Measurements of modal symmetry in subwavelength plasmonic slot waveguides

We excite a guided plasmonic mode in slot waveguides of subwavelength width. With a phase- and polarization-sensitive near-field microscope, we measure the electric field of the mode for a range of slot widths from 40 to 120 nm. The field is experimentally found to be antisymmetric across the
slot gap. Numerical calculations confirm this symmetry. Calculations also show a confinement of the field to a lateral size ∼10 times smaller than the free-space wavelength.