Infrared Propagating Electromagnetic Surface Waves Excited by Induction

Propagating inhomogeneous electromagnetic waves called surface plasmon polaritons (SPPs) can be excited by free-space beams on corrugated conducting surfaces at resonance angles determined by corrugation period, permittivity, and optical frequency. SPPs are coupled to and co-propagate with surface charge displacements. Complete electrical isolation of individual conducting corrugations prevents the charge displacement necessary to sustain an SPP, such that excitation resonances of traveling SPPs are absent. However, SPPs can be excited via electric induction if a smooth conducting surface exists below and nearby the isolated conducting corrugations. The dependence of SPP excitation resonances on that separation is experimentally investigated here at long-wave infrared wavelengths. We find that excitation resonances for traveling SPPs broaden and disappear as the dielectric’s physical thickness is increased beyond ~1% of the free-space wavelength. The resonance line width increases with refractive index and optical thickness of the dielectric.