A new form of optical spectroscopy is demonstrated whereby the local s
cattering from the interaction between a nanometer size probe tip and
a sample is measured as a function of wavelength. We show images of vi
ruses and molecules at molecular spatial resolution and demonstrate ho
w the image contrast varies with wavelength. To first approximation, t
he contrast as a function of wavelength varies in the same way as the
local polarizability of the sample. (C) 1996 American Institute of Phy
sics.