Microscale Arrays of Nanoscale Holes

The observation of enhanced optical transmission (EOT) through metal subwavelength hole arrays has generated significant interest in nanophotonics and contributed insight into the interaction between light and materials at the nanoscale. These fundamental studies have demonstrated that rationally engineered surfaces can manipulate and focus light well below the diffraction limit and have led to emerging applications in photonics and chemical and biological analysis. 9] Metallic subwavelength hole arrays exhibit EOT because surface-plasmon polaritons (SPPs) assist in coupling incident light through the holes; the efficiency of this coupling depends on several factors, such as the size of the holes and spacing of the arrays, the thickness of the films, the material composition of the film, and the refractive index of the surrounding medium. Transmission through subwavelength arrays of rectangular and ovalshaped holes depends on the polarization of the incident light; under specific polarization directions, the transmissivity can be higher for holes with higher aspect ratios. Although most work has focused on subwavelength hole arrays, where both the size of the holes and the spacing of the array are subwavelength in scale, nanoscale holes patterned with microscale spacings (microscale arrays) are becoming important for understanding in detail the optical properties of nanostructured metallic surfaces. Here we report how microscale arrays of nanoholes can exhibit narrow resonance features in their transmission spectra with a full width at half-maximum (FWHM) of <15 nm. These structures—the hole–hole spacing of which is greater than the wavelength of incident light—show unique spectral characteristics compared to subwavelength hole arrays because of high-order Bragg coupling. Angle-resolved transmission spectra displayed in the form of dispersion diagrams confirmed the complexity of the zero-order transmission spectra and were in excellent agreement with predicted models. Furthermore, anisotropic nanoholes patterned into different lattice geometries displayed different transmission spectra and striking, tunable polarization-dependent colors. Singlehole spectroscopy revealed that the polarization-dependent transmittance was dominated by SPP rather than localized surface plasmon (LSP) modes for anisotropic holes. Figure 1 depicts microscale arrays of nanoholes with different shapes in free-standing 170-nm-thick gold films created by a nanofabrication technique called PEEL (a combination of Phase-shifting photolithography (PSP), Etching, Electron-beam deposition, and Lift-off of the film). As reported previously, circular nanoholes were generated starting from circular photoresist posts made by two sequential exposures through a poly(dimethylsiloxane) (PDMS) mask of bas-relief lines 2 mm wide spaced by 2 mm; the second exposure was carried out after the line mask was rotated an angle f=908. Anisotropic holes with high aspect ratios can be generated by performing the second exposure of the PSP step with f=458 or 158 to produce photoresist posts shaped like ellipses or high-aspect-ratio structures, respectively. The posts were then transferred into gold films perforated with arrays of anisotropic holes by carrying out the remaining steps of the PEEL procedure. This approach can produce holes with sharp corners, features that are difficult to construct using focused ion beam (FIB) milling, the most common method to fabricate subwavelength hole arrays. Circular holes with a 250-nm diameter were constructed in a square array (f=908, Figure 1a); 145-nm=500-nm elliptical holes were created in a diamond array (f=458, aspect ratio of 3.5, Figure 1b); and 90-nm=950-nm slitlike holes were fabricated in a sharp-diamond array (f=158, aspect ratio of 10.5, Figure 1c). The shortest distance between holes in these different array structures for any hole shape was approximately 2 mm (defined as 2-mm arrays), and the fractional areas occupied by the holes were 1.18%, 0.87%, and 0.24% for circular, elliptical, and slitlike hole arrays, respectively. As a result of the lithographic procedure, arrays of elliptical and slitlike nanoholes exhibited two slightly different shapes (Figure 1b,c, insets). After drying on glass substrates, gold films perforated with arrays of nanoholes were remarkably smooth and highly uniform over areas greater than one square inch (Figure 1d). To delineate between contributions of individual holes and hole–hole interactions in the transmission, we analyzed the transmittance of single nanoholes on a spacing of 25 mm (defined as 25-mm arrays) and arrays of nanoholes patterned on a smaller spacing of approximately 2 mm (Figure 2). Holes separated by 25 mm can be considered isolated since this distance exceeds the SPP propagation length at the gold/air interface. We performed single-hole spectroscopy by collecting the transmission from a single hole (area focused onto spectrometer 2 mm) under white-light illumi[*] M. H. Lee, J. Henzie, Prof. T. W. Odom Department of Chemistry Northwestern University 2145 Sheridan Road, Evanston, IL 60208 (USA) Fax: (+1)847-491-7713 E-mail: todom@northwestern.edu