NANO LETTERS
Refractive Index Sensing Using Quasi One-Dimensional Nanoslit Arrays
2009 Vol. 9, No. 7 2584-2588
Min Hyung Lee,† Hanwei Gao,‡ and Teri W. Odom*,†,‡ Department of Chemistry, Department of Materials Science and Engineering, Northwestern UniVersity, 2145 Sheridan Road, EVanston, Illinois 60208-3113 Received March 11, 2009; Revised Manuscript Received May 20, 2009
ABSTRACT This letter describes the optical properties of quasi one-dimensional (1D) Au nanoslit arrays on a microscale pitch. The transmission spectra exhibited multiple minima that were well characterized by 1D surface plasmon polariton Bloch wave modes. We found that all higher order modes showed a linear response to small changes of refractive index (RI) with sensitivities up to 560 nm per RI unit, which is comparable to that of two-dimensional nanohole arrays. By calibrating the RI response of the nanoslit arrays, we could use the multiple modes to determine the RI of unknown, nonabsorbing solutions.
Nanoscale metallic gratings are of intense interest because of optical phenomena such as enhanced optical transmission (EOT),1 beaming of light,2 and negative refraction.3 Surface plasmon polaritons (SPPs) generated under free-space photon illumination play a major role in these processes. Specifically, surface plasmons can propagate through subwavelength channels and contribute to EOT, while photons can only tunnel inefficiently through holes with lateral dimensions smaller than the half wavelength.4,5 There has also been direct evidence from near-field optical measurements to demonstrate that SPPs are critically involved in EOT through twodimensional (2D) nanohole arrays.6 Although one-dimensional (1D) nanoslit arrays have reduced dimensionality, the transmission mechanism can be more complicated. For example, 1D arrays of nanoslits can support waveguide modes, which can open additional channels for optical transmission,7-9 while circular subwavelength holes do not support photon propagation because of confinement in both lateral dimensions. Because of the coexistence of waveguide and SPP modes, 1D nanoslit arrays can suppress10 or enhance transmission7 depending on the geometry, the materials, and the excitation wavelength. Previously, we reported that transmission through 2D Au arrays of nanoholes on a microscale pitch showed high-order 2D SPP-Bloch wave (SPP-BW) modes. These modes form when SPPs resonantly couple with light whose in-plane momentum matches the Bragg coupling condition for periodic gratings.11 SPP-BW resonances can be tuned by changing the periodicity of the grating, the permittivity of the metal, or the surrounding dielectric media. Here we * To whom correspondence should be addressed. E-mail: todom@ northwestern.edu. † Department of Chemistry. ‡ Department of Materials Science and Engineering. 10.1021/nl900773m CCC: $40.75 Published on Web 06/03/2009
2009 American Chemical Society
investigate quasi-1D nanoslit arrays with a similar microscale pitch and use these plasmonic substrates to determine the refractive index (RI) of nonabsorbing solutions. Although the patterned Au films were perforated with 2D arrays composed of rows of finite-length slits, the light transmission in the angle-resolved spectra was characterized by 1D SPPBW modes because the vertical separation between adjacent rows was greater than the surface plasmon propagation length; hence, we define these slit arrays as quasi-1D. Unlike waveguide modes that were reported in infinitely long nanoslits,7 we did not observe any effects of waveguide modes in our nanoslit arrays with finite lengths. Tuning and tailoring SPP resonances are critical for designing sensitive, label-free chemical/biological sensors.12-14 Figure 1 shows a scanning electron microscopy (SEM) image of quasi-1D nanoslit arrays in 150 nm thick Au films generated by a nanofabrication technique called PEEL (a combination of Phase-shifting photolithography, Etching, Electron-beam deposition, and Lift-off of the film).15 Waferscale arrays of nanoslits were created from photoresist templates with high aspect-ratio structures. Such patterns were made by two sequential exposures through a poly(dimethylsiloxane) mask of 2 µm bas-relief lines spaced by 2 µm. The second exposure was performed after the line mask was rotated 5°.11 The resist features were then transferred using PEEL into Au films to produce nanoslits with dimensions of 90 nm × 2000 nm (aspect ratio >1:20). The primitive lattice spacing a0 of the slits was 4 µm along the x-direction with a unit cell consisting of a two-slit basis (neighboring slits separated by 1.8 µm, Figure 1A); the slits were separated by 9.5 µm along the y-direction. Angle-dependent transmission spectra of the nanoslit arrays were measured using glass substrates (sub) and
Figure 1. (A) SEM images of quasi-1D nanoslit arrays in 150 nm thick Au films (a0 depicts the unit cell). (B) Optical micrograph of a large-area (3 cm × 2 cm) Au film perforated with nanoslit arrays.
superstrates (sup) with different refractive indices. The incident excitation angle θ ranged from 0 to 80°, and the degree step size was 0.5°. Collimated white light with a divergence angle
