衍射极限尺度下的亚波长电磁学

Subwavelength electromagnetics below the diffraction limit

作为波的本性之一,衍射是现代物理学的重要研究内容.衍射导致自由空间中波的能量不能被无限小地聚集,从而为成像、光刻、光存储、光波导等技术设定了一个原理性的障碍——衍射极限.对于电磁波和光波而言,尽管通过提高介质的折射率可以压缩衍射效应,但由于自然界中材料的折射率有限,该方法存在很大限制.近年来,随着表面等离子体光学的兴起,表面等离子体在超越传统衍射极限方面的能力和应用前景受到了学术界的关注.本文从亚波长电磁学的角度出发,介绍衍射极限研究的历史,综述了突破衍射极限的理论方法.首先,利用金属介质表面等离子体激元的短波长特性,可将等效波长压缩一个数量级以上,在纳米尺度实现光波的聚焦或定向传输;更进一步,通过人为设计超构材料和超构表面,利用结构化金属和介质中的局域谐振、耦合等特殊电磁响应,可实现亚波长局域相位调制、超宽带色散调控、近完美吸收、光子自旋轨道耦合等,从而突破传统理论的诸多局限,为下一代电磁学和光学功能器件奠定重要基础.

As a fundamental property of waves,diffraction plays an important role in many physical problems.However,diffraction makes waves in free space unable to be focused into an arbitrarily small space,setting a fundamental limit（the so-called diffraction limit） to applications such as imaging,lithography,optical recording and waveguiding,etc.Although the diffraction effect can be suppressed by increasing the refractive index of the surrounding medium in which the electromagnetic and optical waves propagate,such a technology is restricted by the fact that natural medium has a limited refractive index.In the past decades,surface plasmon polaritons（SPPs） have received special attention,owing to its ability to break through the diffraction limit by shrinking the effective wavelength in the form of collective excitation of free electrons.By combining the short wavelength property of SPPs and subwavelength structure in the two-dimensional space,many exotic optical effects,such as extraordinary light transmission and optical spin Hall effect have been discovered and utilized to realize functionalities that control the electromagnetic characteristics（amplitudes,phases,and polarizations etc.） on demand.Based on SPPs and artificial subwavelength structures,a new discipline called subwavelength electromagnetics emerged in recent years,thus opening a door for the next-generation integrated and miniaturized electromagnetic and optical devices and systems.In this paper,we review the theories and methods used to break through the diffraction limit by briefly introducing the history from the viewpoint of electromagnetic optics.It is shown that by constructing plasmonic metamaterials and metasurfaces on a subwavelength scale,one can realize the localized phase modulation and broadband dispersion engineering,which could surpass many limits of traditional theory and lay the basis of high-performance electromagnetic and optical functional devices.For instance,by constructing gradient phase on the metasurfaces,the traditional laws of reflection and refraction can be rewritten,while the electromagnetic and geometric shapes could be decoupled,both of which are essential for realizing the planar and conformal lenses and other functional devices.At the end of this paper,we discuss the future development trends of subwavelength electromagnetics.Based on the fact that different concepts,such as plasmonics,metamaterials and photonic crystals,are closely related to each other on a subwavelength scale,we think,the future advancements and even revolutions in subwavelength electromagnetics may rise from the in-depth intersection of physical,chemical and even biological areas.Additionally,we envision that the material genome initiative can be borrowed to promote the information exchange between different engineering and scientific teams and to enable the fast designing and implementing of subwavelength structured materials.