Spectroscopic Pump-Probe-Reflectometry of NIR Excited Silicon

During laser mater interaction of ultrashort pulsed laser radiation =1950 nm, <50 fs) with silicon, the resulting nonlinear excitation of electrons by NIR radiation also affects to the optical properties in the visual spectral range. Imaging pump-probe reflectometry enables the measurement of the transient reflectivity for different probe wavelengths (420 nm < < 1000 nm, < 100 fs) and time delays up to Δt=500 ps after irradiation. Assigning the spatial coordinates to local fluences allows a fluence dependent interpretation as well. Below the fluence for material modification ℎ, a reduction of reflectivity was detected which become more and more significant with an increasing probe wavelength. For fluences above ℎ, the reflectivity increases rapidly after irradiation and features a local minimum between Δt =2 ps and Δt =50 ps for all probe wavelengths.

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