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Detectors
Published in C. R. Kitchin, Astrophysical Techniques, 2020
At THz frequencies (tens to hundreds of microns wavelengths) lenses have recently been constructed from metal meshes embedded within a dielectric such as polypropylene. By using several meshes of variable grid sizes the refractive index can be varied radially resulting in a flat lens (sometimes called a ‘Wood’s lens’86). The Lüneburg lens (Section 1.2) is a radio-frequency version of a Wood’s lens in the shape of a sphere and with the refractive index varying radially from its centre to its surface. Yet another design for a flat lens relies upon variable phase shifts to the radiation across its surface. It is potentially usable from the NIR to the submillimetre region and could be largely aberration-free. At present only available in the laboratory, it comprises a layer of gold a few nanometres thick deposited onto a silicon substrate. The gold layer is etched into closely spaced V-shaped ridges which act as antennas, receiving and then re-emitting the signal and so introducing a brief delay to it. The delays are tuned across the device’s surface so that the radiation is brought to a focus, just as with a ‘normal’ lens. Highly corrected and relatively cheap lens systems may thus become available within a few years through the use of some or all of these techniques and possibly for use in many regions of the spectrum.
Broadband Performance of Lenses Designed with Quasi-Conformal Transformation Optics
Published in Douglas H. Werner, Broadband Metamaterials in Electromagnetics, 2017
Jogender Nagar, Sawyer D. Campbell, Donovan E. Brocker, Xiande Wang, Kenneth L. Morgan, Douglas H. Werner
Despite its incredible imaging properties, the Luneburg lens is not commonly used in optical applications since it requires a detector that conforms to the surface of the sphere, while most detectors in practice are planar. In 2008, D. Schurig proposed an analytical transformation that flattens the Luneburg lens [52]. The mathematical description of the transformation is given as follows: ρ′=ρφ′=ϕz′=12(z+R2-p2) $$ \begin{gathered} \rho ^{'} = \rho \hfill \\ \varphi ^{'} = \phi \hfill \\ z^{'} = \frac{1}{2}(z + \sqrt {R^{2} - p^{2} } ) \hfill \\ \end{gathered} $$
Sound reception system by an acoustic meta-lens
Published in Journal of International Maritime Safety, Environmental Affairs, and Shipping, 2021
Sang-Hoon Kim, Byeong-Won Ahn, Kyung-Min Park, Gung Su Lim, Mukunda P. Das
A main feature of the Luneburg meta-lens is its multi-focusing property as shown in Figure 2 (Smolyaninova et al. 2015). The focusing region come from the geometry of the lens depends on the frequency. The refractive index of the ALL depends on the density of the medium. We designed and fabricated an ALL using the common gradient index lens method (Torrent and Sanchez-Dehesa 2007; Climente, Torrent, and Sanchez-Dehesa 2010; Kim S.-H., 2014). It is a direct geometric application of transformation optics. The refractive index profile of the Luneburg lens is given as, . Where R is the radius of the lens and . It was derived from Fermat’s principle and the calculus of variation (Luneburg 1944; Gutman 1954; Morgan 1958). The refractive index of ALL can be rewritten in the discrete form as , where N is the number of layers inside the lens and i =0,1, 2, …, N-1.