SURFACE-PLASMON COUPLED X-APERTURES FOR OPTICAL FIELD ENHANCEMENT AND LOCALIZATION
Maxim Abashin, Amit Agrawal, and Henri Lezec
It was recently demonstrated that in the mid-IR spectral region (λ ~ 8.8 µm) a cross-shaped aperture (X-antenna) in a metal film can act as a resonant optical antenna producing an ultra-enhanced near-field spot . Here we show that by proper optimization of geometrical parameters, X-antennas can be adapted for operation at visible and near-IR wavelengths – spectral regions of key importance for surface-enhanced Raman scattering (SERS) analysis and bio-sensing. Using finite-difference time-domain (FDTD) simulations, an X-antenna was designed for maximum resonant response in the near infrared. The basic geometry consists of a cross-shaped aperture formed by two perpendicular 200-nm-long slits in a 150-nm-thick Ag film. A slit width w = 20 nm yields resonant wavelengths lR1 = 610 nm and lR2 = 870 nm along with a resonant intensity (|E|2) enhancement factor gR ~ 66.
Taking subwavelength apertures in noble-metal films, such as circular holes or slits, and arranging them into periodic arrays has been shown to significantly enhance both local field intensity and transmission at specific frequencies . Following this general approach, we further increase the field-enhancement factor of individual X-antennae by arranging them into a periodic square array in a continuous metal film. Using FDTD simulations, the array periodicity P is optimized to achieve phase-matching between the local resonance of each aperture and Surface Plasmons Polaritons (SPPs) travelling on the array. An optimized value P = 500nm yields enhancement of gR ~ 1200 and nanoscale confinement (~l0/40) of the intensity at the facing apexes of the antenna.
We fabricate a periodic X-antenna array with above-described parameters using Focused Ion Beam (FIB) milling. Two reference arrays were also fabricated on the same sample: 1) an array of randomly positioned X-antenna and 2) a periodic array (P = 500 nm) of 200-nm-diameter circular holes. By averaging-out of coherent interactions between apertures, random positioning of the X-apertures enables high signal-to-noise determination of the spectral response of individual antennae. The transmission spectrum for randomly-distributed X-antennas displays evidence of a broad resonance centered at λ = 785 nm. In contrast, the spectrum for the square lattice of X-antennae displays prominent features similar in shape to those of the square lattice of circular holes (though significantly more intense when normalized to aperture area). A strong transmission peak appears at a wavelength λ0 = 820 nm, of intensity approximately 8 times larger than that of the random X-aperture array.
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