Vortex beams are a unique type of laser beam that is useful in a variety of laser applications. However, before we deep dive into what actually a vortex beam is and what are its characteristics and applications, we need to understand that this laser beam is not a naturally occurring beam. In fact, the most naturally occurring laser beams are Gaussian laser beams. The conversion of a Gaussian beam into a doughnut-shaped ring is essential for several applications, hence the need for vortex beams. This beam conversion helps a vortex beam achieve a distinct wavefront structure with an orbital angular momentum. The phase of the vortex beam stays unchanged along the radial coordinate, and varies only in the angular coordinate . A vortex beamโs phase pattern mirrors the twirling motion of a fluid vortex or whirlpool, very similar to what we see in fluids draining through a hole. The term ‘vortex’ originates from this phenomenon.
Optical Vortex Phase Plate
An optical vortex phase plate is a window-like optical element that helps control the transmitted beamโs phase and develop a unique phase pattern. The structure of the phase plate consists of a helical structure or spiral staircase with a unique optical path difference at each step. This is why the vortex phase plate is popularly known as a spiral phase plate. There is a tool called a vortex calculator that can help you to analyze vortex behavior and calculate parameters related to vortices.
When we use a lens to focus a vortex beam, it generates a focused spot that looks different from the typical focused spot produced by a Gaussian beam output of conventional lasers. The spiral phase structure of the optical vortex beam helps generate a unique intensity pattern. You can see a ring-shaped structure at the focused spot that surrounds a zero-irradiance central area. Although a specific combination of Laguerre-Gaussian modes can produce such an irradiance pattern, vortex phase plates are more convenient and efficient to produce this kind of beam pattern.ย
Applications
Some of the major applications of a vortex beam include-
- Quantum optics
- Solar coronagraphs in astronomy
- Lithography
- High-resolution microscopy
- Optical tweezers for trapping and manipulating the particles
In quantum computation, a vortex beam with a topological charge or angular momentum is useful to encode quantum information. In fluorescence microscopy, with the help ofย vortex lenses an imaging system can obtain a high resolution exceeding the diffraction limit established for the operational wavelength. We can simply use two beams with different wavelengths to break the resolution limit in fluorescent microscopy. Oneย gaussian laser beam stimulates the fluorescence, or lights up the sample,while the other beam passes through the phase plate and generates a donut that depletes (suppresses) the fluorescence, resulting in only the hole of the donut generating light. This method is applied in stimulated emission depletion microscopy (STED), helping us to see smaller details than usual. Another crucial application of vortex beams can be observed in material processing, including cutting and drilling materials.
