The SFG apparatus is a complex laser system based on a high-power picosecond Nd:YAG laser and an optical parametric generator/amplifier (OPG/OPA). The fundamental output (1064 nm) of a Nd:YAG laser is frequency doubled to produce the 532 nm visible beam and is used to drive an OPO/OPA. The tunable (1300 to 4000 cm-1) IR beam is generated from a series of non-linear crystals through OPG/OPA and difference frequency mixing. The sum-frequency (SF) spectra are obtained by overlapping the visible and IR beams on the polymer surface at incident angles of 60° and 55°, respectively. The SF signal from the polymer surface is collected by a photomultiplier tube (PMT) and processed using gated integrator. Surface vibrational spectra are obtained by measuring the SF signal as a function of the input IR frequency. A schematic of the experimental setup is shown in Figure 2.
To obtain surface specificity, the analysis is only performed in reflection mode.
Figure 2. Schematic diagram of the SFG laser system.