Microscopic absorption measurement with enhanced sensitivity by using focused surface plasmons

We propose a technique to enhance absorbance of a sample with ultra small volume by using surface plasmons excited in a microscopic region on a flat metal surface. In this technique, the absorbance is measured from the variation of reflected intensity at around the propagating constant of excited surface plasmons in a spatial frequency spectrum. We demonstrate that a wider and deeper dip appears in the spatial frequency spectrum by placing an absorbing droplet sample instead of a non-absorbing sample. An experimental result employing methylene-blue water solution as a sample reveals absorbance enhancement by a factor of 19 from a measurement by attenuated total reflection method.