  Measurements of the deflection induced by thermal noise have been performed
on a rectangular atomic force microscope cantilever in air. The detection
method, based on polarization interferometry, can achieve a resolution of 1E-14
m/rtHz in the frequency range 1 kHz ? 800 kHz. The focused beam from the
interferometer probes the cantilever at different positions along its length
and the spatial modes' shapes are determined up to the fourth resonance,
without external excitation. Results are in good agreement with theoretically
expected behavior. From this analysis accurate determination of the elastic
constant of the cantilever is also achieved.
