological charge can be dynamically changed between 1, 2, 3, 4, 6, 8 and 12. The device has been calibrated and characterised at 632.8 nm but can be employed at any wavelength in the visible and near infrared spectrum, just modifying the driving parameters of the electrodes. The experimental results have been compared to predictions derived from simulations. An excellent correspondence between theoretical and experimental result has been found in all cases.

Surface structures with unconventional patterns and shapes generated by femtosecond structured light fields

We present an investigation on ultrashort laser surface structuring with structured light fields generated by various q-plates. In particular, q-plates with topological charges q = 1, 3/2, 2, 5/2 are used to generate femtosecond (fs) vector vortex beams, and form complex periodic surface structures through multi-pulse ablation of a solid crystalline silicon target. We show how optical retardation tuning of the q-plate offers a feasible way to vary the fluence transverse distribution of the beam, thus allowing the production of structures with peculiar shapes, which depend on the value of q. The features of the generated surface structures are compared with the vector vortex beam characteristics at the focal plane, by rationalizing their relationship with the local state of the laser light. Our experimental findings demonstrate how irradiation with fs complex light beams can offer a valuable route to design unconventional surface structures.

Role of beam waist in Laguerre-Gauss expansion of vortex beams

Laguerre-Gauss (LG) modes represent an orthonormal basis set of solutions of the paraxial wave equation. LG modes are characterized by two integer parameters n and ℓ that are related to the radial and azimuthal profile of the beam. The physical dimension of the mode is instead determined by the beam waist parameter w₀: only LG modes with the same w₀ satisfy the orthogonality relation. Here, we derive the scalar product between two LG modes with different beam waists and show how this result can be exploited to derive different expansions of a generic beam in terms of LG modes. In particular, we apply our results to the recently introduced circular beams by deriving a previously unknown expansion. We finally show how the waist parameter must be chosen in order to optimize such expansion.