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

Topological photonics is a rapidly emerging field of research in which geometrical and topological ideas are exploited to design and control the behavior of light. Drawing inspiration from the discovery of the quantum Hall effects and topological insulators in condensed matter, recent advances have shown how to engineer analogous effects also for photons, leading to remarkable phenomena such as the robust unidirectional propagation of light, which hold great promise for applications. Thanks to the flexibility and diversity of photonics systems, this field is also opening up new opportunities to realize exotic topological models and to probe and exploit topological effects in new ways. This article reviews experimental and theoretical developments in topological photonics across a wide range of experimental platforms, including photonic crystals, waveguides, metamaterials, cavities, optomechanics, silicon photonics, and circuit QED. A discussion of how changing the dimensionality and symmetries of photonics systems has allowed for the realization of different topological phases is offered, and progress in understanding the interplay of topology with non-Hermitian effects, such as dissipation, is reviewed. As an exciting perspective, topological photonics can be combined with optical nonlinearities, leading toward new collective phenomena and novel strongly correlated states of light, such as an analog of the fractional quantum Hall effect.

Topological Photonics

The term photonic crystals appears because of the analogy between electron waves in crystals and the light waves in artificial periodic dielectric structures. During the recent years the investigation of one-, two-and three-dimensional periodic structures has attracted a widespread attention of the world optics community because of great potentiality of such structures in advanced applied optical fields. The interest in periodic structures has been stimulated by the fast development of semiconductor technology that now allows the fabrication of artificial structures, whose period is comparable with the wavelength of light in the visible and infrared ranges.

http://ieeexplore.ieee.org/iel5/6354/16969/00781867.pdf

Photonic crystals

It is now shown that coupled optical microcavities bear all the hallmarks of parity–time symmetry; that is, the system’s dynamics are unchanged by both time-reversal and mirror transformations. The resonant nature of microcavities results in unusual effects not seen in previous photonic analogues of parity–time-symmetric systems: for example, light travelling in one direction is resonantly enhanced but there are no resonance peaks going the other way.

Parity–time-symmetric whispering-gallery microcavities

In recent years, notions drawn from non-Hermitian physics and parity–time (PT) symmetry have attracted considerable attention. In particular, the realization that the interplay between gain and loss can lead to entirely new and unexpected features has initiated an intense research effort to explore non-Hermitian systems both theoretically and experimentally. Here we review recent progress in this emerging field, and provide an outlook to future directions and developments. This Review Article outlines the exploration of the interplay between parity–time symmetry and non-Hermitian physics in optics, plasmonics and optomechanics.

Non-Hermitian physics and PT symmetry

Effective manipulation of cavity resonant modes is crucial for emission control in laser physics and applications. Using the concept of parity-time symmetry to exploit the interplay between gain and loss (i.e., light amplification and absorption), we demonstrate a parity-time symmetry-breaking laser with resonant modes that can be controlled at will. In contrast to conventional ring cavity lasers with multiple competing modes, our parity-time microring laser exhibits intrinsic single-mode lasing regardless of the gain spectral bandwidth. Thresholdless parity-time symmetry breaking due to the rotationally symmetric structure leads to stable single-mode operation with the selective whispering-gallery mode order. Exploration of parity-time symmetry in laser physics may open a door to next-generation optoelectronic devices for optical communications and computing.

http://science.sciencemag.org/content/sci/346/6212/972.full.pdf

Single-mode laser by parity-time symmetry breaking.

Invisibility by metamaterials is of great interest, where
optical properties are manipulated in the real permittivity–
permeability plane. However, the most effective approach
to achieving invisibility in various military applications is
to absorb the electromagnetic waves emitted from radar to
minimize the corresponding reflection and scattering, such that
no signal gets bounced back. Here, we show the experimental
realization of chip-scale unidirectional reflectionless optical
metamaterials near the spontaneous parity-time symmetry
phase transition point where reflection from one side is
significantly suppressed. This is enabled by engineering the
corresponding optical properties of the designed paritytime
metamaterial in the complex dielectric permittivity
plane. Numerical simulations and experimental verification
consistently exhibit asymmetric reflection with high contrast
ratios around a wavelength of of 1,550 nm. The demonstrated
unidirectional phenomenon at the corresponding parity-time
exceptional point on-a-chip confirms the feasibility of creating
complicated on-chip parity-time metamaterials and optical
devices based on their properties.

Experimental demonstration of a unidirectional reflectionless parity-time metamaterial at optical frequencies

Nearly one century after the birth of quantum mechanics, parity–time symmetry is revolutionizing and extending quantum theories to include a unique family of non-Hermitian Hamiltonians. While conceptually striking, experimental demonstration of parity–time symmetry remains unexplored in quantum electronic systems. The flexibility of photonics allows for creating and superposing non-Hermitian eigenstates with ease using optical gain and loss, which makes it an ideal platform to explore various non-Hermitian quantum symmetry paradigms for novel device functionalities. Such explorations that employ classical photonic platforms not only deepen our understanding of fundamental quantum physics but also facilitate technological breakthroughs for photonic applications. Research into non-Hermitian photonics therefore advances and benefits both fields simultaneously. General concepts and recent developments of parity–time symmetry in classical photonics are reviewed.

Non-Hermitian photonics based on parity-time symmetry

Optical communications and computing require on-chip nonreciprocal light propagation to isolate and stabilize different chip-scale optical components. We have designed and fabricated a metallic-silicon waveguide system in which the optical potential is modulated along the length of the waveguide such that nonreciprocal light propagation is obtained on a silicon photonic chip. Nonreciprocal light transport and one-way photonic mode conversion are demonstrated at the wavelength of 1.55 micrometers in both simulations and experiments. Our system is compatible with conventional complementary metal-oxide-semiconductor processing, providing a way to chip-scale optical isolators for optical communications and computing.

http://science.sciencemag.org/content/sci/333/6043/729.full.pdf

Nonreciprocal Light Propagation in a Silicon Photonic Circuit

We demonstrate room-temperature pulsed laser emission from optically pumped metallo-dielectric cavities that are smaller than their emission wavelength in all three dimensions. The cavity consists of an aluminium/silica bi-layer shield surrounding an InGaAsP disk in which the thickness of the silica layer is optimized to minimize the gain threshold of the laser. The lasers are pumped using a 1,064-nm pulsed fibre laser with a pulse width of 12 ns and repetition rate of 300 kHz. Lasing emission at 1.43 µm is observed from a laser with slightly elliptical gain core with major and minor diameters of 490 and 420 nm, respectively. Owing to the isolation provided by the aluminium shield, this laser design approach can be used to create arrays of uncoupled lasers with emitter densities that are close to the Rayleigh resolution limit. Room-temperature lasing from metallo-dielectric cavities that are smaller than their emission wavelength in all three dimensions is reported. The cavity consists of an aluminium/silica bi-layer shield that surrounds an InGaAsP disk. The gain threshold of the laser is minimized by optimizing the thickness of the silica layer.

Liang Feng

Papers

Single-mode laser by parity-time symmetry breaking.

Experimental demonstration of a unidirectional reflectionless parity-time metamaterial at optical frequencies

Non-Hermitian photonics based on parity-time symmetry

Nonreciprocal Light Propagation in a Silicon Photonic Circuit

Room-temperature subwavelength metallo-dielectric lasers