HIGHLY SENSITIVE OPTICAL MEASUREMENT TECHNIQUES BASED ON ACOUSTOOPTICDEVICES

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.