AXIAL AND LATERAL DISPLACEMENT MEASUREMENTS OF A MICROSPHERE BASED ONTHE CRITICAL-ANGLE METHOD

A method has been developed for optically measuring nanometer-scale di splacements of transparent and metal-coated microspheres in both the a xial (vertical) and lateral (horizontal) directions. This method works by detecting changes in the internal reflection of a laser beam refle cted from the microsphere after it passes through critical-angle prism s. For weakly reflective 10-mu m-diameter polystyrene microspheres in water (relative refractive index n = 1.2), the detection resolutions a s estimated from the full-width at half maximum (FWHM) noise level in the frequency region above 500 Hz in the axial and lateral directions were experimentally found to be as good as 1.7 and 1.1 nm, respectivel y. Furthermore, the lateral displacement resolution (0.2 nm) of a 10-m u m-diameter metal-coated microsphere was better than the axial displa cement resolution (0.9 nm) of a conventional flat mirror by more than a factor of four.