The equipment needed for the
SHG experiment is shown diagrammatically in figure 5.
5. SHG apparatus showing the control of the polarisation of the
fundamental and harmonic waves
equipment is similar to that used in our recent studies20
The frequency doubled beam from a Q-switched Nd:YAG, (532 nm, 5 ns
pulse width and 20 Hz repetition rate) impinges on the surface of the
sample at 60o to the surface normal. The polarisation of
the input beam is controlled by a half-wave plate and polariser and a
yellow Schott filter removes any second harmonic signal generated by
the optics, and beam is then focused onto the sample surface by a
convex lens (f = 100 mm). The beam is reflected off the surface
and passes through a re-collimating lens and analysing polariser.
UG5 filters are used to block the fundamental and the SHG signal is
focussed on to the slit of the monochromator, and detected by a UV
PMT. The signal is processed using a SRS pre-amp and boxcar and
captures with a National Instruments A/D card and recorded on a PC
automatically entered into the ?raw data? database (MySQL)
together with the relevant metadata. Environmental data on the room
and the laser is also now recorded and logged.