Surface-Plasmon-Assisted Guiding of Broadband Slow and Subwavelength Light in Air

A class of axially uniform waveguides is introduced, employing a new mechanism to guide light inside a low-index dielectric material without the use of photonic band gap, and simultaneously exhibiting subwavelength modal size and very slow group velocity over an unusually large frequency bandwidth. Their basis is the presence of plasmonic modes on the interfaces between dielectric regions and the flat unpatterned surface of a bulk metallic substrate. These novel waveguides allow for easy broadband coupling and exhibit absorption losses limited only by the intrinsic loss of the metal.

Figure 1

$\omega \mathrm{\text{−}}k$ diagrams (solid curves) for conventional layered surface-plasmon structures (insets $A–D$) with ${ϵ}_{\mathrm{hi}}=4$ and ${ϵ}_{\mathrm{low}}=1$ (air). Layer thicknesses $d/{\lambda }_p=0.015$, 0.02, and 0.025 are used for $C$ and $D$ ($\mathrm{\text{solid}}+\mathrm{\text{dotted}}$ curves). The light lines $\omega /{\omega }_p=\sqrt{ϵ}k/k_p$ (“vertical” dashed lines) and the cutoff frequencies ${\omega }_c(ϵ)/{\omega }_p=1/\sqrt{1+ϵ}$ (horizontal dashed lines) are shown.

Figure 2

$\omega \mathrm{\text{−}}\beta$ diagrams (red curves calculated via the effective-index method) for SP-based waveguide structures [insets (i)] with $w/{\lambda }_p=0.25$ and $d/{\lambda }_p=0.02$. The SP modes $A–D$ (from Fig. 1) of the individual vertical slices (thick solid black curves) and the ${ϵ}_{\mathrm{hi}}$ light line (dashed black curve) limit the regions where fully confined modes can exist. The $H_x$ component of the first two modes at (a) $\omega /{\omega }_p=0.705$ and (b) $\omega /{\omega }_p=0.65$ is shown [insets (ii) and (iii) calculated via a finite-difference frequency-domain eigenmode solver].

Figure 3

$\omega \mathrm{\text{−}}\beta$ diagrams (red curves) for SP-based waveguide structures [insets (i)] with $w/{\lambda }_p=0.25$ and $d/{\lambda }_p=0.02$ and the $H_x$ component of their first two modes at (a) $\omega /{\omega }_p=0.444$ and (b) $\omega /{\omega }_p=0.47$ [insets (ii) and (iii)].