An optical measurement technique is presented that permits a direct me
asurement of the differential transmission or reflectivity of a sample
. The technique is based on the use of an acousto-optic device to modu
late rapidly the incident angle or wavelength of the probe beam. Detec
tion of the resulting modulated signal by means of a lock-in amplifier
gives a direct measure of the differential optical properties of the
sample. It is demonstrated that this direct measurement of the differe
ntial can strongly enhance normally undetectable optical features, suc
h as weakly coupled, Otto geometry surface plasmon polaritons. A devel
opment of the technique, which uses the optical analog of a phase-lock
ed loop, is demonstrated to have an angular resolution of 6 x 10(-6) d
eg. This permits the detection of the shift in the critical angle caus
ed by a change of 10(-6) in the refractive index of a gas mixture.