Vortex Plates

Vortex plates

A vortex plate is a special type of diffractive optical element that imparts a phase modulation to an incoming beam. The unique aspect of the vortex plate is that this phase modulation varies only with the angular coordinate. The phase along the radial coordinate remains constant so it looks like a vortex. Now, given that any diffractive optical element is a piecewise structure, that is, is not a continuous function, a magnified image of the vortex plate will resemble a spiral staircase, so they also referred to vortex plates as spiral phase plates.

Perhaps the most well-known property of vortex plates is that their far field can be described as having a doughnut shape. This shape could also be obtained with a combination of Gaussian modes but a vortex plate is much more versatile and compact. This sort of beam shape can be useful in many applications. For instance, in optical tweezers the vortex plate can provide an extra degree of freedom for light manipulation. In microscopy and lithography, where the imaging properties are given by the Point Spread Function of the overall optics involved, a vortex plate can open new possibilities to explore. In fact, there is one technique in microscopy that is already harnessing the properties of vortex plates. This technique is called stimulated emission depletion, or STED, and it uses two optical channels with different wavelengths. Both are used to illuminate the sample at the exact location to generate fluorescence, but in one of them a vortex plate is placed along its optical train. This will create a ring pattern of that wavelength that encircles the spot from the other wavelength. The bounded area is generally smaller than the diffraction limit. Hence, resolution beyond the diffraction limit can be obtained.

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Another remarkable property of the beam coming out of a vortex phase plate is that it is considered to be “self-healing”. This means that the beam is not affected by clipping induced by apertures not being big enough.  All other types of beams can be greatly affected by this as a result of diffraction.

The output beam of a vortex plate is said to have acquired an angular momentum.  Along the same line, there is a parameter referred to as the topological charge which indicates the number of full phase cycles (2π radians) that the vortex plate can impart to the beam. These concepts are becoming more common in advanced physics fields like quantum optics and quantum computing.

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