Special issue-Metasurface empowered manipulation of wavefront
2020, 49(9): 20201028.
doi: 10.3788/IRLA20201028
In recent years, metasurface has received extensive attention in the field of classical light control, and excellent results have been obtained. At the same time, the application of metasurface in nonlinear optics and quantum optics has also attracted more and more interest. The basic principles and applications of nonlinear metasurface and quantum metasurface were introduced, and the related reports in recent years were summarized, including the harmonic generation and enhancement, the relationship between harmonic generation and symmetry, the nonlinear phase control and holography, and the generation, measurement and manipulation of entangled photon based on metasurface. Finally, the potential application of metasurface in these two fields were prospected.
2020, 49(9): 20201029.
doi: 10.3788/IRLA20201029
Exceptional points are special points in non-Hermitian systems, and there are many novel physical phenomena in the parameter space near the exceptional points. In recent years, metasurface has been a popular topic in physics. A large number of devices with superior performance have been designed based on metasurface platform. The appearance of the metasurface provides an easy platform for the study of exceptional points. By precisely controlling the structural parameters of the metasurface, it is convenient to study the parameter space of the exceptional points. The research on exceptional points in non-Hermitian metasurface also provides a foundation platform for studying new laws of physics. Firstly, the basic theory of exceptional points and the exceptional point in metasurface was introduced. Secondly, the recent research on the exceptional points in metasurface was introduced. Finally, the current problems needing to be solved were analyzed and the development of the field in the future was prospected.
2020, 49(9): 20201030.
doi: 10.3788/IRLA20201030
Polarization is one of the inherent characteristics of light, but the polarization information of electromagnetic waves is lost by traditional intensity and spectral detection technology. At the same time, not only the devices and technologies based on polarimetry have the problem of limited field of view, but also the measuring systems are complicated. In this paper, a compact large field-of-view polarimeter was designed based on dielectric metasurface, which realized the detection of the angle and polarization state of the incident light. The device was composed of 2×2 quadratic phase metasurfaces, each of which realized the symmetry transformation for a specific polarization, that is, the rotational symmetry of the incident angle was converted into the translational symmetry of focus in the focal plane. The theory of the symmetry transformation of the quadratic phase made it possible to characterize the angle of incidence by measuring the offset of the focus over a wide angle range (−40°—+40°). On this basis, the influence of oblique incidence on the measurement of Stokes parameters was elaborated, and the modified Stokes formula was obtained. The Stokes parameters of the incident light can be calculated by utilizing the intensities of the four focal points and the modified Stokes formula. The measured Stokes parameters agree well with the theoretical values, when the field-of-view is 0°, 20°, and 40°.
2020, 49(9): 20201031.
doi: 10.3788/IRLA20201031
Metasurface is an artificially ultrathin material with two-dimensional nanostructure array, which can achieve flexible modulation on amplitude, phase and polarization of light field in a sub-wavelength scale, providing a new possibility for the miniaturization and integration of modern optical devices. With the development of optical imaging, display and so on, the requirement of miniaturized optical devices with high efficiency in visible light band is becoming conspicuous. In recent years, optical metasurfaces fabricated by dielectric materials with high refractive indices and low losses have been extensively studied, showing application prospects in achromatic metalens, polarization-dependent holographic display, et al. Around the research on the metasurface of dielectric, firstly, the generalized Snell's law and the modulation principle of nanostructures in dielectric metasurface on amplitude, phase and polarization of light field were introduced. Then the research progress of dielectric metasurfaces in holographic display and structural light field generation, based on single- and multi-parameters modulation of light field was reviewed. At last, the possible challenges and prospects of dielectric metasurfaces were discussed.
2020, 49(9): 20201032.
doi: 10.3788/IRLA20201032
The design of large scale, high numerical aperture, and broadband achromatic flat lens is a bottleneck of imaging technology and also a big challenge in metalens researches in recent years. The major reason is that there are some internal constraints between these parameters. In this paper, considering both the group dispersion theory and the phase distribution of the flat lens, the semi-quantitative relationship between these parameters was derived. Then, the achromatic flat lenses (including metalenses and multi-level diffractive lenses) with different parameters were designed by using directly binary search and topology optimization. It was found that under the condition of maintaining an efficiency of 80%, when doubling the size of flat lens (i.e., the diameter), the numerical or achromatic bandwidth was cut in half, and the thickness of lens increased linearly with lens scale. The results definitely show that these two types of flat lenses have the same internal constraint relations in parameters, that is, as that the lens size has negative correlation with the numerical aperture and achromatic bandwidth, while positive correlation with the thickness of the lenses. This result is in coincidence with the theoretical prediction.
2020, 49(9): 20201033.
doi: 10.3788/IRLA20201033
Metasurface is an ultrathin planar device composed of artificial microstructures, which can be used to manipulate the amplitude, phase, and polarization of electromagnetic (EM) waves. Metasurface has the advantages of small volume, light weight, highly integrated, flexible manipulation of EM waves, so it plays an important role in the field of EM wave spectrum and wavefront modulation. In this paper, the research progresses of metasurface for wavefront modulation in the terahertz (THz) waveband were reviewed. The amplitude and phase modulation mechanisms of three kinds of microstructure units in the metasurface, including the microstructure based on Pancharatnam-Berry (PB) phase, localized surface plasmon resonance (LSPR) and Mie resonance were summarized, and the methods for realizing metasurface with high efficiency were discussed. After that, the pure phase and complex amplitude modulation methods for designing the wavefront modulation metasurface were introduced. Specifically, the typical functions, including single function and multifunction and tunable function, of the wavefront modulation metasurfaces in the THz waveband were reviewed. In the early research, metasurfaces were used to realize beam focusing, beam deflection, holographic imaging, and special beam generation such as vortex beam, Airy beam, and Lorentz beam in the THz region. In order to improve the utilization of a THz component, multifunctional metasurfaces, such as metasurfaces with polarization and wavelength multiplexing were proposed. With the requirement of dynamic control of the THz wavefront, some active metasurfaces were proposed and demonstrated. There were two kinds of active metasurfaces. One of the active metasurfaces was formed by combining the metasurface with semiconductor or phase transition materials, and the other was the all-optical metasurface formed by a silicon wafer with pump beam. The all-optical metasurfaces can be reused without reprocessing. The THz wavefront can be modulated dynamically by adjusting the image of the metasurface projected on the silicon wafer. Thus, the all-optical metasurface had the ability to dynamically control the beam steering and focusing, and it can be applied in THz communication, THz radar and other fields. At the end, the development trend and application prospects of the metasurfaces for wavefront modulation in the THz waveband were discussed.
2020, 49(9): 20201034.
doi: 10.3788/IRLA20201034
Metasurface is an artificial layered material with a subwavelength or wavelength-scale thickness. By adjusting the size, shape, orientation angle, displacement, etc. of the metasurface nanostructures, dimensions of electromagnetic wave including frequency, amplitude, phase, polarization, wavelength, etc. can be flexibly and effectively controlled. Metasurface has various characteristics including the ultra-thin thickness, broadband, low loss, easy processing, flexible design and powerful functionalities. This paper reviewed metasurfaces capable of one-dimensional, two-dimensional, multi-dimensional, and active light field manipulations and their applications in information encryption and anti-counterfeiting. An outlook was given for the future development trend of multi-dimensional metasurfaces. Compared with the traditional information encryption and anti-counterfeiting technology, the metasurface has the superior advantages in sub-wavelength pixels, precise controlling and ultra-secure factor in information encryption and anti-counterfeiting. Its bright future to replace the traditional information encryption and anti-counterfeiting technology was envisioned, and its broad application prospects were underpinned.
2020, 49(9): 20201035.
doi: 10.3788/IRLA20201035
The metasurface is composed of carefully arranged sub-wavelength units in a two-dimensional plane, which provides a new paradigm for designing ultra-compact optical elements and shows great potential in miniaturizing optical systems. In less than ten years, metasurfaces have caused extensive concern in multidisciplinary fields due to their advantages of being ultra-light, ultra-thin and capable of manipulating various parameters of light waves to achieve multi-functional integration. However, in the optical band, high-degree-of-freedom, aperiodic, and densely arranged metaunits put forward many extreme parameter requirements for fabrication, such as extremely small size, extremely high precision, high aspect ratio, difficult-to-process materials, cross-scale, etc. This poses a great challenge for metasurfaces from laboratory to practical applications. Here, the principles, characteristics and latest developments for micro-nano fabrication of metasurfaces in recent years were summarized, including small-area direct writing methods, large-area template transfer methods, and some emerging fabrication methods. Finally, the current challenges and future development trends of metasurface fabrication were summarized and prospected.
2020, 49(9): 20201036.
doi: 10.3788/IRLA20201036
Computer-generated holography and nanoprinting are two typical applications of optical metasurfaces. Recently, merging holography and nanoprinting into a multifunctional metasurface becomes an emerging research hotspot, which has prospective applications in multi-folded anti-counterfeiting, information decoding and multiplexing, multi-channel image display and VR/AR, etc. In this paper, based on the features of metasurface-based nanoprinting and computer-generated holography, the advances in the research of merging them were classified and characterized in detail. Specifically, the merging methods were discussed in detail which included orthogonal-polarization multiplexing scheme, in-plane arrangement scheme, multilayer-stacking scheme, simultaneous spectrum and phase control scheme, complex amplitude modulation scheme, orientation degeneracy scheme. The future development of multifunctional metasurfaces was prospected.
2020, 49(9): 20201037.
doi: 10.3788/IRLA20201037
Metasurfaces refer to the optical nanoantenna arrays composed of subwavelength structures. Nanoantennas can have resonances under appropriate excitation conditions, to achieve near-field enhancement, and thus enhancing nonlinear optical effects. Compared with conventional nonlinear optical crystals, metasurfaces are more compact, with the possibility for on-chip integration. Due to the subwavelength light-matter interaction length, for applications such as nonlinear harmonic generation, metasurfaces does not suffer from the limitation of phase-matching. In addition, metasurfaces own the subwavelength spatial resolution. By designing and arranging the meta-atoms, metasurfaces can realize flexible control of the phase, polarization, and amplitude of the harmonic wave. Recent works on nonlinear metasurfaces for applications in optical frequency conversion, nonlinear wavefront control and ultrafast all-optical modulation were reviewed, the challenges and perspectives for the practical application of metasurfaces were presented.
2020, 49(9): 20201038.
doi: 10.3788/IRLA20201038
The invisibility cloak as a longstanding fantastic dream for humans is now within the realm of possibility, thanks to the development of metamaterials. Transformation-optics-based invisibility cloaks have been proposed and realized in many frequency ranges by utilizing gradient-index metamaterials. However, due to the large size of device and difficulty in fabrication, transformation-optics-based invisibility cloaks are significantly limited in practical applications. Recently, metasurfaces as the 2D counterpart of metamaterials have attracted tremendous interests because of its thin thickness and strong capability in manipulating the electromagnetic waves. Ultrathin invisibility cloaks based on metasurfaces release the demand on bulky sizes and extreme parameters, thus promoting further development of invisibility cloaks. This review overviewed recent progress in ultrathin invisibility cloaks based on metasurfaces, focusing particularly on the working principles, implementation methods, advantages and disadvantages. Finally, some advice was put forward on the trends of this fast-developing research field.
2020, 49(9): 20201039.
doi: 10.3788/IRLA20201039
With the development of nanophotonics, optical structures, such as optical microcavity, waveguide structure, photonic crystal, subwavelength gratings and metasurfaces, can realize light transmission and manipulation at nanoscale, which promotes the development of optical integration. Subwavelength grating has been widely studied by scientists because of its simple structure and low cost. It has gradually formed a mature theoretical system of grating analysis model when applied to various optical devices. Combined with the coupling of periodic structure and scattering modulation characteristics of meta-atoms, the metagrating derived from subwavelength gratings can improve the efficiency by using periodic Bragg scattering, thus avoiding the efficiency reduction and energy loss caused by the phase discretization. Scientists have studied and designed metagratings, and more physical phenomena and applications have been explored. In this paper, the basic theory, design and application of subwavelength gratings and metagratings were summarized. Based on the basic principle, the characteristics of subwavelength gratings and metagratings were discussed, the theoretical and unit design methods were also outlined and their applications in biosensing, spectral control of filter and absorption film were introduced. Finally, the future development was prospected.
2020, 49(9): 20201040.
doi: 10.3788/IRLA20201040
The rapid development of nanotechnology has promoted the processing and manufacturing of micro-nano structures, scientific research and industrial applications. The investigation on optical properties of micro-nano structures has recently been one of the hotspots in the field of optics, which has driven emerging disciplines such as nanophotonics, surface plasmonic optics, metasurface/metamaterial optics, topological photonics, and non-Hermitian optics. It provides the important technical fundamentals for full control of light with high precision. This article focused on the edge detection in all-optical image processing. The fundamentals, principles, technologies and applications of micro-/nano-scale structures and devices were discussed to realize optical mathematical computing (such as differential, convolution), followed by a detailed prospect about its future applications in ultrafast image processing, high-contrast microscopic imaging, convolutional neural networks and intelligent optics.
2020, 49(9): 20201041.
doi: 10.3788/IRLA20201041
Polarization is an inherent property of light. By deconstructing the relation between the incident and transmitting polarization states, the crucial information about the composition and structure of the interacting materials can be obtained, and thus polarization information shows high research value and application potential. In recent years, research areas, such as polarization imaging and polarization information, are flourishing and attracting wide attention of scholars all over the world. A key of these research fields is the acquisition of polarization information with high efficiency. However, conventional methods for acquisition of polarization information have various shortcomings, restricting the development of polarization information. Metasurface technology provides people with an opportunity to artificially change the phase, amplitude and polarization of light wave, with characteristics of miniaturization and integration. Firstly the polarization-information acquisition and the circular-polarization detector based on Archimedes spiral structure were introduced. Then the progress of polarization-information acquisition based on metasurface in recent years, including metal metasurface polarization detector, all-dielectric metasurface polarization detector, and polarization imagings was summarized. At last, the research status and development trend of the polarization-information acquisition technology based on metasurface were discussed.
