The photo is sourced from rscf.ru
Scientists from the Institute of Automation and Control Processes of the Far Eastern Branch (FAB) of the Russian Academy of Sciences (RAS) have described changes in monocrystalline silicon surface during laser processing. The authors of the study placed the crystal in a methanol solution and applied laser pulses lasting quadrillionths of a second (15 orders of magnitude less than a second) to the sample, varying the number of pulses hitting each square micrometre (millionth of a square metre) of the surface from five to fifty.
The experiment showed that with a small number of pulses, three-dimensional nanostructures emerge on the surface of the crystal, forming a pattern of parallel convex stripes. When 25–30 laser pulses are applied to each square micrometer of silicon, the stripe pattern turns into a labyrinth of irregularly shaped convexities. The scientists believe that this effect is caused by the material heating up and partially melting under intensive laser processing, which leads to changes in the surface structure.
“We slightly changed the laser processing technique accepted by the scientific community: we did it when the material was not in an air environment but in a liquid environment, i.e., in methanol. This made it possible to prevent the oxidation of silicon, to avoid any debris getting onto the surface of the material, and to form regular and dense nanostructures,” Sergey Syubaev, junior researcher at the Institute of Automation and Control Processes FAB RAS, is quoted as saying by the Russian Science Foundation.
The authors also found how the patterns on the surface of the crystal change depending on the polarisation of the laser beam, a feature that reflects how the vectors (directions of propagation) of the electric and magnetic fields of a light wave are oriented in space. For instance, if oscillations of the electric field vector occur in a single plane, the laser can form both 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 convexities are formed on the surface of the crystal. Finally, when the polarisation changes, the oscillations of the electric field vector become perpendicular to the axis of the light beam, and the laser beam assumes the shape of a donut: when it irradiates the surface, nanostructures resembling spikes of wheat emerge.
The researchers assessed the ability of the resulting samples to absorb light. They found that all patterns reflected light, losing no more than 5% of the light. To prove in practice that laser processing makes single-crystalline silicon more sensitive to light than the original samples, the authors designed a photodetector on the basis of the material. The sensitivity of the device to infrared radiation turned out to be twice that of a detector using a conventional silicon crystal.
