Highly sensitive optical measurement techniques based on acousto-optic devices

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Abstract
An optical measurement technique is presented that permits a direct measurement of the differential transmission or reflectivity of a sample. The technique is based on the use of an acousto-optic device to modulate rapidly the incident angle or wavelength of the probe beam. Detection 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 differential can strongly enhance normally undetectable optical features, such as weakly coupled, Otto geometry surface plasmon polaritons. A development of the technique, which uses the optical analog of a phase-locked loop, is demonstrated to have an angular resolution of 6 × 10−6 deg. This permits the detection of the shift in the critical angle caused by a change of 10−6 in the refractive index of a gas mixture.