Diffraction limit of electromagnetic waves
编者按：
自1873年阿贝提出衍射极限以来，该问题就一直是学术界的研究热点和难点。近年来，随着近场光学、表面等离子体亚波长光学、非线性光学等学科的发展，人们开始在亚波长尺度重新认识衍射极限。本专题针对如何突破衍射极限，以及突破衍射极限之后的新问题，邀请本领域专家学者阐述最新进展和研究趋势，以期进一步促进相关理论、技术及应用的发展。
2017, 66 (14): 144201.
doi: 10.7498/aps.66.144201
Abstract +
Surface plasmons as the collective electrons oscillation at the interface of metal and dielectric materials, have induced tremendous applications for the nanoscale light focusing, waveguiding, coupling, and photodetection. As the development of the modern technology, cathodoluminescence (CL) has been successfully applied to describe the plasmon resonance within the nanoscale. Usually, the CL detection system is combined with a high resolution scanning electron microscope (SEM). The fabricated plasmonic nanostructure is directly excited by the electron beam, and detected by an ultra-sensitive spectrometer and photodetector. Under the high energy electron stimulation, all of the plasmon resonances of the metallic nanostructure can be excited. Because of the high spatial resolution of the SEM, the detected CL can be used to analyze the details of plasmon resonance modes.
In this review, we first briefly introduced the physical mechanism for the CL generation, and then discussed the CL emission of single plasmonic nanostructures such as different nanowires, nanoantennas, nanodisks and nanocavities, where the CL only describes the individual plasmon resonance modes. Second, the plasmon coupling behavior for the ensemble measurement was compared and analyzed for the CL detection. Finally, the CL detection with other advanced technologies were concluded. We believe with the development of the nanophotonics community, CL detection as a unique technique with ultra-high energy and spatial resolution has potential applications for the future plasmonic structure design and characterization.
2017, 66 (14): 147901.
doi: 10.7498/aps.66.147901
Abstract +
Sapphire has shown broad application prospects in military and medical fields, due to its high hardness, excellent corrosion resistance and high transmission in the infrared band. However, these characteristics have also brought about lots of difficulties in machining or chemical etching the material. Femtosecond laser processing with excellent characteristics including small heat-affected zones and high processing resolution ratio, has become an emerging field. Therefore, it has important application prospects and has found increasingly wide applications in the fields of material modification and high-quality fabrication of three-dimensional micro-nano structures and devices. In this paper, we propose a method in which femtosecond laser processing based on multi-photon absorption is used to process sapphire beyond the optical diffraction limit. In this work, femtosecond laser with a central wavelength of 343 nm is focused on the sapphire and the surface of sapphire is scanned with the high-precision piezoelectric positioning stages. Nano structures each with a width of about 61 nm are obtained, and the minimum space between the nano structures could be as short as about 142 nm. Further, the influences on the processing resolution from laser power and scanning speed are investigated and the generation mechanism for the nano-ripple structure is discussed. Finally, femtosecond laser processing on the sapphire with a resolution beyond the optical diffraction limit is achieved. This work provides a reference for processing the hard and brittle materials by femtosecond laser.
2017, 66 (14): 140702.
doi: 10.7498/aps.66.140702
Abstract +
In the field of optical imaging, the conventional imaging resolution is about 200 nm due to the diffraction limit. The higher resolution is urgently needed for further developing scientific research. Therefore, how to break through this limitation to acquire high quality and high resolution image has become a hot research topic. The microspheres with the size of tens of micrometers exhibit the ability to improve the imaging resolution of the conventional optical microscope by locating them directly on the sample surface. Due to its simplicity, the microsphere optical nanoscope technology is widely studied. This paper introduces the research background of the optical microscope and the research progress of microsphere optical nanoscope technology. At the same time, approaches to adjusting the photonic nanojet generated by the microspheres by fabricating concentric ringing, central mask, and surface coating of microspheres are reviewed. The possible reasons for this improved resolution are discussed. The applications and development of the microsphere ultra-microscopic technology in the future are discussed.
2017, 66 (14): 144202.
doi: 10.7498/aps.66.144202
Abstract +
Surface plasmon polariton has attracted more and more attention and has been studied extensively in the recent decades, owing to its ability to confine the electro-magnetic field to a sub-wavelength scale near the metal-dielectric interface. On one hand, the tightly confined surface plasmonic modes can reduce the size of integrated optical device beyond the diffraction limit; on the other hand, it provides an approach to enhancing the interaction between light and matter. With the development of experimental and numerical simulation techniques, its investigation at a quantum level has become possible. In the recent experiments, scientists have realized quantum interference between single plasmons in a nanoscale waveguide circuit and achieved the strong coupling between photons and single molecules by using plasmonic structure, which demonstrates its superiority over the traditional optics. Here, we review the theoretical and experimental researches of surface plasmon polariton in the field of quantum information processing. First, we introduce the experiments on the basic quantum properties of surface plasmons, including the preservation of photonic entanglement, wave-particle duality and quantum statistical property. Second, we review the research work relating to the generation, manipulation and detection of surface plasmons in a quantum plasmonic integrated circuit. Then, we present the research of the interaction between surface plasmons and single quantum emitters and its potential applications. Finally, we make a discussion on how the intrinsic loss affects the quantum interference of single plasmons and the coupling between quantum emitters. The collision and combination of quantum optical and plasmonic fields open up possibilities for investigating the fundamental quantum physical properties of surface plasmons. It can be used to make ultra-compact quantum photonic integrated circuits and enhance the interaction strength between photons and quantum emitters.
2017, 66 (14): 144205.
doi: 10.7498/aps.66.144205
Abstract +
The diffraction limit of traditional optical device greatly restricts the further development of optical super-resolution systems. It is a great challenge to overcome the diffraction limit at a device level, and achieve label-free far-field super-resolution imaging. Optical super-oscillation provides a new way to realize super-resolution since it allows the generation of arbitrary small structures in optical fields in the absence of evanescent waves. The researches of optical super-oscillation and super-oscillatory optical devices have grown rapidly in recent decades. Optical super-oscillation and super-oscillatory optical devices have been demonstrated theoretically and experimentally to show great potential applications in label-free far-field optical microscopy, far-field imaging and high-density data storage. In this paper, we gives a broad review of recent development in optical super-oscillation and super-oscillatory optical devices, including basic concepts, design tools and methods, testing techniques for super-oscillatory optical field, and their applications.
2017, 66 (14): 144206.
doi: 10.7498/aps.66.144206
Abstract +
Due to the fundamental laws of wave optics, the spatial resolution of traditional optical microscopy is limited by the Rayleigh criterion. Enormous efforts have been made in the past decades to break through the diffraction limit barrier and in depth understand the dynamic processes and static properties. A growing array of super-resolution techniques by distinct approaches have been invented, which can be assigned to two categories: near-field and far-field super-resolution techniques. The near-field techniques, including near-field scanning optical microscopy, superlens, hyperlens, etc., could break through the diffraction limit and realize super-resolution imaging by collecting and modulating the evanescent wave. However, near-field technique suffers a limitation of very short working distances because of the confined propagation distance of evanescent wave, and certainly produces a mechanical damage to the specimen. The super-resolution fluorescence microscopy methods, such as STED, STORM, PALM, etc., could successfully surpass the diffractive limit in far field by selectively activating or deactivating fluorophores rooted in the nonlinear response to excitation light. But those techniques heavily rely on the properties of the fluorophores, and the labelling process makes them only suitable for narrow class samples. Developing a novel approach which could break through the diffraction limit in far field without any near-field operation or labelling processes is of significance for not only scientific research but also industrial production. Recently, the planar metalenses emerge as a promising approach, owing to the theoretical innovation, flexible design, and merits of high efficiency, integratable and so forth. In this review, the most recent progress of planar metalenses is briefly summarized in the aspects of sub-diffractive limit focusing and super-resolution imaging. In addition, the challenge to transforming this academic concept into practical applications, and the future development in the field of planar metalenses are also discussed briefly.
2017, 66 (14): 144208.
doi: 10.7498/aps.66.144208
Abstract +
Electromagnetic metamaterials are artificial structures engineered on a subwavelength scale to have optical properties that are not observed in their constituent materials and may not be found in nature either, such as negative refractive index. They have enabled unprecedented flexibility in manipulating light waves and producing various novel optical functionalities. Since the beginning of this century, with the development of nanofabrication and characterization technologies, there has been aroused a tremendous growing interest in the study of electromagnetic metamaterials and their potential applications in different fields including super-resolution imaging, optical biosensing, electromagnetic cloaking, photonic circuits and data storage. Electromagnetic metasurfaces are two-dimensional metamaterials composed of subwavelength planar building blocks. Although metasurfaces sacrifice some functionalities compared with their bulk counterparts, they provide us with distinct possibility to fully control light wave with ultrathin planar structures. Based on Huygens principle, the metasurfaces are able to arbitrarily manipulate the phases, amplitudes or polarizations of optical waves. For example, metasurfaces made of gold nanoantenna-arrays are able to create phase discontinuities for light propagating through the interfaces and drastically change the flows of reflected and refracted light at infrared frequencies. Comparing traditional dielectric optic elements, the thickness values of metasurface-based optical devices are much smaller. In addition to the control of free-space incident light, metasurfaces can also be used to precisely control and manipulate surface electromagnetic waves. In this review, we introduce the generalized Snell's law and the fundamental principles to modulate phase by using metasurfaces. Research progress of a variety of imaging technologies based on metasurfaces is then presented, including plasmonic metasurface, all-dielectric metasurface and metal/insulator hybrid metasurface. Finally, we summarize several frontier problems associated with metasurface, which maybe provide some references for the future researches and applications.
2017, 66 (14): 148701.
doi: 10.7498/aps.66.148701
Abstract +
Structure illumination microscopy (SIM) is a novel imaging technique with advantages of high spatial resolution, wide imaging field and fast imaging speed. By illuminating the sample with patterned light and analyzing the information about Moir fringes outside the normal range of observation, SIM can achieve about 2-fold higher in resolution than the diffraction limit, thus it has played an important role in the field of biomedical imaging. In recent years, to further improve the resolution of SIM, people have proposed a new technique called plasmonic SIM (PSIM), in which the dynamically tunable sub-wavelength surface plasmon fringes are used as the structured illuminating light and thus the resolution reaches to 3-4 times higher than the diffraction limit. The PSIM technique can also suppress the background noise and improve the signal-to-noise ratio, showing great potential applications in near-surface biomedical imaging. In this review paper, we introduce the principle and research progress of PSIM. In Section 1, we first review the development of optical microscope, including several important near-field and far-field microscopy techniques, and then introduce the history and recent development of SIM and PSIM techniques. In Section 2, we present the basic theory of PSIM, including the dispersion relation and excitation methods of surface plasmon, the principle and imaging process of SIM, and the principle of increasing resolution by PSIM. In Section 3, we review the recent research progress of two types of PSIMs in detail. The first type is the nanostructure-assisted PSIM, in which the periodic metallic nanostructures such as grating or antenna array are used to excite the surface plasmon fringes, and then the shift of fringes is modulated by changing the angle of incident light. The resolution of such a type of PSIM is mainly dependent on the period of nanostructure, thus can be improved to a few tens of nanometers with deep-subwavelength structure period. The other type is the all-optically controlled PSIM, in which the structured light with designed distribution of phase or polarization (e.g. optical vortex) is used as the incident light to excite the surface plasmon fringes on a flat metal film, and then the fringes are dynamically controlled by modulating the phase or polarization of incident light. Without the help of nanostructure, such a type of PSIM usually has a resolution of about 100 nm, but benefits from the structureless excitation of plasmonic fringes in an all-optical configuration, thereby showing more dynamic regulation and reducing the need to fabricate nanometer-sized complex structures. In the final Section, we summarize the features of PSIM and discuss the outlook for this technique. Further studies are needed to improve the performance of PSIM and to expand the scope of practical applications in biomedical imaging.
2017, 66 (14): 148702.
doi: 10.7498/aps.66.148702
Abstract +
Optical microscope has been giving impetus to the development of modern technology. As the advancement of these techniques, high resolution microscopy becomes crucial in biological and material researches. However, the diffraction limit restricts the resolution of conventional microscopy. In 1968, confocal microscopy, the first pointwise scanning superresolution method, appeared. It improves the imaging resolution, enhances the contrast, and thus breaks through the diffraction limit. Since then many superresolution methods have come into being, among which the pointwise scanning superresolution method earns reputation for its high imaging resolution and contrast. The stimulated emission depletion microscopy becomes the most prominent method with an achievable resolution of about 2.4 nm and then widely used. Besides, the newly developed fluorescence emission difference microscopy (FED) and the saturated absorption competition microscopy (SAC) have their advantages of non-constraint on fluorescent dyes, low saturated beam power, simplified optical setups, while they achieve a resolution of lower than /6. Further explorations of FED will be keen on vivo biological observations by using it, while that of SAC can concentrate on enhancing the resolution on a nanoscale and reducing the signal-to-noise ratio. In addition, the Airyscan technique in which a detector array is used for image acquisition, can serve as a complementary tool to further enhance the imaging quality of pointwise scanning superresolution method. The detector-array enables both the narrowed size of pinhole and the increasing of the acquired signal intensity by 1.84 folds. The other methods, e.g. superoscillation lens and high-index resolution enhancement by scattering, have the potentialities to obtain superresolved image in material science or deep tissues. After being developed in the past three decades, the superresolution methods now encounter a new bottleneck. Further improvement of the current methods is aimed at imaging depth, and being used more practically and diversely. In this review, we detailedly describe the above pointwise scanning superresolution methods, and explain their principles and techniques. In addition, the deficiencies and potentialities of these methods are presented in this review. Finally, we compare the existing methods and envision the next generation of the pointwise scanning superresolution methods.
2017, 66 (14): 144101.
doi: 10.7498/aps.66.144101
Abstract +
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.
2017, 66 (14): 144207.
doi: 10.7498/aps.66.144207
Abstract +
Laser is recognized as one of the top technological achievements of 20th century and plays an important role in many fields, such as medicine, industry, entertainment and so on. Laser processing technology is one of the earliest and most developed applications of laser. With the rapid development of nanoscience and nanotechnology and micro/nano electronic devices, the micro/nanofabrication technologies become increasingly demanding in manufacturing industries. In order to realize low-cost, large-area and especially high-precision micro-nanofabrication, it has great scientific significance and application value to study and develop the laser fabrication technologies that can break the diffraction limit. In this article, the super resolution laser fabrication technologies are classified into two groups, far-filed laser direct writing technologies and near-field laser fabrication technologies. Firstly, the mechanisms and progress of several far-field laser direct writing technologies beyond the diffraction limit are summarized, which are attributed to the lasermatter nonlinear interaction. The super-diffraction laser ablation was achieved for the temperature-dependent reaction of materials with the Gaussian distribution laser, and the super-diffraction laser-induced oxidation in Metal-Transparent Metallic Oxide grayscale photomasks was realized by the laser-induced Cabrera-Mott oxidation process. Besides, the multi-photon polymerization techniques including degenerate/non-degenerate two-photon polymerization are introduced and the resolution beyond the diffraction limit was achieved based on the third-order nonlinear optical process. Moreover, the latest stimulated emission depletion technique used in the laser super-resolution fabrication is also introduced. Secondly, the mechanisms and recent advances of novel super diffraction near-field laser fabrication technologies based on the evanescent waves or surface plasmon polaritons are recommended. Scanning near-field lithography used a near-field scanning optical microscope coupled with a laser to create nanoscale structures with a resolution beyond 100 nm. Besides, near-field optical lithography beyond the diffraction limit could also be achieved through super resolution near-field structures, such as a bow-tie nanostructure. The interference by the surface plasmon polariton waves could lead to the fabrication of super diffraction interference fringe structures with a period smaller than 100 nm. Moreover, a femtosecond laser beam could also excite and interfere with surface plasmon polaritons to form laser-induced periodic surface structures. Furthermore, the super-resolution superlens and hyperlens imaging lithography are introduced. Evanescent waves could be amplified by using the superlens of metal film to improve the optical lithography resolution beyond the diffraction resolution. The unique anisotropic dispersion of hyperlens could provide the high wave vector component without the resonance relationship, which could also realize the super resolution imaging. Finally, prospective research and development tend of super diffraction laser fabrication technologies are presented. It is necessary to expand the range of materials which can be fabricated by laser beyond the diffraction limit, especially 2D materials.
2017, 66 (14): 144209.
doi: 10.7498/aps.66.144209
Abstract +
In the last few decades, nanoscience and nanotechnology have been growing with breath taking speed, and how to break through the diffraction limit and tame the light on a nanoscale have become the major challenges in optics. In this field, several super-resolution optical nanoscopy techniques have been developed, leading to a series of breakthroughs in physics, chemistry, and life sciences. In the work, we give a retrospect of the newly developed techniques in diffraction theory of linear optical systems, including the solid immersion lens, structured light illumination microscopy, scanning near-field optical microscopy, metamaterial-based wide field near-field imaging technique and super-oscillatory lens. Brief discussion on their principles, advantages and applications is also provided.
2017, 66 (14): 147101.
doi: 10.7498/aps.66.147101
Abstract +
Nanophotonics focuses on the study of the behavior of light and the interaction between light and matter on a nanometer scale. It has often involved metallic nanostructures which can concentrate and guide the light beyond the diffraction limit due to the unique surface plasmons (SPs). Manipulation of light can be accomplished through controlling the morphologies and components of metallic nanostructures to incur special surface plasmons. However, it is still a severe challenge to achieve exquisite control over the morphologies or components of metallic nanostructures: chemical methods can provide anisotropic but highly symmetric metallic nanostructures; lithographic methods have a limited resolution, especially for three-dimensional metallic nanostructures. By comparison, DNA self-assembly-based fabrication of metallic nanostructures is not restricted to these confinements. With the high-fidelity Waston-Crick base pairing, DNA can self-assemble into arbitrary shapes ranging from the simplest double strands to the most sophisticated DNA origami. Due to the electrostatic interactions between negatively charged phosphate backbones and positively charged metal ions, DNA of any shapes can affect the metal ions or atoms to a certain degree. Depending on the shape and base, DNA self-assembly nanostructures can exert different influences on the growth of metallic nanoparticles, which in turn gives rise to deliberately controllable metallic nanostructures. Besides, DNA self-assembly nanostructures can act as ideal templates for the organization of metallic nanoparticles to construct special metallic nanostructures. In this case, DNA-modified metallic nanoparticles are immobilized on DNA self-assembly nanostructures carrying complementary sticky ends. The geometry and component arrangements of metallic nanostructures both can be precisely dictated on the DNA nanostructures by programming the sticky end arrays. Complicated metallic nanostructures which can be hardly fabricated with conventional chemical or lithographic methods have been readily prepared with the DNA self-assembly-based fabrication method, thereby greatly promoting the development of nanophotonics. Therefore, the studies of DNA self-assembly-based fabrication of metallic nanostructures and related nanophotonics have received rapidly growing attention in recent years. This review first gives a brief introduction of the mechanism for breaking the diffraction limit of light with metallic nanostructures based on SPs. Then we give a systematic review on DNA self-assembly-based fabrication of metallic nanostructures and related nanophotonics, which is divided into several parts according to the different pathways by which DNA self-assembly can influence the morphologies or components of metallic nanostructures. Finally, the remaining problems and limitations for the existing DNA self-assembly-based fabrication of metallic nanostructures are presented and an outlook on the future trend of the field is given as well.
2017, 66 (14): 147803.
doi: 10.7498/aps.66.147803
Abstract +
In linear optical regime, many novel optical functions have been demonstrated by using ultrathin photonic metasurfaces. The main concept of metasurface is to appropriately assembly the spatially variant meta-atoms on a subwavelength scale, and realize the manipulations of polarization, phase and amplitude of light. Recently, the nonlinear optical properties of photonic metasurfaces have attracted a lot of attention. In this review, we discuss the design, material selection, symmetry consideration of the meta-atoms, as well as the applications such as nonlinear chiral optics, nonlinear geometric Berry phase and nonlinear wavefront engineering. Lastly, we point out the challenges and potentials of nonlinear photonic metasurfaces for manipulating the light-matter interactions.
2017, 66 (14): 148703.
doi: 10.7498/aps.66.148703
Abstract +
The diffraction of the finite aperture in the optical imaging system restricts further improvement of the resolution of optical microscopy, which is called the diffraction limit. Since raised by Ernst Abbe in 1873, the problem of diffraction limit has been one of the foci of academic research. In recent years, with the rapid development of related fields such as the development of optoelectronic devices including high energy lasers and high sensitivity detectors and the development of new fluorescent probes, the problem of diffraction limit in optical microscopy ushered in a new opportunity, and super-resolution microscopy (SRM) has made remarkable achievements in the past decade. The basic principles of diffraction limited resolution in both space and frequency domains are reviewed, and on this basis, the mechanisms for the various SRM technologies to circumvent the diffraction limit and improve the resolution are explained in detail. The development trends and research directions of various SRM techniques are also introduced. As a new and important development trend of SRM, correlative super-resolution microscopy and its recent progress are reviewed, including correlative studies on SRM and time-lapse live cell fluorescence microscopy, fluorescence lifetime imaging microscopy, spectrometry and spectroscopy, electron microscopy, atomic force microscopy, etc. The role and significance of various correlative super-resolution microscopy are discussed. The future development of super-resolution microscopy and correlative super-resolution microscopy is also prospected.
2017, 66 (14): 148704.
doi: 10.7498/aps.66.148704
Abstract +
Structured illumination microscopy (SIM) is one of the most promising super-resolution techniques, owing to its advantages of fast imaging speed and weak photo bleaching. The quality of the SIM image is greatly dependent on the contrast of the sinusoidal fringe illumination patterns. Low fringe contrast illumination will seriously affect the super-resolution result and lead to additional artifacts. The generation of fringe patterns with high contrast is the key requirement in hardware for the SIM technique. This can be done by the interference of two laser beams diffracted from the phase gratings addressed on a spatial light modulator. Meanwhile, for maximal interference contrast, precise polarization control to maintain s-polarization for different fringe orientations is critical. In this paper, we review several typical polarization control methods in SIM, and propose a new method by using a zero-order vortex half-wave retarder (VHR). Compared with the other methods, the presented VHR-based polarization control method is very efficient in terms of simple system configuration, ease of use, and high light energy utilization efficiency near to 100%.
2017, 66 (14): 144210.
doi: 10.7498/aps.66.144210
Abstract +
Self-accelerating beam is a kind of light beam capable of self-bending in free space without any external potential, of which a typical one is the well-known Airy beam. Such a beam has gained great attention for its extraordinary properties, including nondiffracting, self-accelerating and self-healing, which may have versatile applications in the delivery and guiding of energy, information and objects using light, such as particle manipulation, micro-machining, optical routing, super-resolution imaging, etc. However, since Airy beam can only propagate along parabolic trajectory, which reduces the flexibility in practical applications, thus how to design accelerating beams propagating along arbitrary trajectory is still a crucial problem in this area. One scheme is to keep on finding other analytical solutions of the wave equation besides Airy beam, such as semi-Bessel accelerating beams, Mathius beams, and Weber beams, moving along circular, elliptical, or parabolic trajectories, but it becomes increasingly difficult to find out any more solutions. A more effective solution to this problem is based on the caustic method, which associates the predesigned trajectory with an optical caustics and then obtains the necessary initial field distribution by performing a light-ray analysis of the caustics. This method has been implemented in real space and Fourier space based on Fresnel diffraction integral and angular-spectrum integral, respectively. It has been found recently that they can be unified by constructing Wigner distribution function in phase space. Based on the caustic method, accelerating beams were constructed to propagate along arbitrary convex trajectories in two-dimensional space at first. With continuous development of this method, the types of accelerating beams available have been extending from convex trajectories to nonconvex trajectories, from two-dimensional trajectories to three-dimensional trajectories, and from one main lobe to multiple main lobes, which opens up more possibilities for emerging applications based on accelerating beams. In future, previous researches and applications based on Airy beams will certainly be generalized to all these new types of accelerating beams, and owing to the great flexibility in designing accelerating beams, more application scenarios may emerge in this process with huge development potential. Thus in this paper, we review the principle and progress of the caustic method in designing accelerating beams.
2017, 66 (14): 147802.
doi: 10.7498/aps.66.147802
Abstract +
Since electromagnetic waves were discovered, effectively controlling them has been a goal and radiators with better characteristics have always been chased by researchers. However, limited by the electromagnetic properties of nature materials, traditional radiation technology is reaching its bottleneck. For example, traditional microwave antenna has the disadvantages of large volume, heavy weight, narrow operating frequency band, etc., and cannot satisfy the development requirement of modern communication systems. Therefore, the state-of-art radiation technology meets the challenge of minimizing the size and broadening the bandwidth of radiators, and constructingmulti-functional and reconfigurable antennas. In recent years, metamaterials have aroused great interest due to the extraordinary diffraction manipulation on a subwavelength scale. Fruitful bizarre electromagnetic phenomena, such as negative refraction index, planar optics, perfect lens, etc. have been observed in metamaterials, and the corresponding theories improve the fundamental principle systems of electromagnetics. Based on these novel theories, a series of new radiators has been proposed, which has effectively overcome the difficulties in traditional radiation technology and broken through the limits of natural electromagnetic materials. The relating theory and technology may greatly promote the development of electromagnetics, optics, materials.
In this article, we mainly review the recent progress in the novel electromagnetic radiation technology based on metamaterials, which is named meta-antenna, including the principle of diffraction manipulation of metamaterial to control the amplitude, phase and polarization of the incident electromagnetic waves. Subsequently, a series of radiation devices is introduced, including the new phased array antenna on the concept of phase manipulating metamaterial, and the high directivity antenna based on zero refraction index metamaterial and photonic crystal, and the low RCS antenna simultaneously has the functions of gain enhancement and stealth ability. Besides, the polarization manipulation characteristics of metamaterial are also reviewed. The anisotropic and chiral metamaterials are analyzed, and several polarizers with broadband characteristics and reconfigurable ability are introduced. Furthermore, due to the importance as future radiation sources, nanolasers that work on a subwavelengh scale are demonstrated. Finally, we point out the current problems and future trend of the radiation technology based on metamaterials.
2017, 66 (14): 147804.
doi: 10.7498/aps.66.147804
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Super diffraction imaging has been a research hotspot for a long time. We realize the super diffraction imaging with a metasurface structure, which is consisted of asymmetrically split rings. Based on the wave vector selectivity of the metasurface, radiation can be transmitted through it only in a narrow range of the incident angular. The metasurface acts as a high frequency spatial filter, reduces the diffraction effect, and obtains the super diffraction resolution. Numerical simulation results demonstrate the validity of this method.
2017, 66 (14): 148101.
doi: 10.7498/aps.66.148101
Abstract +
Lithography is one of most important technologies for fabricating micro- and nano-structures. Limited by the light diffraction limit, it becomes more and more difficult to reduce the feature size of lithography. Surface plasmon polariton (SPP) is due to the interaction between electromagnetic wave and oscillation of free-electron on metal surface. For the shorter wavelength, higher field intensity and abnormal dispersion relation, the SPP would play an important role in breaking through the diffraction limit and realizing nanolithography. In this paper, we theoretically and experimentally study the optical nonlinear effect of SPP (two-SPP-absorption) in the photoresist and its application of nanolithography with large field. First, the concept and features of two-SPP-absorption are introduced. Like two-photo-absorption, the two-SPP-absorption based lithography is able to realize nanopatterns beyond the diffraction limit: 1) the absorption rate quadratically depends on the light intensity, which can further squeeze the exposure spot; 2) the pronounced power threshold provides a possibility for precisely controlling the linewidth by manipulating the illumination power. Nevertheless, unlike the two-photo-absorption lithography which focuses light onto a single spot and scans point by point, the two-SPP-absorption method could obtain the subwavelength field pattern by simply illuminating the plasmonic mask. The subwavelength field pattern due to the short wavelength of SPP would further result in the overcoming-diffraction-limit resist pattern. Besides, the highly concentrated SPP field leads to the strong electromagnetic field enhancement at the metal-dielectric interface, which could reduce the input power density of exposure source or enlarge the exposure area. Then the two-SPP absorption is realized under the illuminations of femtosecond lasers with vacuum wavelengths of 800 nm and 400 nm. Meanwhile, the interference periodic patternis realized and it is observed that the linewidth could be adjusted by controlling the exposure dose. The minimum linewidth of resist pattern is only one tenth of the vacuum wavelength. By utilizing the features of two-SPP-absorption, namely shorter wavelength, enhanced field and threshold effect, the lithography field could be of millimeter size, which is about four to five orders of magnitude larger than the characteristic size of nanostructure. Therefore, this two-SPP-absorption scheme could be used for large-area plasmonic lithography beyond the diffraction limit with the help of various plasmonic structures and modes.
2017, 66 (14): 148705.
doi: 10.7498/aps.66.148705
Abstract +
Enormous efforts have been made to manipulate the light-matter interactions, especially in sub-diffraction-limited space, leading to miniaturized and integrated photonic devices. In physics, an elementary excitation, called polariton, which is the quantum of the coupled photon and polar elementary excitation wave field, underlies the light-matter interaction. In the dispersion relation, polaritons behave as anti-crossing interacting resonance. Surface polaritons provide ultra-confinement of electromagnetic field at the interface, opening up possibilities for sub-diffraction-limited devices, and various field enhancement effects. In the electromagnetic spectra, terahertz (THz) regime was called THz gap before the 1990s, but has now been thrust into the limelight with great significance. This review is devoted to the emerging but rapidly developing field of sub-diffraction-limited THz photonics, with an emphasis on the materials and the physics of surface polaritons. A large breadth of different flavours of materials and surface polaritonic modes have been summarized. The former includes metallic, dielectric, semiconductor, two-dimensional (2D) materials, metamaterials, etc.; the latter covers surface phonon-, plasmon-, and hybrid polaritons. In the THz regime, 2D surface plasmon polariton and artificial surface phonon polaritons offer more attractive advantages in ability to obtain low-loss, tunable, ultracompact light-matter modes.
