Effects of pulse delay in metal micromachining with femtosecond laser pulses

Single and double femtosecond laser pulses with different time delays were used to investigate surface morphology changes in Cr thin films. A double pulse was generated by varying the beam path length of a split pulse and recombining the two beams. With a single pulse, a nanometer-sized droplet was formed on the irradiated surface; the area was dependent on the laser power. With mul-tiple pulses at a fixed repetition rate, the subwavelength ripples aligned perpendicular to the polari-zation direction of the laser light with a spacing of about 200 nm. Curiously, the initial ripple was in the same direction as the laser polarization for the first few pulses. Using a double pulse with differ-ent polarization directions and a time separation of zero, the ripples formed in the sum vector direc-tion of both beams. Increasing the time separation removed the ripples effectively. After a few tens of ps, the second pulse did not affect the surface changes. The changes in surface morphology were attributable to interference or disturbances between the hot electron oscillations induced by the two pulses and the diffusion time of the electron energy. A double pulse with different polarization direc-tions within the diffusion time of the electron energy is a useful method for decreasing the size of the ablated area and eliminating ripples in micromachining.