Create reconfigurable liquid crystal axicon-type lenses

These new lenses, with applications in science and industry, can change their depth of focus by electronic control without the need for moving parts.

An axicon is a type of lens that, unlike normal lenses, allows light to be focused on a volume instead of a point. When projected onto a flat surface, the light appears as a ring, which increases or decreases in size as the lens is zoomed in or out. Axicons are used in various fields of science and industry, such as physics research, optometry, and astronomy. They are usually made of glass or polymers and have a fixed volume of focus (or depth of focus). However, researchers at the Polytechnic University of Madrid (UPM) in Spain have now developed axicons that can change their depth of focus by electronic control without the need for moving parts. And they have achieved it by using liquid crystal for its manufacture, a material whose molecules are reoriented under the action of an electric field.

In contrast to glass lenses where the focus is modified with lenses of greater or lesser curvature, the lenses developed at the Center for Advanced Materials and Devices for Information and Communication Technologies (CemdaTIC) of the UPM achieve a similar effect. varying the orientation of the liquid crystal, that is, changing the electronic signals, but maintaining a constant thickness. Because another difference is that these reconfigurable lenses are flat, instead of having the traditional conical shape of the axicons.

Apart from designing, manufacturing and verifying the operation of the devices, the researchers considered and verified the possibility of using the novel lenses for the generation of optical vortices, ring-shaped laser light beams that are currently attracting much interest for their use in quantum communications. Their research has shown that they can not only change the topological charge of these optical vortices but also be used to generate so-called perfect optical vortices, those in which the radius of their optical singularity does not depend on their topological charge.

Images obtained by microscope in which it can be seen how the axicon developed by the researchers changes its configuration by the application of different electrical signals. (Images: UPM)

Apart from quantum communications, there are other possible applications of liquid crystal reconfigurable axicons. Javier Pereiro García, assistant professor at the Higher Technical School of Telecommunications Engineers (ETSIT) and one of the authors of the research, mentions the use of lasers to cut or shape parts. “The fact that you can set the focus volume could allow you to drill holes of different depths,” he says. And for the same reason, in cases of corneal surgery, where the use of axicons is common, it would make it possible to modify the ring-shaped cuts. The researcher also refers to optical tweezers, an instrument that uses a laser beam, very useful, for example, for microbiologists. “The light beam produced by these axicons can be used to capture and controllably move microscopic objects.”

The study is titled “Flat variable liquid crystal diffractive spiral axicon enabling perfect vortex beams generation”. And it has been published in the academic journal Scientific Reports. (Source: UPM)