Nonresonant Broadband Funneling of Light via Ultrasubwavelength Channels

Enhancing and funneling light efficiently through deep subwavelength apertures is essential in harnessing light-matter interaction. Thus far, this has been accomplished resonantly, by exciting the structural surface plasmons of perforated nanostructured metal films, a phenomenon known as extraordinary optical transmission. Here, we present a new paradigm structure which possesses all the capabilities of extraordinary optical transmission platforms, yet operates nonresonantly on a distinctly different mechanism. Our proposed platform demonstrates efficient ultrabroadband funneling of optical power confined in an area as small as $\sim (\frac{\lambda }{500}{)}^2$, where optical fields are enhanced, thus exhibiting functional possibilities beyond resonant platforms. We analyze the nonresonant mechanism underpinning such a phenomenon with a simple quasistatic picture, which shows excellent agreement with our numerical simulations.

Figure 1

The proposed paradigm structure consists of Au patterned film of thickness $d_{\mathrm{Au}}$ resting on a $d_{\mathrm{sub}}$ thick substrate. The unit cell of the underlying square lattice is magnified and depicted on the right panel (top view), with the associated geometric features designated.

Figure 2

Transmission (T) versus free space wavelength in microns. Five variations of the structure depicted in Fig. 1 are considered, all having $d_{\mathrm{Au}}=50\mathrm{nm}$, $d_{\mathrm{sub}}=500\mathrm{nm}$, and $w_y=200\mathrm{nm}$. ($w_{x1}$ and $w_{x2}$ for each case are given in the text. The adjacent icons depict the corresponding unit cell features to scale.)

Figure 3

Spatial field distribution of the electric field amplitude, normalized to an input field of $1\mathrm{V}/\mathrm{m}$, at the middle of the Au film [26] for the structures with (a) $w_{x1}=15\mathrm{nm}$, $w_{x2}=100\mathrm{nm}$; (b) $w_{x1}=50\mathrm{nm}$, $w_{x2}=100\mathrm{nm}$; (c) $w_{x1}=w_{x2}=15\mathrm{nm}$; and (d) $w_{x1}=w_{x2}=100\mathrm{nm}$. The remaining structural parameters are as in Fig. 2.

Figure 4

Plotted versus free space wavelength: (a) ratio of the electric field amplitudes in the small and large slits; (b) normalized electric field amplitude in the large slit with respect to source amplitude; and (c) normalized electric field amplitude in the small slit with respect to source amplitude. The dark (light) solid lines represent the result for the double-groove structure of Fig. 3a [Fig. 3b]. For comparison, the result for the 15 nm WG is shown in (c) with a long-dashed line. The dashed (dotted) lines represent the expected result from a quasistatic analysis in the long-wavelength limit for the DG structures of Fig. 3a [Fig. 3b]. The schematic on the right depicts a simple quasistatic picture of the charge response. The monitoring points are taken at the middle of the Au film. Their locations with respect to the structural unit cell, for the $E_S$ and $E_L$ fields, are also indicated.

Figure 5

(a) Spatial map of the Poynting vector, at the middle of the Au film [27], for the $10\mu \mathrm{m}$ incident wavelength for the DG structure of Fig. 3a (normalized to the incident Poynting vector). (b) Corresponding ratio of the total power funneled through the small-gap region, $F_{P_s}$, versus free space wavelength. (c) Comparison of the PCF versus free space wavelength for the DG structure (solid line) of (a) with the 15 nm WG result (dashed line).