Abstract:We demonstrate a millimeter-wave range metamaterial fabricated from cuprate superconductor. Two complementary metamaterial structures have been studied, which exhibit Fano resonances emerging from the collective excitation of interacting magnetic and electric dipole modes.Our interest in superconducting metamaterials is driven by the desire to develop low loss media supporting high quality resonances. Such resonances may be achieved in metamaterials with broken structural symmetry supporting Fano resonances [1] where the quality factor can be controlled by design and is only limited by the Joule losses. Cuprate superconductors show lower surface conductivity than copper at frequencies below 200 GHz even at liquid nitrogen temperatures (see Fig. 1a) and is a prime choice for developing such metamaterials. Here we report the first experimental data on observation of Fano resonances in superconducting metamaterials and demonstrate that their quality factor may be controlled by temperature. All our measurements were performed using a free-space setup, which is based on mm-wave test system equipped with horn antennas (see Fig. 1b) and liquid hellium cryostat.Our metamaterials were fabricated by etching arrays of both positive and negative forms of asymmetrically-split rings in 330 um thick film of high-temperature superconductor YBCO deposited on a low-loss sapphire substrate, as show in Figs. 2a and 2b. Electromagnetic properties of the superconducting structures were studied at temperatures above and below the critical temperature T c = 87.4 K in 75 -110 GHz range of frequencies. Our measurements clearly showed the appearance of the Fano resonances upon superconducting phase transition. The results of the measurements are presented on Figs. 2c and 2d, where we plot changes in the transmission spectra of the cuprate metamaterials with decreasing temperature (down to 77 K) relative to their room temperature state. Fano resonances in metamaterials with broken structural symmetry appear as a result of excitation of the so-called trapped mode (electromagnetic mode that is weakly coupled to free-space [1]), and can be seen to fully develop in
A fiber-optic acoustic pressure sensor based on a large-area nanolayer silver diaphragm is demonstrated with a high dynamic pressure sensitivity of 160 nm∕Pa at 4 kHz frequency. The sensor exhibits a noise limited detectable pressure level of 14.5 μPa∕Hz 1∕2 . Its high dynamic pressure sensitivity and simple fabrication process make it an attractive tool for acoustic sensing and photo-acoustic spectroscopy. . Although the pressure sensitivity of the sensor based on the graphene diaphragm can potentially be very high thanks to small thickness, it is usually difficult to prepare and process single-or few-layered graphene diaphragms with large diameters and transfer them onto the sensor head. In addition, optical reflectivity of the graphene layer is relatively small compared to metal films. In this Letter, we demonstrate an acoustic sensor based on a large area silver diaphragm 150 nm in thickness and 2.4 mm in diameter. While the thickness is comparable to the graphite-based diaphragms previously reported [6], the diameter is an order of magnitude larger. The acoustic pressure test shows that such a sensor based on a large area silver diaphragm has a sensitivity of up to 160 nm∕Pa at 4 kHz frequency. To our best knowledge, this is the highest acoustic pressure sensitivity ever reported for fiber-optic acoustic sensors based on diaphragms. The sensor is able to detect acoustic pressure as weak as 14.5 μPa∕Hz 1∕2 . The acoustic pressure sensing head based on the silver diaphragm comprises a standard polarization maintaining fiber (PM fiber), a ceramic split sleeve, and the silver diaphragm, as shown in Fig. 1(a). Figure 1(b) is the image of the proposed sensing head. A picture of the ceramic split sleeve, on which the diaphragm is attached, is also shown in the bottom inset of Fig. 1(b), and the inner diameter of this split sleeve is 2.4 mm. The PM fiber end facet was polished with an angle of 8°to reduce the Fresnel reflection from the fiber end surface. The fabrication process of the sensing head is similar to that reported in our previous paper [5]. The distance between the fiber end and the diaphragm is approximately 150 μm adjusted by a high precision linear stage (Newport ILS-250CC) with a displacement resolution of 0.5 μm.The silver thin film used in this sensor is approximately 150 nm thick with >2.4 mm diameter that covers one end of the split sleeve. The top inset of Fig. 1(b) is an SEM image of the silver diaphragm. This sensing diaphragm has several advantages, including good stability and high reflectivity over the near infrared wavelength range. In general, the pressure sensitivity depends both on the material and the thickness of the diaphragm. In fact, the pressure sensitivity of an acoustic sensor based on an edge-clamped circular diaphragm is proportional to R 4 ∕t 3 , where R is the radius and t is the thickness of the diaphragm [7]. Therefore a diaphragm with large
We propose a kind of planar chiral optical metamaterial consisting of two layers of connected I-shape resonators arranged by a twist angle of 90°. Numerical simulation results demonstrate that our scheme can realize a mutual polarization conversion and dual-band asymmetric transmission for linearly polarized waves in the optical regime. For the forward propagation, the x-to-y and y-to-x polarization conversions in the proposed bilayered metamaterial result from the concentric and eccentric C-shaped dimers, respectively. The current distributions of bilayered metamaterials at the resonant frequencies are presented to interpret the dual-band asymmetric transmission. The polarization conversion efficiency and resonant frequencies can be modified via parametric study.
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