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New high-resolution displacement sensor based on surface plasmon resonance

In this work we report on a novel angular and positional sensor based on the phenomenon of attenuated total reflection that occurs at a metal-dielectric interface when the conditions for the excitation of a surface plasma wave in Kretschmann configuration are satisfied. The reflectivity of the metallic surface exhibits a very sharp dip at an angle of incidence corresponding to the phase-matching condition for the coupling of energy from the incident beam to the resonant surface mode. The typical width of the resonance is a few mrad, thus making feasible the direct measurement of small angular movements by just detecting the intensity of the reflected light. By means of a simple optical setup this sensitivity can be exploited to build a position-sensitive detector capable of nanometric resolution. Tests have been carried out on several Ag depositions. The angular resolution obtained has been in the 0.2 to 0.4 arcsecs range; the sensitivity to linear displacements has been tested monitoring the motion of piezoelectric actuators and is better than 5 nm over a range of a few microns. We have verified that the proposed method does not require beams of high optical quality and permits in principle a considerable simplification over interferometric systems. Well-established technological processes might be used for its implementation, keeping its cost at a competitive level with respect to other devices of the same potential sensitivity.