For measurements designed to accurately determine layer thickness, there is a natural trade-off between sensitivity to optical thickness and lateral resolution due to the angular ray distribution required for a focused beam. We demonstrate a near-field imaging approach that enables both sub-wavelength lateral resolution and optical thickness sensitivity. We illuminate a sample in a total internal reflection geometry, with a photo-activated spatial modulator in the near-field, which allows optical thickness images to be computationally reconstructed in a few seconds. We demonstrate our approach at 140 GHz (wavelength 2.15 mm), where images are normally severely limited in spatial resolution, and demonstrate mapping of optical thickness variation in inhomogeneous biological tissues.
We present a microwave absorbing structure comprised of an array of subwavelength radius copper disks, closely spaced from a ground plane by a low loss dielectric. Experiments and accompanying modeling demonstrate that this structure supports electromagnetic standing wave resonances associated with a cylindrical cavity formed by the volume immediately beneath each metal disk. Microwave absorption on resonance of these modes, at wavelengths much greater than the thickness of the structure, is dictated almost entirely by the radius of the disk and permittivity of the dielectric, being largely independent of the incident angle and polarization.
The wing-scale microstructures associated with two species of Papilio butterfly are described and characterized. Despite close similarities in their structures, they do not exhibit analogous optical effects. With Papilio palinurus, deep modulations in its multilayering create bicolor reflectivity with strong polarization effects, and this leads to additive color mixing in certain visual systems. In contrast to this, Papilio ulysses features shallow multilayer modulation that produces monocolor reflectivity without significant polarization effects.
This work presents the first detailed study of the optical dielectric function of optically thick TiN x films using grating coupling of radiation to surface plasmon-polaritons. Angle-dependent reflectivities are obtained in the wavelength range 500–875 nm and by comparison with grating modelling theory, we determine both the imaginary and the real parts of the dielectric function. This method provides an alternative to traditional characterization techniques (e.g. Kramers-Kronig analysis) that may require additional information about film thickness, or the sample's optical properties in other parts of the electromagnetic spectrum. We have fitted the determined dielectric function to a model based on a combination of interband absorptions and free-electron response evaluating both the plasma energy and the relaxation time.
In this Letter, the transmission properties of a nonperiodic array of slots arranged in the form of a Fibonacci sequence are investigated. By arranging the slots in this manner, an additional periodicity can be utilized, resulting in corresponding resonance features in the transmitted signal. By investigating the transmission response of a perforated metallic sheet over a broad frequency range (6--40 GHz), it is shown that this simple one-dimensional chain supports two periodicities, one due to the regular periodic separation and one due to average spacing---which is related to the golden ratio. This response replicates the resonant behavior of a two-dimensional periodic array with a single nonperiodic array also creating new families of diffraction lobes in the far-field region.
Abstract In this work, the electromagnetic response of a mathematically interesting shape—a Möbius strip—is presented, along with a ring resonator for comparison. Both resonators consist of a central lossy dielectric layer bounded by perfectly conducting layers. For the case of the Möbius strips, the computational results show that there are a family of half-integer wavelength modes within the dielectric layer. These additional modes result in increased absorption, and a corresponding reduction in the radar cross section. Interestingly, rotational scans show that these modes can be excited over a large angular range. This investigation gives an understanding of the electromagnetic response of these structures, paving the way for future experiments on Möbius strip resonators.
Between the design and final realization of a working metasurface lies the potential for a myriad of complications: fabrication tolerances, material permittivity uncertainties, alignment issues and localized defects to name just a few. Global methods of characterizing an entire surface are often incapable of separating these candidates and typically one must resort to the simulation of a wide parameter space to begin to understand experimental discrepancies. In this work we introduce a new imaging technique that is able to locate and discern the resonant frequencies of individual antennas in a complex microwave metasurface. This is achieved with a microwave single-pixel camera using patterned optical excitation of a silicon layer adjacent to the metamaterial to achieve super-resolution. This approach allows us to locate and diagnose fabrication defects, spectrally characterize individual meta-atoms, and visualize inhomogeneous broadening across our samples with below λ/20 resolution, over large areas and in near real-time.
A metamaterial surface formed by three slot gratings at 60° to each other has two possible high-symmetry arrangements. One forms equilateral triangular metal patches, the other a combination of hexagons and small equilateral triangles. When spaced above a ground plane with a thin dielectric spacer both structures give strong microwave absorption at certain resonant frequencies, which is largely angle independent. The results of microwave reflectivity measurements are here presented for the two distinct sample geometries and compared with predictions from finite element method models.
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