Guiding and Focusing of Electromagnetic Fields with Wedge Plasmon Polaritons

We study theoretically electromagnetic modes guided by metallic wedges at telecom wavelengths. These modes are found to exhibit superior confinement while showing similar propagation loss as compared to other subwavelength guiding configurations. It is also shown that mode focusing can be realized by gradual modification of the wedge geometry along the mode propagation direction.

Figure 1

(a) WPP dispersion relation. Black thick line, SPP mode on a flat surface; red or gray line (squares), $\mathrm{WPP}(\infty )$ mode supported by a nontruncated wedge. Insets: time-averaged electric field of $\mathrm{WPP}(\infty )$ mode at wavelengths $\lambda =0.6\mu \mathrm{m}$ and $\lambda =1.6\mu \mathrm{m}$. The lateral size of the insets is $0.5\mu \mathrm{m}$. (b) Modal size (red or gray dashed line) and propagation length (black solid line) of $\mathrm{WPP}(\infty )$ mode as a function of $\lambda$. Inset: diagram of the truncated wedge.

Figure 2

Transverse electric field of (a) CPP mode, and (b) WPP mode, both at $\lambda =1.5\mu \mathrm{m}$. The geometry of both structures is identical (see main text) exchanging the metallic and vacuum regions. The panel’s lateral size is $2\mu \mathrm{m}$.

Figure 3

WPP modal characteristics as a function of wedge height or angle. In (a) and (b) the height is varied and the wedge angle is constant, $\varphi =20°$. In (c) and (d) the wedge angle is varied (the wedge is not truncated). The radius of curvature is $r=10\mathrm{nm}$ in all cases. (a) Solid line, effective index of WPP mode. (b) Red or gray dashed line, modal size of WPP; solid line, propagation length of WPP. (c) Solid line, effective index of $\mathrm{WPP}(\infty )$ mode. (d) Red or gray dashed line, modal size of $\mathrm{WPP}(\infty )$; solid line, propagation length of $\mathrm{WPP}(\infty )$.

Figure 4

Electric field amplitude in the $\mathrm{WPP}\to \mathrm{SPP}$ transition (geometries are detailed in the main text). (a) and (b) longitudinal cross sections for structure II [dashed line in (a) is the location of longitudinal section (b)]. (c), (d), (e), and (f) transverse cross sections ($XY$ plane, at various $z$ coordinates) for structure II [dashed lines in (b) show the position of these transverse cross sections]. (g) and (h) longitudinal cross sections for structure III. The Cartesian axes are shown in Fig. 1b. The wavelength is $\lambda =1.5\mu \mathrm{m}$. The size of all panels along $X$ and $Y$ directions is $1.4\mu \mathrm{m}$.

Figure 5

Focusing of the $\mathrm{WPP}(\infty )$ modal field as the wedge angle $\varphi$ is slowly decreased from $\varphi =180°$ (SPP mode on a flat surface) to $\varphi =20°$. The panels display the transverse electric field at a wavelength of $1.5\mu \mathrm{m}$. The lateral size of the panels is $2\mu \mathrm{m}$. The radius of curvature of the tip is $r=10\mathrm{nm}$. The color scale or shading of the various panels is not the same.