Formation of colorized silicon by femtosecond laser pulses in different background gases

A single-crystal silicon(111) wafer surface fixed on an x-y translation stage is scanned with a focused femtosecond laser beam at a wavelength of 800 nm under different atmospheres (air, vacuum, and nitrogen). Different colors from different angles on the surface of the silicon then appear. From the result of the experiments, periodic ripple surface structures emerge on the surface of colorized silicon, and the phenomenon is more obvious in vacuum and nitrogen than in air. The periods of the surface structures on silicon are not the same in the different atmospheres. Under vacuum, the period is the longest and is closer to the wavelength of the laser irradiation. Different from metals, the range of energy density is smaller when the colorized silicon appears with femtosecond laser pulses. Through SEM, TEM, and AFM, we observe in detail the microstructures of colorized silicon that forms in air, vacuum, and nitrogen and analyze the possible physical mechanism. Finally, research into the optical reflection of the colorized silicon indicates that the reflectivity is not higher than 30% in the 250-800 nm range.