Channel Plasmon-Polariton Guiding by Subwavelength Metal Grooves

We report on realization of channel plasmon-polariton (CPP) propagation along a subwavelength metal groove. Using imaging with a near-field microscope and end-fire coupling with a tapered fiber connected to a tunable laser at telecommunication wavelengths (1425–1620 nm), we demonstrate low-loss (propagation length $\sim 100\mu \mathrm{m}$) and well-confined (mode width $\cong 1.1\mu \mathrm{m}$) CPP guiding along a triangular $0.6\mu \mathrm{m}$-wide and $1\mu \mathrm{m}$-deep groove in gold. We develop a simple model based on the effective-index method that accounts for the main features of CPP guiding and provides a clear physical picture of this phenomenon.

Figure 1

The S-SPP (symmetric) mode effective index and its propagation length as a function of slit width $w$ for gold (at the light wavelength $\lambda =1.55\mu \mathrm{m}$) and silver ($\lambda =0.633\mu \mathrm{m}$). The inset shows the slit configuration and S-SPP magnetic field orientation along with the example of S-SPP field distribution across a $1\mu \mathrm{m}$-wide gold slit ($\lambda =1.55\mu \mathrm{m}$).

Figure 2

The effective indexes of CPP modes and their propagation lengths as a function of groove depth $d$ for different groove angles $\theta$. The inset shows the groove configuration and dominant orientation of CPP electric field [17].

Figure 3

Scanning electron microscope images of grooves (a) N1 and (b) N2. Optical microscope images of (c) the coupling arrangement (the tapered fiber is seen to the left and the illuminated groove to the right) and (d) the light propagation ($\lambda =1.55\mu \mathrm{m}$) along groove N2 (from left to right) at a distance of $\sim 200\mu \mathrm{m}$ from the in-coupling groove edge.

Figure 4

Grayscale (a) topographical and near-field optical images ($36\times 9\mu {\mathrm{m}}^2$) taken with groove N2 at $\lambda \cong$ (b) 1.44, (c) 1.5, and (d) $1.57\mu \mathrm{m}$. The CPP propagation is from left to right.

Figure 5

Cross sections of the topographical (stars) and near-field optical (filled and open circles) images of Fig. 4 [Figs. 4a, 4c, respectively] averaged along 10 lines along the propagation direction or perpendicular to it. The exponential dependence fitted by the least-square method to the signal dependence along the propagation direction is also shown.