Figure 4.

The polarization of the laser beam was passively modulated by doubling an 80 MHz Ti:sapphire laser to 160 MHz using two polarizing beam splitters and a 6.25 ns delay stage. The resulting 160 MHz train of light pulses produced orthogonally interleaved polarizations, with modulation at 80 MHz. The laser served as the master clock for all electronics. The laser was galvanometer/sample scanned across the sample, and the resulting SHG was separated and detected as H and V polarizations. When the LIA was used for measurements, its DC output was recorded with the same digitizer.