In situ monitoring of optical characteristics of pulsed picosecond laser-irradiated diamond-like carbon thin films using an optical parametric generator and amplifier

In situ reflectivity measurements were carried out at room temperature and atmospheric pressure to follow dynamic structural variations occurring during pulsed laser irradiation of diamond-like carbon (DLC) thin films. Laser irradiation consisted of second harmonic (λ=532 nm) picosecond pulses of a Nd–YAG laser, leading to a modification of the carbon layer structure. Irradiation at this wavelength usually leads to graphitization, i.e. an increase of the number and/or size of sp2 clusters dispersed in the amorphous carbon structure of DLC films. One of the consequences of such a transformation is a pronounced change in the optical properties. Upon graphitization, the optical absorption strongly increases in the near-infrared region. In order to monitor such changes we used a probe beam tunable in this wavelength region. An optical parametric generator (OPG) and amplifier (OPG/OPA) with low energy pulses tunable in the near-infrared range between 700 nm and 2 μm was used to record reflectivity changes at several wavelengths. Such an in situ characterization reveals how the structural transformations proceed in the early phase of graphitization.