A surface-plasmon-based optical sensor using acousto-optics

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Abstract
In this work the construction of a wavelength-tunable optical sensor with a fixed angle of incidence, incorporating an acousto-optic tunable filter (AOTF) is described. The AOTF is used to control the wavelength of a p-polarized light beam incident on a gold-coated diffraction grating. A surface plasmon resonance (SPR) is observed as a deep minimum in the intensity of the reflected beam as the incident wavelength is incremented. Slight modification of this arrangement allows measurement of the differential reflectivity profile with respect to wavelength. By locking to the zero differential corresponding to the SPR reflectivity minimum and monitoring the AOTF drive frequency (typically about 100 MHz) the SPR minimum position is then measured to within a precision of 0.0005 nm. The sensitivity of this system was found to be equal to a change in the refractive index of a gas of 1*10-6. Furthermore, by adding a chemically active overlayer to the system a concentration of 0.01 ppm NO2 in N2 was detected.