ITMO Scientists Propose Improved Method For Laser-Plasma Processing Of Glass

The new method makes it possible to create thin elements on glass surfaces that are essential in the production of micro-optical components and phase elements for laser systems. An article on the subject was published in the Journal of Materials Processing Technology.

To control a laser beam in a lab, scientists need lots of different equipment: special lenses, diffraction gratings, waveguides. They are all hard to manufacture. Controlled etching of a nanorelief on a glass surface turns a glass plate into a functional optical element that can transform laser emissions. Such transformed beams are widely used in “optical tweezers”, precision materials processing,and high-resolution microscopy.

As of today, the best results were obtained using photolithography. But it is an expensive procedure due to the great number of steps it involves; it also calls for a lot of time which is spent on the processing of millimeter-size areas.

Scientists at ITMO University are actively developing the laser-initiated microplasma (LIMP) glass-processing method. It’s used with industrial laser systems equipped with fiber-optic lasers. The system’s key feature has to do with the position of the glass on the target’s surface. Laser emission passes through a transparent material but is intensively absorbed by the target, on the surface of which plasma emerges. Its interaction with glass results in the forming of a micro- or nano-relief.

Traditionally, the target was pressed firmly against the glass. But at some point, a problem emerges: the relief formed on the glass starts to change the laser beam. The closer the sample becomes to a ready lens or diffraction grating, the more it affects the beam’s form and size. This restricts the method’s processing resolution. The laboratory’s team succeeded in solving this problem.

ITMO researchers decided to modify the method by positioning the glass and the graphite not close by but rather with a gap between them.

Using plasma at a distance is not always bad
As a result, the scientists showed that a specific gap can reduce the size of the laser beam that hits the target. This, in turn, makes the plasma plume on graphite’s surface smaller, as well.

The theoretical calculations of the optical system with regard to the microlens and the plasma spraying angle correlate with the experimental data of the research. For one, the scientists have assessed the geometric efficiency of the improved method. It was 27%, which means that most of the plasma plume does not reach the glass. These calculations will be useful when creating the method’s energy model.

Method relevance
In laser micro- and nanoprocessing circuits, one can’t do without special phase optical converters that are needed to change the shape of the initial laser beam or split it. The team of ITMO’s International Laboratory “Laser Micro- and Nanotechnologies” has designed and manufactured such elements using the LIMP method following a detailed study of the features of laser plasma’s effect on glass surfaces. Among the elements they created are microlens arrays, phase arrays, spiral particles, and phase masks.

Result
The research on the focusing parameters in the LIMP method helped answer such important questions as “what should be the requirements for focusing parameters?” and “does the air gap affect the processing results?”. What’s more, the proposed model opens the way to building a full-fledged thermophysical model, which will eventually make it possible to predict the results of glass processing with the LIMP method and control it better.

The research was carried out as part of a grant from the Russian Science Foundation (project № 20-71-10103).