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Vortex Beam

A vortex beam, or optical vortex beam, stands distinct from typical light beams due to its unique wavefront. While most light beams possess a flat or radially symmetric wavefront, a vortex beam showcases a wavefront with a constant phase along the radial coordinate and a phase singularity in the middle , creating a donut shape . Typically, Vortex beams don’t occur naturally; they require an optical element known as a vortex phase plate to superimpose this distinct phase pattern on the wavefront. Essentially, a vortex plate is a type of diffractive optical element that introduces a phase modulation pattern. The phase pattern varies exclusively with the angular coordinate, but remains constant along the radial coordinate, culminating in a vortex-like appearance. Magnifying the vortex plate reveals an image reminiscent of a spiral staircase, where each step denotes a varied optical path difference. Hence, vortex plates are often termed spiral phase plates.

Properties of the Vortex Plate:

The far-field of a vortex plate assumes a doughnut shape corresponding to laguerre-gauss modes.  These modes are stable donut shapes, unlike gaussian mode combinations that are sometimes used to create donut shapes.

The emanating beam from the vortex plate gains angular momentum post traversal. The topological of the plate charge serves as an indicator, denoting the number of full phase cycles (2π radians) the vortex plate imposes on the beam .In general, the higher the topological charge, the higher is the laguerr-guess mode generated and the larger are the donut angles. These principles find resonance in quantum optics, quantum computing, and other sophisticated areas of physics.

Vortex plates are widely utilized in lithography and microscopy due to their prowess in light manipulation.This is due to the fact that after traversing the vortex plate, the beam exhibits a “self-healing” quality. Consequently, it remains unaltered even when clipped by small apertures, whereas other beams are susceptible to diffraction effects.

Optical Vortex Beam

Distinct from a traditional laser beam, the wavefront of an optical vortex beam is exceptional. Focusing this beam through a positive lens doesn’t yield the same pinpoint as a conventional Gaussian beam. Instead, the resulting beam profile manifests as a ring structure encircling an area void of irradiance. Although such an irradiance pattern can be derived from a specific combination of Gaussian modes, leveraging an optical vortex phase plate for this beam pattern proves more robust.

Optical vortex beams are pivotal in multiple domains. For instance, quantum computation is a burgeoning arena employing beams with phase vortexes and angular momentum. In STED fluorescence microscopy, the introduction of an optical vortex phase beam enhances the imaging system, allowing it to achieve a resolution surpassing the diffraction limit set for the operational wavelength. In material processing such as drilling of small vias using scanners , vortex phase plates are often used to generate small spots with more flat-top distribution compared to a gaussian spot.