Non-linear absorption of focused femtosecond laser pulses at 1.3 μm inside silicon: Independence on doping concentration

We investigate experimentally local non-linear absorption when 1.3 μm wavelength femtosecond pulses are tightly focused inside n-type doped silicon. We show that 130 fs pulses with only few nanojoule energy is enough to initiate free-carrier generation. Our results also demonstrate that the laser energy deposition is independent on the doping concentration for substrates with free-carrier densities up to 1018 cm−3. For deep focusing experiments, the energy deposition can remain confined in micron-scale focal regions provided we perform the experiments with focused beam corrected for spherical aberration. The high degree of control observed in the experiments and the independence on doping are major assets for future 3D-micromachining technology developments based on this approach.