  The guided modes of sub-wavelength diameter air-clad optical fibers exhibit a
pronounced evanescent field. The absorption of particles on the fiber surface
is therefore readily detected via the fiber transmission. We show that the
resulting absorption for a given surface coverage can be orders of magnitude
higher than for conventional surface spectroscopy. As a demonstration, we
present measurements on sub-monolayers of 3,4,9,10-perylene-tetracarboxylic
dianhydride (PTCDA) molecules at ambient conditions, revealing the
agglomeration dynamics on a second to minutes timescale.
