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A negative permeability material at red light

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Abstract

A negative permeability in a periodic array of pairs of thin silver strips is demonstrated experimentally for two distinct samples. The effect of the strip surface roughness on negative permeability is evaluated. The first sample, Sample A, is fabricated of thinner strips with a root mean square roughness of 7 nm, while Sample B is made of thicker strips with 3-nm roughness. The real part of permeability, μ́, is -1 at a wavelength of 770 nm in Sample A and -1.7 at 725 nm in Sample B. Relative to prototypes simulated with ideal strips, larger strip roughness acts to decrease |μ́| by a factor of 7.8 in Sample A versus a factor of 2.4 decrease for Sample B.

©2007 Optical Society of America

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Figures (5)

Fig. 1.
Fig. 1. (a). Ideal unit cell for the array of coupled silver nano-strips of width w that are separated by an alumina spacer of the same width; here t is the thickness of both strips and d is the thickness of the spacer. The strips are infinite in y and periodic in x with period p. (b) The real part of ϵ and μ shown for the cell with w = 140 nm, t = 35 nm, d = 40 nm, and p = 300 nm. The optical constants of bulk silver [15] are taken for the strips. The refractive index of the substrate is 1.52. (c) The actual cross-section of samples obtained after fabrication (left half). Right half shows that the total B-field between the strips is opposite to the incident H-field at the magnetic resonance.
Fig. 2.
Fig. 2. (a). FE SEM picture of the periodic array of coupled silver strips (Sample A). (b) Transmission and reflection spectra of Sample A measured at normal incidence with TE polarization, here λd is the diffraction threshold. The experimental spectra are compared to the results of numerical modeling. The optical constants of silver strips are taken from the experimental data for bulk silver [15]. (c) Transmission, absorption and reflection spectra of Sample A at normal incidence with TM polarization compared to spectra obtained from simulations. In this case, e of the silver strips was adjusted to match excessive losses. (d) The real part of the effective permeability (μ́) and effective permittivity (ϵ́)
Fig. 3.
Fig. 3. (a). FE SEM picture of Sample B. (The inset shows an AFM image of the sample.) (b) Comparison of the loss-adjustment factor a obtained for Samples A and B. Sample A demonstrates more excessive loss in comparison to bulk metal [15] and Sample B. (c) Transmission, absorption and reflection spectra of Sample B at normal incidence with TM polarization compared to spectra obtained from simulations. In this case, ϵ″ of the silver strips was adjusted to match excessive losses. (d) The real part of the effective permeability (μ́) and effective permittivity (ϵ́). A minimum μ́ of - 1.7 is obtained at 725 nm.
Fig. 4.
Fig. 4. Section analysis obtained from AFM scans of Sample A and Sample B (shown in panels (a) and (b) respectively).
Fig. 5.
Fig. 5. (a). Transmission, absoption and reflection spectra of Sample B at normal incidence with TM polarization compared to spectra obtained from simulations; here ϵ″ of silver was adjusted using a factor of α = 3 . (b) The real part of the effective values of μ, and ϵ́ obtained using the wavelength-independent factor α = 3 .
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