Optical measurement of picometer displacements of transparent microscopic objects (original) (raw)

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Author Affiliations

Winfried Denk1and Watt W. Webb2

1Winfried Denk now is with the IBM Research Laboratory, Säumerstr. 4, CH 8803- Switzerland

2School of Applied and Engineering Physics, Clark Hall, Cornell University, Ithaca, New York 14853, where this work was performed. USA

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Abstract

An optical laser differential interferometer, based on a modified differential interference contrast microscope, has been developed to measure the thermal motion of microscopic protrusions (stereocilia) that are the sites of mechanoelectrical transduction in auditory hair cells. The measurement sensitivity was limited at high frequencies mainly by shot noise, at intermediate frequencies by acoustic interference, and at low frequencies by thermal drift. The power spectral density of the instrumental noise was found to be as low as 1 pm/Hz in the shot noise regime. We could, thus, measure the Brownian motion of hair bundles over the frequency range from 1 Hz–100 kHz. Experimental data that test and demonstrate the sensitivity and spatial discrimination of the instrument were found to be in agreement with theoretical estimates.

© 1990 Optical Society of America

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