Controlling the electromagnetic properties of materials, going beyond the limit that is attainable with naturally existing substances, has become a reality with the advent of metamaterials. The range of various structured artificial 'atoms' has promised a vast variety of otherwise unexpected physical phenomena, among which the experimental realization of a negative refractive index has been one of the main foci thus far. Expanding the refractive index into a high positive regime will complete the spectrum of achievable refractive index and provide more design flexibility for transformation optics. Naturally existing transparent materials possess small positive indices of refraction, except for a few semiconductors and insulators, such as lead sulphide or strontium titanate, that exhibit a rather high peak refractive index at mid- and far-infrared frequencies. Previous approaches using metamaterials were not successful in realizing broadband high refractive indices. A broadband high-refractive-index metamaterial structure was theoretically investigated only recently, but the proposed structure does not lend itself to easy implementation. Here we demonstrate that a broadband, extremely high index of refraction can be realized from large-area, free-standing, flexible terahertz metamaterials composed of strongly coupled unit cells. By drastically increasing the effective permittivity through strong capacitive coupling and decreasing the diamagnetic response with a thin metallic structure in the unit cell, a peak refractive index of 38.6 along with a low-frequency quasi-static value of over 20 were experimentally realized for a single-layer terahertz metamaterial, while maintaining low losses. As a natural extension of these single-layer metamaterials, we fabricated quasi-three-dimensional high-refractive-index metamaterials, and obtained a maximum bulk refractive index of 33.2 along with a value of around 8 at the quasi-static limit.
The effects of anisotropic dielectric properties of ferroelectric Ba 1Ϫx Sr x TiO 3 ͑BST͒ films on the characteristics of the interdigital ͑IDT͒ capacitors have been studied in microwave regions at room temperature. Ferroelectric BST films with ͑001͒, ͑011͒, and ͑111͒ orientation were epitaxially grown on ͑001͒, ͑011͒, and ͑111͒ MgO substrates, respectively, by the pulsed laser deposition method. The microwave properties of orientation engineered BST films were investigated using interdigital capacitors. The calculated dielectric constant tunability with 40 V dc bias variation and the calculated dielectric quality factor values for IDT capacitors based on ͑001͒, ͑011͒, and ͑111͒ oriented BST films at 9 GHz with no dc bias were about 47%, 55%, 43%, and 12, 14, 21, respectively.
Compositionally graded (Bax, Sr1−x)TiO3 (BST) thin films were deposited on MgO substrates by pulsed laser ablation. The microwave properties of the graded BST thin films were investigated at microwave frequencies with coplanar waveguide (CPW) meander-line phase shifters as a function of the direction of the composition gradient with respect to the substrate at room temperature. CPW phase shifters using graded BaTiO3(BT)→SrTiO3(ST) and ST→BT thin films exhibited a differential phase shift of 73° and 22° at 10 GHz with a dc bias voltage of 150 V, respectively. The figure of merit of the phase shifters at 10 GHz was 14.6 deg/dB for the graded BT→ST film, and 10 deg/dB for the graded ST→BT film. The graded BT→ST thin films showed a larger phase tuning than the ST→BT thin films. The microwave properties of the graded BST films depended strongly on the crystalline structure and the direction of the composition gradient with respect to the substrate.
Terahertz time‐domain spectroscopy has been used to study the optical and dielectric properties of three chalcogenide glasses: Ge30As8Ga2Se60, Ge35Ga5Se60, and Ge10As20S70. The absorption coefficients α(ν), complex refractive index n(ν), and complex dielectric constants ɛ(ν) were measured in a frequency range from 0.3 THz to 1.5 THz. The measured real refractive indices were fitted using a Sellmeier equation. The results show that the Sellmeier equation fits well with the data throughout the frequency range and imply that the phonon modes of glasses vary with the glass compositions. The theory of far‐infrared absorption in amorphous materials is used to analyze the results and to understand the differences in THz absorption among the sample glasses.
Calculations of the two-photon ionization probability rates of atomic caesium are reported for incident photon energies between 19400 and 22000cm-'. In this work we have taken into account the electric-dipole plus quadrupole approximation. The discrepancy between the theoretical results and experimental measurements is not explained in spite of the introduction of the quadrupole contribution as was suggested elsewhere.
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