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Invisible plasmonic meta-materials through impedance matching to vacuum

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Abstract

We report on perfect transmission in two-dimensional plasmonic matamaterials in the terahertz frequency range, in which zeroth order transmittance becomes essentially unity near specific resonance frequencies. Perfect transmission may occur when the plasmonic metamaterials are perfectly impedance matched to vacuum, which is equivalent to designing an effective dielectric constant around εr=-2 . When the effective dielectric constant of the metamaterial is tuned towards εr and the hole coverage is larger than 0.2, strong evanescent field builds up in the near field, making perfect transmission possible.

©2005 Optical Society of America

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Figures (4)

Fig. 1.
Fig. 1. (a) Schematic view of plasmonic meta-materials with periodic arrays of square holes and a SEM image. (b) An effective medium converted by an effective surface impedance in the structure displayed in (a). (c) At a specific frequency, the effective surface impedance becomes equal to the vacuum impedance and this medium has a perfect transmission.
Fig. 2.
Fig. 2. (a) Schematics of the Femtosecond machining system used for manufacturing our samples. (b) Schematics of our terahertz transmission experiments. (c) Time trace of the incident terahertz beam (black line) and the transmitted beam (red line) for a typical sample. (d) Fourier transform of (c).
Fig. 3.
Fig. 3. (a) Transmission spectra of the transmitted field amplitude for four samples with different sample coverages of 0.1, 0.17, 0.2 and 0.25, respectively, from top to bottom. At the bottom the sample with the coverage 0.25 is shown. (b) and (c) THz time traces for samples with sample coverage of 0.1 and 0.25 respectively. The source signals (top) are quasi-monochromatic THz waves at 0.66 THz tailored by the pulse shaping.
Fig. 4.
Fig. 4. (a). The effective dielectric constant plotted versus the frequency for coverages of 0.25 (red line), 0.2 (green line), 0.17 (orange line), and 0.1 (blue line). (b) Peak transmission frequency ft at which the transmittance becomes unity, plotted against the sample coverage (from Eq. 5). Only for a coverage larger than 0.19 is ft smaller than Rayleigh minimum as indicated by the gray line. (c) Transmittance for the hole sample with coverage 0.3, along with the sample image (inset). (d) Peak field amplitude plotted against the coverage for the square hole (filled squares) and the circular hole (filled circles) samples. The gray line represents Eq. (3), truncated at the amplitude of unity, calculated for f=0.66 THz.

Equations (5)

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ε eff = π 2 8 1 β ( 1 f c 2 f 2 )
Z = Z 0 1 1 + ε m
Z eff = Z 0 1 1 + ε eff
Z eff = Z 0 for ε eff = 2
f t = f c 1 + 16 β π 2
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