Interferometric measurement of melt depth in silicon using femtosecond infrared Cr:forsterite laser

Interferometric microscopy technique combined with high power infrared Cr:forsterite laser system was applied to investigate femtosecond laser induced melting of silicon. Optically polished wafer of single crystalline silicon of 400 μm thickness was irradiated with 100 fs pump pulses at second harmonic wavelength of 620 nm. We used infrared probe pulses at main wavelength of 1240 nm, whose photon energy was less than the band gap width E_g = 1.12eV of silicon, and the penetration depth of probe essentially exceeded the sample thickness. Unlike many previous experiments with Ti:sapphire lasers it allowed us to probe the heated area from the rear side of the sample and obtain the data on melt depth after laser irradiation.