LETTER pubs.acs.org/NanoLett
Anomalous Light Transmission from Plasmonic-Capped Nanoapertures Kohei Imura,†,‡ Kosei Ueno,‡,§ Hiroaki Misawa,§ and Hiromi Okamoto*,|| †
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Department of Chemistry and Biochemistry, School of Advanced Science and Engineering, Waseda University, Okubo, Shinjuku, Tokyo 169-8555, Japan ‡ PRESTO, Japan Science and Technology Agency, Honcho Kawaguchi, Saitama 332-0012, Japan § Research Institute for Electronic Science, Hokkaido University, Kita-Ku, Sapporo, Hokkaido 001-0021, Japan Institute for Molecular Science and The Graduate University for Advanced Studies, Myodaiji, Okazaki 444-8585, Japan
bS Supporting Information ABSTRACT: We report an anomalous light transmission phenomenon for a nanoaperture on an opaque screen when the aperture is covered with an opaque cap. In conventional optics, light transmission must decrease when the aperture is capped. However, we found that light transmission is enhanced when the nanodisk is in close proximity to the aperture at a wavelength close to the plasmon resonance. This effect even occurs when the disk is larger than the aperture. KEYWORDS: Plasmonic nanostructure, nanodisk, nanoaperture, scanning near-field optical microscope, enhanced light transmission
called surface plasmon polaritons, or simply plasmons).4-8 Interactions between the component materials through the electromagnetic field can be particularly pronounced at frequencies near the plasmon resonances.9-11 As a consequence, capped apertures made of metal nanostructures may show peculiar light propagating characteristics that are not predicted in conventional macroscopic optics. Optical transmission of a single subwavelength aperture has been examined over the past decade,12 and transmission enhancement due to the plasmon excitation in the aperture has been reported.13-16 In the present study, we found unique optical characteristics of nanoscale metallic-capped apertures through spectral measurements of planar gold nanodisks by an aperture-type near-field optical microscope. The apertured probe tip utilized for the near-field microscope is essentially a small aperture opened on a metallic screen, and the configuration is that of a capped aperture when the tip is positioned above a gold nanodisk in close proximity. In this arrangement, the nearfield probe tip and the nanodisk correspond, respectively, to the aperture and to the capping obstacle. We found for this system an anomalous light transmission enhancement phenomenon at wavelengths near the plasmon resonance of the gold nanodisk. The origin of the observed anomalous transmission and the effects of plasmon resonances on the phenomenon will be analyzed based on model calculations.
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n optical systems that consist of macroscopic components (i.e., dimensions much larger than the wavelength of light), rays of light propagate along straight lines through space.1 When the optical systems become reduced in size, light cannot be rigorously treated as a ray propagating along straight lines; for instance, this effect is seen in the diffraction of light.1,2 However, it is still commonly accepted that the propagating light intensity will be reduced when the optical path is covered with an obstacle that absorbs or reflects the light. What happens, then, if the optical system becomes further reduced in size to a scale smaller than the wavelength? In the present article, we report a phenomenon wherein the propagating light intensity actually increases when an obstacle covers the optical path in an optical system composed of metal nanostructures. Our optical system treats the transmission of light through a circular aperture in an opaque metal screen. In a macroscopic system, no light can propagate through the aperture (or at least the intensity will decrease) if the aperture is covered with a closeby opaque obstacle of larger diameter than the aperture. Such an optical configuration, where an aperture is geometrically covered with an opaque obstacle, is called hereafter a “capped aperture”. When the dimensions of the components involved in the system are close to or smaller than the wavelength of light, the system in general does not behave as expected from the geometrical structure. The system shows unique properties particularly when the optical components interact with one another through electromagnetic fields.3 Metal nanostructures create locally enhanced optical fields arising from the collective oscillations of conduction electrons (often r 2011 American Chemical Society
Received: September 27, 2010 Revised: November 19, 2010 Published: January 31, 2011 960
dx.doi.org/10.1021/nl103408h | Nano Lett. 2011, 11, 960–965
Nano Letters
LETTER
Figure 2. (a) A schematic illustration of the geometry for the near-field transmission measurement. (b) Near-field transmission spectrum of a single gold nanodisk (diameter of 150 nm and height of 35 nm) taken at the center of the disk.
(I0 - I)/I0 versus wavelength. The spot size for the far-field measurement was several tens of μm. Figure 1a shows far-field extinction spectra observed for gold circular disks. Extinction peaks are found in the region of wavelengths longer than 600 nm. The peak wavelength depends on the diameter of the disk and shifts to longer wavelengths approximately linearly with an increase in the diameter of the disk. The peak shift versus the disk diameter can be understood in a similar way to that for gold nanorods.18 The extinction peak of the rod shifts toward longer wavelengths with an increase of the aspect ratio (= length/diameter). Theoretically, the peak shift is correlated to the depolarization factor of the scatterer, which is determined by the aspect ratio.18,19 As the aspect ratio gets higher, the depolarization factor becomes smaller and the resonance wavelength becomes longer. For circular disks, the aspect ratio is given by the disk diameter divided by the disk height. The observed peak shift is again related to the depolarization factor, which becomes smaller with an increase of the disk diameter and the peak wavelength becomes longer as a consequence. Another prominent feature in Figure 1a is the strong dependency of the extinction peak height on the disk diameter. The dependency may be correlated to the volume of the disk. Mie scattering theory for spherical particles predicts that the absorption and scattering intensities are proportional to, respectively, the particle volume and the square of the particle volume.19 If we approximate a gold disk as an oblate spheroid, the optical properties of the disk can be simulated on the basis of the extended Gans theory20-22 where the shape of the particle and the damping factor of the plasmon resonances are taken into account. The polarizability of the disk is given by the following equation
Figure 1. Far-field extinction spectra of gold nanodisks (diameters of 50, 100, 150, and 200 nm and height of 35 nm). (a) Experiment. (b) Simulation based on the extended Gans theory (eq 1).
The gold circular disks (diameters of 50-200 nm and thickness of 35 nm) were fabricated on a coverslip by the electron beam lithographic technique (number density of disks: 1 μm-2).17 The geometrical dimensions of the disks were verified by topography measurements with a scanning near-field optical microscope (SNOM) and/or by a scanning electron microscope (SEM). The aperture SNOM was operated under ambient conditions. The near-field optical fiber probe was etched and gold-coated (thickness: ca. 300-400 nm), and then an aperture was created at the tip (purchased from JASCO Corp.). The diameter of the probe aperture was determined to be ca. 100 nm by a SEM image. The near-field probe was maintained in the vicinity of the sample surface by a shear-force feedback mechanism. For transmission measurements, a Xe discharge lamp was used as a light source, and the sample was illuminated through the aperture of the nearfield probe. Photons transmitted through the sample were collected by an objective lens and detected by a polychromatorCCD system. The transmission intensity was measured at each point over the scanned area, and the near-field transmission spectrum was obtained as the ratio of the measured transmitted light intensity at the center of the sample disk, I, and on the bare substrate, I0. The far-field extinction spectrum was obtained by measuring the transmitted light intensity over the domain of the metal nanostructures, I, and over the bare substrate, I0, by plotting
RLSP ¼
961
V εm k2 k3 V -i V lþ ε-εm 2πd 6π
ð1Þ
dx.doi.org/10.1021/nl103408h |Nano Lett. 2011, 11, 960–965
Nano Letters
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Figure 3. Electric fields simulated by the finite-difference time-domain (FDTD) method for the capped aperture system. Upper part: schematic illustrations of the geometries used in the calculations. Bottom part: cross-sectional images of electric-field amplitude distributions calculated (a) without a disk, (b) with a gold disk not in direct contact, and (c) with a gold disk in direct contact. The diameter and the height of the disk are 160 and 40 nm, respectively. The aperture diameter is 50 nm. The regions surrounded by dashed lines are the metallic (gold) parts. The incident wavelength (coming from the top) is 800 nm. Scale bars: 300 nm.
where l is the depolarization factor, ε and εm denote the dielectric constants of gold and the surrounding medium, respectively, and k is the wavevector of the incident light. The variables V and d represent the volume and the diameter of the disk, respectively. The second and third terms in the denominator of eq 1 account for the dynamic depolarization and volume damping,23 respectively. The extinction spectrum of the disk is given by the imaginary part of the polarizability. Figure 1b shows the simulated spectra for the disks. The peak positions, peak widths, and the relative peak intensities of the extinction bands are well reproduced semiquantitatively by the simulation. We found that the volume damping effect is essential to account for the observed bandwidth. The resonance peaks are attributed to the fundamental dipolar plasmon modes based on the analysis from the spectral simulation as well as the near-field images of the plasmon modes. We investigated the light transmission through the aperture of the near-field probe near the gold nanodisks. Figure 2b shows a typical near-field transmission spectrum observed for a single gold nanodisk (diameter of 150 nm and height of 35 nm). The arrangement of the sample and the probe is schematically drawn in Figure 2a. Because the aperture of the probe is smaller than the disk and the distance between the aperture and the disk surfaces is small enough (
