There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

A recently published theory has suggested that a cloak of invisibility is in principle possible, at least over a narrow frequency band. We describe here the first practical realization of such a cloak; in our demonstration, a copper cylinder was "hidden" inside a cloak constructed according to the previous theoretical prescription. The cloak was constructed with the use of artificially structured metamaterials, designed for operation over a band of microwave frequencies. The cloak decreased scattering from the hidden object while at the same time reducing its shadow, so that the cloak and object combined began to resemble empty space.

/pdf/metamaterial-electromagnetic-cloak-at-microwave-frequencies-1qj4zdgl8e.pdf

Metamaterial Electromagnetic Cloak at Microwave Frequencies

Conventional optical components rely on gradual phase shifts accumulated during light propagation to shape light beams. New degrees of freedom are attained by introducing abrupt phase changes over the scale of the wavelength. A two-dimensional array of optical resonators with spatially varying phase response and subwavelength separation can imprint such phase discontinuities on propagating light as it traverses the interface between two media. Anomalous reflection and refraction phenomena are observed in this regime in optically thin arrays of metallic antennas on silicon with a linear phase variation along the interface, which are in excellent agreement with generalized laws derived from Fermat’s principle. Phase discontinuities provide great flexibility in the design of light beams, as illustrated by the generation of optical vortices through use of planar designer metallic interfaces.

http://www.zenogaburro.it/papers/LightPropagation.pdf

Light Propagation with Phase Discontinuities: Generalized Laws of Reflection and Refraction

Metamaterials are artificially fabricated materials that allow for the control of light and acoustic waves in a manner that is not possible in nature. This Review covers the recent developments in the study of so-called metasurfaces, which offer the possibility of controlling light with ultrathin, planar optical components. Conventional optical components such as lenses, waveplates and holograms rely on light propagation over distances much larger than the wavelength to shape wavefronts. In this way substantial changes of the amplitude, phase or polarization of light waves are gradually accumulated along the optical path. This Review focuses on recent developments on flat, ultrathin optical components dubbed 'metasurfaces' that produce abrupt changes over the scale of the free-space wavelength in the phase, amplitude and/or polarization of a light beam. Metasurfaces are generally created by assembling arrays of miniature, anisotropic light scatterers (that is, resonators such as optical antennas). The spacing between antennas and their dimensions are much smaller than the wavelength. As a result the metasurfaces, on account of Huygens principle, are able to mould optical wavefronts into arbitrary shapes with subwavelength resolution by introducing spatial variations in the optical response of the light scatterers. Such gradient metasurfaces go beyond the well-established technology of frequency selective surfaces made of periodic structures and are extending to new spectral regions the functionalities of conventional microwave and millimetre-wave transmit-arrays and reflect-arrays. Metasurfaces can also be created by using ultrathin films of materials with large optical losses. By using the controllable abrupt phase shifts associated with reflection or transmission of light waves at the interface between lossy materials, such metasurfaces operate like optically thin cavities that strongly modify the light spectrum. Technology opportunities in various spectral regions and their potential advantages in replacing existing optical components are discussed.

Flat Optics With Designer Metasurfaces

Optical lumped nanofilters and circuit feedbacks may be envisioned by arranging collections of lumped nanocircuit modules made of photonic metamaterial nanostructures that form multielement optical nanocircuits. Analytical results and full-wave numerical simulations are presented.

Filters and Feedbacks in Metamaterial Nanocircuits

: Polarization is an important feature of electromagnetic waves, and it can provide useful information about the observed scene and objects Without appropriate instruments human eyes cannot effectively sense the polarization of light. However, it is well known that eyes of certain animal species are sensitive to light's polarization. In this project, we explore the possibility of utilizing coherent motion of tiny dots superimposed on an image as means of representing and displaying polarization information on the image, while maintaining other features of conventional images. Our results show that image segments possessing coherently moving dots pop out' in contrast with the rest of the image with randomly moving dots, and have demonstrated that coherent motion can be exploited in providing visual cues for polarization contents. The direction of motion of the dots in each segment of the image can be related to the preferred axis of polarization in that segment, and the degree of coherence can be used for representing the degree of linear polarization. In this report, first a brief background on the polarization vision will be given, and then our efforts on mapping polarization information onto visual cues using coherently moving dots will be presented.

http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA405896&Location=U2&doc=GetTRDoc.pdf

Biologically Motivated Representational Schemes for Mapping Polarization Information into Visual Information

We give an overview of our theoretical works that connect the concept of lumped nanocircuit elements using plasmonic particles with ideas for 3-D photonic metamaterials with negative refraction. Analytical results and numerical simulations are presented.

From Plasmonic Nanocircuit Elements to Volumetric Photonic Negative-Refraction Metamaterials

Exploiting the anti-phase scattering properties of plasmonic materials, we have recently explored the concept of electromagnetic cloaking with plasmonic covers. Here we generalize this concept to multi-layered covers, providing possibility of cloaking at multiple frequencies.

Multi-Frequency Cloaking with Metamaterial Layered Shells

Jay Enoch’s pioneering work with whole-mounted retinas established that vertebrate photoreceptors behave as dielectric waveguides Theoretical analyses lead to the expectation that the distribution of refractive index in the photoreceptor inner segment will affect its waveguiding behavior To determine the distribution of refractive indices in isolated photoreceptors, we examined them with a scanning microinterferometer Our measurements of cones from green sunfish (Lepomis cyanellus) reveal systematic gradients of refractive index within the inner segments For example, the refractive index of single cones (averaged over transverse cross-sections) increases from ca 138 to ca 142 along the inner segment longitudinal axis Such gradients are large enough to affect waveguiding properties Interferometric measurements and analysis of subcellular variations in refractive index will allow us to address a variety of unresolved issues in photoreceptor optics

Nader Engheta

Papers

Filters and Feedbacks in Metamaterial Nanocircuits

Biologically Motivated Representational Schemes for Mapping Polarization Information into Visual Information

From Plasmonic Nanocircuit Elements to Volumetric Photonic Negative-Refraction Metamaterials

Multi-Frequency Cloaking with Metamaterial Layered Shells

Refractive Index Gradients in Sunfish Cones