Compact optical microcavity structures for enhancement of absorption and transmission cross sections of subwavelength plasmonic devices

Resonant subwavelength plasmonic apertures can efficiently concentrate light into deep subwavelength regions, and therefore significantly enhance the optical transmission through the apertures, or the absorption in the apertures. In addition, grating structures, consisting of periodic arrays of grooves patterned on the metal film on both sides of a metal aperture, are commonly used to enhance the coupling of incident light into the aperture through the excitation of surface plasmons. For efficient surface plasmon excitation, however, the period of the grating has to be equal to the surface plasmon wavelength, and several grating periods are required. Thus, such structures need to be several microns long. In this paper, we show that a compact submicron structure consisting of multiple optical microcavities on both the entrance and exit sides of a subwavelength plasmonic slit filled with an absorbing material can greatly enhance the absorption cross section of the slit. We show that such microcavity structures can increase both the coupling of incident light into the slit mode, as well as the resonant absorption enhancement in the slit by fine tuning the reflection coefficients at the two sides of the slit. An optimized submicron structure consisting of two microcavities on each of the entrance and exit sides of the slit leads to similar to 9.3 times absorption enhancement compared to an optimized slit without microcavities at the optical communication wavelength of 1.55 microns. Finally, we show that multiple microcavity structures can also be used to greatly enhance the coupling of free-space radiation into subwavelength plasmonic waveguides.