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We report on drastic changes in the near-field spectrum as it evolves into the far field in periodically corrugated metallic nanoslit arrays. The far-field spectral minimum is located exactly at the near-field spectral maximum, where a quasimonochromatic standing wave pattern is observed in the near field. These results are in excellent agreement with the equipartition of diffraction orders recently proposed [K. G. Lee and Q-Han Park, Phys. Rev. Lett. 95, 103902 (2005)].
Characterizing strongly localized electromagnetic fields on a corrugated metal surface has become an important issue not only from a fundamental science point of view but also from the technological side. Diverse areas such as enhanced transmission,1 negative index of refraction materials,2 superlensing,3 biosensing,4 near-field fabrication,5 and optical nanoantennas6 require a detailed knowledge of both spatial and spectral near-field profiles.7
In atomic, excitonic8 or single plasmonic particles,9 their electromagnetic near-field profile gives valuable information on the homogeneous spectral linewidth as well as on the localization of wavefunctions. In these systems, the near-field profiles differ mainly in intensity in comparison to the far-field, but not in their spectral shapes—at least not in homogeneously broadened systems. In periodic arrays of nano-holes,1,10 nanoslits11 and nanoparticle,12 however, the periodicity provides another length-scale different from the particle size, which can decide whether a particular wavelength component is mainly evanescent or propagating. In using plasmonic crystals in applications such as plasmonic waveguiding,13 biosensing,14 and surface enhanced Raman scattering, any spectroscopic difference between near and far field has important technological implications.
In this letter, we report on the near-to-far field spectral evolution in a one-dimensional plasmonic crystal. We experimentally decompose the near-field spectrum of this system into its evanescent and propagating components. By mapping near-field spatial profiles with subwavelength resolution, we confirm that at the peak of the near-field intensity, all of the diffraction orders are suppressed to almost zero except for the first order, which leads to the formation of a standing wave pattern. Such a behavior has recently been predicted theoretically by Lee and Park in terms of the equipartition of diffraction orders.15 Our experiments show that the near-field spectral information is crucial in determining surface electromagnetic field profile, while resolving the controversial issue of the negative role of surface plasmons.16
A nanoslit array is prepared by electron beam lithography on a 78nm
thick gold film thermally evaporated on a 320μm thick sapphire substrate. The periodicity of the array is measured by Bragg diffraction and also by scanning electron microscopy [inset of Fig. 1(a)], and is found to be d=761nm. The slit width a is approximately 100nm. Measurements of the near-to-far evolution of the transmission spectra are performed by employing a collection mode near-field scanning optical microscope using a metal-coated aperture probe with a 100nm aperture diameter (Nanonics Imaging, Ltd.). In these measurements, the tip-to-sample distance z is varied between 0–100μm and the incident wavelength is scanned between 740 and 840nm using a tunable Ti: sapphire laser [Fig. 1(a)]. The far-field spectrum is recorded using a tungsten lamp [Fig. 1(b)] for comparison with the near-field experiments. A clear transmission dip is seen at the surface plasmon polariton (SPP) resonance, which is determined by the period d as λsp=dεm∕(εm+1)−−−−−−−−−−−√≈780nm. Here εm denotes the gold dielectric function.17 This negative role of surface plasmons on the far-field transmission has been predicted numerically16 and explained theoretically in terms of an “antiresonance”16 or equipartition of diffraction orders.15 Evidently, the effect of the slits is to couple the incident light field to different evanescent SPP orders. For infinitely small slits, the coupling efficiency is the same for all orders, as it is essentially given by the overlap integral of the incident and SPP fields evaluated over the slit width. This equality of the coupling efficiencies has recently been termed equipartition of diffraction orders.15 The transmission and reflection then result from this constant coupling efficiency for each diffraction order, multiplied by a suitable resonance factor. Therefore if one SPP mode is excited on resonance, the transmission and the reflection of all other modes, including the zero order mode propagating into the far field, must vanish.13 In our near-to-far-field measurement, this predicted behavior shows up clearly as we measure the spectrum by varying the tip-to-surface distance.
