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Optical properties of subwavelength metallic structures
Advances in fabrication technologies have produced artificial metallic nanostructures that are significantly smaller than the wavelegnth of light.
The figure shows a cross section of a metal film with an array of slits that are about 0.4 um in width. These structures display remarkable optical transmission properties. One would naively expect the transmission to be limited by diffraction and therefore very small. However, at certain wavelengths where surface waves on the metal are excited, the transmission becomes very effective. In some cases the transmission is even larger than the fractional area of the holes.
These nanostructures have potential applications in a variety of disciplines, including miniature photonic circuits, biochemical sensing, microscopy and high density optical data storage.
This work is supported by NSF-ECS-0621944.
Optical transmission through double-layer metallic subwavelength slit arrays
We measured transmission of infrared radiation through double-layer metallic grating structures. Each metal layer contains an array of subwavelength slits and supports transmission resonance in the absence of the other layer. The two metal layers are fabricated in close proximity to allow coupling of the evanescent field on individual layers. The transmission of the double layer is found to be surprisingly large at particular wavelengths, even when no direct line of sight exists through the structure as a result of the lateral shifts between the two layers. Numerical simulations using rigorous coupled wave analysis help to explain the strong dependence of the peak transmission on the lateral shift between the metal layers. Our experimental data and numerical simulations indicate that the spatial distribution of the electromagnetic fields at the surface of a single, isolated metal layer plays an important role in determining the resonant mode and optical transmission in the double-layer structure.
The transmission for TM polarization (electric field perpendicular to slits) through the single-layer slit array (dotted curve in Fig. 1a) peaks at a wavelength of ~3.1 um, with maximum transmission as large as 38% even though the fractional area of the slits is less than 20% and the wavelength is about 10 times larger than the slit widths. The solid curve in Fig. 1a shows the measured optical transmission for a double-layer sample (right-hand inset in Fig. 1(a)) that contains two metal layers in close proximity to each other with the two metal layers are displaced laterally by about half the grating period. Even though no direct line of sight exists from the input surface to the output surface, the peak optical transmission exceeds the single-layer case, with the transmission peak shifted to slightly longer wavelengths.
We found that transmission through the double-layer structure depends strongly on the lateral shifts of the pattern on the two metal layers. Figure 2 compares the transmission spectrum for three samples with lateral shifts of 0, 0.9, and 0.5 um, respectively. The filled circles in the inset of Fig. 2 show the dependence of the peak transmission (Tpeak) on the lateral shifts of the two metal layers for six different samples. When the lateral shift between the two layers is close to zero (perfect alignment) or is 1 um (half the grating pitch), the peak transmission is large. The smallest peak occurs at lateral shifts around 0.5 um, corresponding to a quarter of the grating period.
We perform numerical simulations by using rigorous coupled-wave analysis (RCWA) to investigate the origin of the large transmission through the double layers and its dependence on the lateral shift between the layers (Figure 1b). The location of the transmission peaks at ~3.1 um, and other details are in good agreement with our data. In addition, the RCWA calculations reproduced the dependence of the peak transmission as a function of the lateral shift between the two metal layers (dotted curve in the inset of Fig. 2).
Figure 3 shows the electromagnetic fields obtained from RCWA calculations. When the two metal layers are far apart, each layer has electromagnetic field distribution at resonance similar to Fig. 3a. As the two layers are brought closer, the evanescent fields of the two layers begin to couple. If the top layer is perfectly aligned (Fig. 3b) or misaligned (Fig. 3d) with the bottom layer, the positions with maximum magnetic and electric field on the two layers overlap, leading to strong coupling and high transmission. In contrast, when the top layer is misaligned with the bottom layer by one quarter of the period (Fig. 3c), the field maxima on the bottom layer now coincide with the field minima on the top layer. As a result, the evanescent field coupling between the two layers is weak and the transmission is low.
The strong transmission of light through bilayer subwavelength metallic slits opens up a new dimension in the design and operation of plasmonic devices.