Research has shown that patterns on crystals can double the optical sensitivity of photodetectors

Scientists from the Institute of Automation and Control Process at the Far East Branch of the Russian Academy of Sciences described the changes on the surface of monocrystalline silicon during laser processing. The author of this study placed the crystal in a methanol solution and applied a laser pulse lasting one thousandth of a second to the sample, with a pulse count ranging from five to fifty per square micrometer of surface.

Experiments have shown that under a small amount of pulse, three-dimensional nanostructures appear on the crystal surface, forming parallel convex stripe patterns. When 25-30 laser pulses are applied to silicon per square micrometer, the stripe pattern becomes a maze composed of irregularly shaped protrusions. Scientists believe that this effect is due to the heating and partial melting of materials under intensive laser processing, resulting in changes in surface structure.

"We have slightly changed the laser processing technology accepted by the scientific community: when the material is not in an air environment but in a liquid environment, that is, in methanol, we have achieved this. This makes it possible to prevent silicon oxidation, prevent any debris from entering the material surface, and form regular and dense nanostructures," said Sergei Shubayev, a junior researcher at the FAB RAS Institute for Automation and Control Processes, quoted by the Russian Science Foundation.

The author also discovered how the patterns on the crystal surface change according to the polarization of the laser beam, which reflects the direction of the electric and magnetic field vectors of light waves in space. For example, if the oscillation of the electric field vector occurs on a single plane, the laser can form parallel lines and spherical structures on the surface. When the electric field vector rotates in a plane perpendicular to the direction of light propagation, only spherical convex surfaces are formed on the crystal surface. Finally, when polarization changes, the oscillation of the electric field vector becomes perpendicular to the axis of the beam, and the laser beam takes on a donut shape: when it shines on the surface, nanostructures resembling wheat spikes appear.

The researchers evaluated the ability of the obtained samples to absorb light. They found that all patterns reflected light and lost no more than 5% of the light. In order to demonstrate in practice that laser processing makes monocrystalline silicon more sensitive to light than the original sample, the author designed a photodetector based on the material. The sensitivity of this device to infrared radiation is twice that of detectors using traditional silicon crystals.

Source: Laser Net