In an optical heterodyne surface plasma wave detecting method and apparatus,
that contains correlated P1 and P2 wave components (TM waves)
is directed to a total reflective component such that two surface plasma waves
are generated at an interface of a metal film and a test object. Light reflected
from the total reflective component is detected to obtain an optical heterodyne
test signal that is compared with an optical heterodyne reference signal to determine
changes in at least one of amplitude and phase of the optical heterodyne signal
relative to the optical heterodyne reference signal.