Guiding, Focusing, and Sensing on the Subwavelength Scale Using Metallic Wire Arrays

We show that tapered arrays of thin metallic wires can manipulate electromagnetic fields on the subwavelength spatial scale. Two types of nanoscale imaging applications using terahertz and midinfrared waves are enabled: image magnification and radiation focusing. First, the tapered wire array acts as a multipixel TEM endoscope by capturing an electromagnetic field profile created by deeply subwavelength objects at the endoscope’s tip and magnifying it for observation. Second, the image of a large mask at the endoscope’s base is projected onto a much smaller image at the tip.

Figure 1

Schematics of two subwavelength endoscopes based on a $3\times 3$ array of metal wires embedded in a straight (left) or tapered (right) metal-coated fiber. Both endoscopes are terminated by square single-mode rectangular waveguides on both ends.

Figure 2

Dispersion curves for two types of electromagnetic waves supported by an infinite square array of metallic wires (diameter $w=\lambda /15$, period $d=\lambda /10$, vacuum wavelength $\lambda \equiv 2\pi c/\omega$) imbedded in a dielectric with ${ϵ}_d=2.25$. Blue lines: wires are PECs. Red lines: gold wires with ${ϵ}_{\mathrm{Au}}=-916+228i$ corresponding to $\lambda =5\mu \mathrm{m}$.

Figure 3

Transport of the image of a metal sphere (diameter $D=\lambda /10$) by a straight subwavelength endoscope shown in Fig. 1(left) consisting of a $3\times 3$ wire array placed inside a square subwavelength metallic waveguide of the width $W=\lambda /3$. The sphere’s center is at $z_{\mathrm{obj}}=-0.7D$, $x=x_{\mathrm{obj}}$, $y=y_{\mathrm{obj}}$. Shown are the color-coded $|{\overrightarrow{E}}_{\perp }{|}^2$ profiles in the imaging plane $z=4\lambda /3$ for (a) ($x_{\mathrm{obj}}=-d/2$, $y_{\mathrm{obj}}=0$), and (b) ($x_{\mathrm{obj}}=d/2$, $y_{\mathrm{obj}}=d/2$). Arrows represent the electric field.

Figure 4

Applications of a tapered endoscope from Fig. 1 (right): image magnification and demagnification by a factor 5. (a) Image magnification: image of a small metal sphere (diameter $D_{\mathrm{small}}=\lambda /25$) placed just above the tip at ($x_{\mathrm{obj}}=-D_{\mathrm{small}}/2$, $y_{\mathrm{obj}}=0$) is transported to the base plane. (b) Image demagnification: image of a larger metal sphere (diameter $D_{\mathrm{large}}=\lambda /5$) placed just below the base at ($x_{\mathrm{obj}}=D_{\mathrm{large}}/2$, $y_{\mathrm{obj}}=0$) is transported to the tip.