In this paper, a tunable extrinsic chiral metasurface with giant circular dichroism in the optical frequency band is proposed. The unit cell of the metasurface consists of two symmetrical square silver split rings and a GST film sandwiched between them. Compared with the works reported in the existing literature, the CD of the metasurface is larger and the tuning range is wider. In the frequency range of 50 THz~300 THz, the CD of this extrinsic chiral metasurface is up to 0.85 when the GST is amorphous. Due to the large loss of the crystalline GST, the extrinsic chiral response is weakened and the maximum CD is 0.52 in the crystalline state. The GST switches between two phase states (amorphous-crystalline) and enables the frequency tuning range to reach about 70 THz. Further studies have shown that the CD can be tuned by changing the incident angle and the geometric parameters of the GST layer. When θ = 0° and φ= 0°, the CD is zero in all frequency bands, which means when the wave is incident normally, there is no intrinsic chiral characteristics when the wave is incident normally. The electric field distributions at the resonance point in different phase states are also investigated. This work provides a way to realize devices such as efficient polarization modulation devices, circular polarizers, and polarization filters in the optical frequency band.We propose a metasurface with tunable circular dichroism extrinsic chiral based on the phase change material Ge2Sb2Te5 (GST). The metasurface is composed of two symmetrical square silver split ring resonators and a GST intermediate layer arranged periodically. Combined with the oblique incident light, this metasurface is capable of achieving electromagnetic properties similar to that of the chiral structure. Numerical simulation results show that in the frequency range of 50 THz~300 THz, the maximum circular dichroism (CD) of the metasurface is 0.85 in the amorphous state of GST and 0.52 in the crystalline state of GST. When the GST switches between two states (amorphous-crystalline), a tunable frequency of about 70 THz is achieved. Compared with the reported works, the CD of this metasurface is larger and the tuning range is wider. By studying the electric field distribution, the origin of the circular dichroism is explained; and the effects of incidence angles and structural parameters on the CD of this metasurface are investigated. This research will have potential applications in efficient polarization modulation devices, circular polarizers, and polarization filters in the optical frequency band.

With the rapid development of lithium-niobate-on-insulator (LNOI) thin film technology and related surface micro-nano manufacturing technology in recent years, a series of high-quality and high-performance photonic functional devices on lithium niobate chip have been realized, such as compact modulators with ultra-high performance, broadband frequency combs, as well as high-efficiency optical frequency converters and single-photon sources. Great progress has been made in nonlinear optical frequency conversion, electro-optic modulation and optical passivity. In this paper, we briefly introduce several micro-nano processing technologies that have the potential to produce high-quality lithium niobate metasurfaces, and summarize the recent research progress in optical frequency conversion, electro-optic modulation, optical passivity and other aspects of lithium niobate metasurfaces, and prospected the potential research directions in the field of micro-nano optics.As derivatives of 3D metamaterials, artificial metasurface structures with sub-wavelength thicknesses can flexibly manipulate light-matter interactions in a compact manner, which is beneficial for the fabrication of multi-functional and ultracompact photonic devices. Therefore, metasurface structures are of great significance for micro-nano photonics and integrated photonics. The ferroelectric crystal lithium niobate is regarded as one of the most promising multifunctional integrated photonic platforms due to its wide transparent window spanning from the visible to the mid-infrared band as well as large nonlinear optical and electro-optic coefficients. In recent years, research on integrated photonics devices based on lithium-niobate-on-insulator (LNOI) thin films has also been developed rapidly. In this paper, several micro-nano processing technologies that have the potential to prepare high-quality lithium niobate metasurfaces are summarized. At the same time, the research progress of lithium niobate metasurface structures in recent years is introduced, and its future research directions are prospected.

Overview: Surface plasmon polaritons (SPPs) directional excitation is the basis for the development of on-chip integrated photonic systems, such as the super-resolution imaging, the nano lithography, and the high sensitivity biosensors. It is difficult for traditional directional structures, such as prisms, nano slits and grooves to satisfy the accurate phase-matching condition required for SPPs excitation, resulting in an unsatisfactory coupling efficiency, a low extinction ratio, and high loss and noise. In recent years, the directional excitation of surface plasmon polaritons based on the catenary metasurface began to be valued because of the continuous and linear geometric phase control ability. However, the research of SPPs directional excitation with linearly polarized light is less than that of circularly polarized light. In this paper, all excitation is explained according to the multi-level scattering theory and the Huygens-Fresnel principle. The simulation results show that at the resonance wavelength (836 nm), the SPPs directional excitation is effectively achieved due to the stronger electric dipole excited by SPPs resonances. At the same time, in the effective bandwidth range (820 nm~870 nm) of unit catenary nanoparticle, the electric dipole scattering intensity and spectral extinction ratio curve both show the trend of increasing first and then decreasing. Therefore, there is a positive correlation between the electric dipole scattering intensity and spectral extinction ratio curve. The above analysis shows that the dipole intensity is the main factor affecting the directional extinction ratio. The designed directional excitation of surface plasmon polaritons with linearly polarized light based on the catenary nanoparticle metasurface is effective. The peak extinction ratio is up to 27 dB (corresponding to the incident wavelength of 820 nm), and the bandwidth above 10 dB is about 47 nm (798 nm~845 nm). Therefore, these results are helpful for the research and development of the catenary multifunctional devices which has great potential in the design of SPP directional excitation devices. Besides, it is also a planar integrated device, which can provide new ideas for chip-level photonic device or system design. Moreover, the method in this paper is also suitable for circularly polarized light, therefore it can be referenced in the design of other multi-functional integrated photonic devices such as multi-directional beam splitters and polarization detectors.Surface plasmon polaritons (SPPs) directional excitation is the basis for the development of on-chip integrated photonic systems. And SPPs directional excitation based on the catenary metasurface is a hot and frontier field in recent years, however, the SPPs directional excitation with linearly polarized light is less than that of circularly polarized light. In this paper, we design a catenary nanoparticle metasurface to realize the SPPs directional excitation with linearly polarized light. The spectral extinction ratio curve and electric field distribution under the incident of x-polarized light are calculated with the finite difference time domain. The physical mechanism of SPPs directional excitation is explained according to the multi-level scattering theory and the Huygens-Fresnel principle. The simulation results show that the SPPs directional excitation with linearly polarized light based on the catenary nanoparticle metasurface is effective, and the peak extinction ratio is up to 27 dB (corresponding to the incident wavelength of 820 nm), and the bandwidth above 10 dB is about 47 nm (798 nm~845 nm). Therefore, these results are helpful for the research and development of the catenary multifunctional devices.

Overview: Spectral imaging detection technology has been widely used in many fields, such as remote sensing, medical diagnosis, food safety testing, environmental monitoring, and other fields due to its advantages of accurate and non-contact detection. However, conventional spectral imaging systems usually suffer from the large volume, long sampling time, and low energy efficiency. Metasurface is an artificial two-dimensional material that can flexibly control the amplitude, phase and spectrum of electromagnetic waves. Metasurfaces have been used in spectral detection, holography, metalens, and other fields due to its compact structure and the capacity to flexibly control the electromagnetic waves. Benefiting from the advantages of small size, compact structure, and easy integration, miniature spectral detection technologies based on metasurfaces have been widely studied in recent years. The miniature spectral detection systems usually utilize the broadband spectral properties of metasurfaces and compressive sensing algorithms to achieve computational spectral imaging detection with lightweight. However, the existing designs of the metasurfaces-based miniature spectral detection system usually lack the quantitative analysis of the relationship between the average correlation values of the metasurfaces transmission spectra and the reconstruction quality. The random selection method used in the existing design process cannot guarantee the optimal reconstruction quality. Different from the traditional methodology of using the maximum linear independence criterion to select the broadband filters, this paper quantitatively analyzes the relationship between the average correlation value of the metasurfaces transmission spectra and reconstruction quality, and proposes a methodology for miniature spectral detection based on metasurfaces, which provides a route for the subsequent design and optimization of the metasurfaces. In order to verify the advantages of the proposed methodology, ten broadband spectra and image spectra were selected from many spectra. Compared with the random selection design methodology, the proposed methodology can improve the reconstruction fidelity of broadband spectral and image signals. The fidelity of the broadband spectral reconstruction can be increased by 13.17%, and the reconstruction fidelity of the image spectral signals has also been improved to a certain extent. In addition, this paper also verifies the spectral properties of the metasurfaces-based miniature spectral detection technology, showing that the system has good reconstruction effect for broadband, narrowband and image spectral signals, and has the advantages of compact structure and small volume.Benefiting from the advantages of small size, compact structure, and easy integration, miniature spectral detection technologies based on metasurfaces have been widely studied in recent years. However, the existing designs of the metasurfaces-based miniature spectral detection system usually lack the quantitative analysis of the relationship between the average correlation values of the metasurfaces transmission spectra and the reconstruction quality. The random selection method used in the existing design process cannot guarantee the optimal reconstruction quality. This paper quantitatively analyzes the relationship between the average correlation value of the metasurfaces transmission spectra and reconstruction quality, and proposes a design methodology for miniature spectral detection based on metasurfaces. In addition, this paper also verifies the spectral properties of the metasurfaces-based miniature spectral detection technology. Compared with the random selection design methodology, the proposed methodology can improve the reconstruction fidelity of broadband spectral and image signals.

Overview: Metasurface is a new kind of artificial two-dimensional material. Its working principle is to flexibly control the amplitude, phase and polarization of the incident electromagnetic wave by using the local interaction between the subwavelength scale unit cell and electromagnetic wave. Compared with traditional optical devices, devices based on metasurfaces have the advantages of compact structure, diverse functions, and easy integration. Therefore, metasurface has become a research hotspot in optics and photonics. At present, the electromagnetic manipulation devices based on the metasurfaces have achieved many novel functionalities, such as perfect absorption, anomalous deflection, focused imaging, electromagnetic cloak, and high efficiency holography. However, there are still some key problems to be solved in this field such as low working efficiency and narrow bandwidth. In recent years, the emergence of catenary electromagnetics provides new ideas and methods to solve these problems. In fact, catenary was first used in engineering and architecture to describe the shape of a soft rope suspended under the uniform gravitational force between two horizontal points. The use of catenary equations to solve problems in the field of electromagnetism has only recently been discovered by researchers. In this paper, we proposed a metasurface absorber based on a twisted catenary structure in the near-infrared band. The local electric field enhancement effect of the structure is different when the incident electromagnetic wave is with opposite spins, which can achieve efficient chiral selective absorption. The simulation results show that the circular dichroism is larger than 78% at the working wavelength. At the same time, the designed structure also has good angular stability, and can still get larger than 60% circular dichroism absorption in the case of oblique incidence at different azimuth angles. Besides, a possible method of information encryption using this kind of structure is proposed. Different information can be read by controlling the handedness of incident electromagnetic wave. This work further enriches the content of catenary electromagnetics, and has certain research value in the fields of chiral imaging and chiral sensing.As a kind of artificial two-dimensional material, metasurfaces have drawn wide attentions in recent years due to their ultra-thin profile and flexible electromagnetic manipulation capability. Therefore, how to further improve the working efficiency of metasurface devices has become a hotspot in this field. Catenary electromagnetics as an emerging metasurface design principle provides new ideas and methods for designing efficient metasurfaces. Here, we proposed a metasurface absorber based on twisted catenary structure that can achieve efficient spin-selective absorption. The simulated results indicate that the perfect absorption can be achieved for left-handed circularly polarized incidence at the working wavelength, while the absorption for right-handed circularly polarized incidence is below 22%. The corresponding circular dichroism is larger than 78%. Besides, the physical mechanism for the chiral absorption is analyzed and a promising application for information encryption is also discussed. This work may find potential applications in chiral imaging and chiral sensing.

Overview: Image edge extraction is a widely used and rapidly developing technology, playing an important role in medical imaging, enhanced vision, automatic driving and other fields. In recent years, there has been growing interest in developing miniature metasurface devices to obtain image edge information. Currently, it has been reported that discrete metasurface edge detection devices are used to obtain image edge information, but discrete metasurfaces often maintain a high energy efficiency only near the preset wavelength, and the energy efficiency decreases when deviating from the preset wavelength, which will limit the operating bandwidth of the metasurface optical computing device. Here, an optical differential device is designed by using a metasurface composed of quasi-continuous nanostrips to realize one-dimensional images edge detection. By changing the spatial orientation of quasi-continuous nanostrips, the device achieves geometric phase in the range of 0~2π, and maintains high energy efficiency over a wide wavelength range. The optical path system consists of two linear polarizers and two lenses with the same focal length, of which two lenses are placed in a confocal position to form a classical 4f optical system. The designed quasi-continuous metasurface edge detection device is placed on the Fourier plane of the 4f optical system. The original image is located on the object plane of the 4f optical system (at the front focal plane of the lens 1), and the object edge information is finally obtained on the image plane of the 4foptical system (at the rear focal plane of the lens 2). The simulation results show that the designed sample can achieve high average energy efficiency edge detection in the whole visible and near-infrared bands. Specifically, the quasi-continuous meta-device can obtain a clear image of object edge in the wavelength range of 400 nm~1000 nm, the energy efficiency of the device reaches 90.27% at the wavelength of 600 nm, and the average energy efficiency is 64.57% at the wavelength of 400 nm~1000 nm. Compared with the traditional edge detection devices based on discrete metasurface, the quasi-continuous devices have higher broadband average energy efficiency. Hopefully, this work enjoys many research merits in signal processing, optical communication and machine vision.In this paper, we design an optical differential device based on quasi-continuous metasurface and realize one-dimensional edge detection of an optical image. By changing the spatial orientation of quasi-continuous nanostrips, the device achieves geometric phase in the range of 0~2π, and maintains high energy efficiency over a wide wavelength range. The simulation results show that when the illumination wavelength increases from 400 nm to 1000 nm, the quasi-continuous meta-device can achieve clear images for the target edge. The maximum energy efficiency is 90.27% (the incident wavelength is 600 nm) and the average energy efficiency is 64.57% (the incident wavelength changes from 400 nm to 1000 nm). It can be expected that the proposed method can promote the application of quasi-continuous metasurface in image information processing and ultrafast optical computation.

Overview: Photonic integrated circuits (PIC) serve as an essential and promising candidate to eventually replace electronic circuits for the next-generation information processing. However, traditional PIC devices based on optical waveguides are usually bulky and lack full control at the subwavelength scale to achieve arbitrary wavefront-shaping functionalities. Recently, the invention of on-chip metasurface promotes the connection between guided and free-space optics and realizes the arbitrary conversion of guided waves and free-space light. As a new type of on-chip nanophotonic device, the introduction of metasurface onto the optical waveguide has made significant progress and exhibited multi-functional conversion from the guided waves to free-space, including directional beam-steering emitters, mode-conversion, on-chip lensing, optical router, and on-chip holography, etc. These on-chip nanophotonics devices provide new avenues for photonic chip-scale devices and miniature on-chip systems. For instance, meta-holography is an emerging and universal strategy based on engineered nanoantennas array to construct an optical-field image. However, on-chip far-field holographs are limited for realizing multiplexing for multiple directions due to a lack the arbitrary-encoding capability because their detour phases are complementarily related when the source propagates and excites the on-chip array from either positive or negative direction. Here, we propose and experimentally demonstrate a quad-fold multiplexed far-filed holographic display optics device based on an on-chip metasurface. This optics device is composed of silicon nanopillar arrays on top of a planar waveguide of Si3N4, in which a relatively thick layer of silica serves as the bottom cladding substrate. By mixing the detour phase and Pancharatnam-Berry (PB) phase, the on-chip metasurface could couple the guided waves into free space in circular polarization. The phase degeneracy in the positive and negative directions could be decoupled by selecting the desired circular polarization. Subsequently, utilizing a simulated annealing phase optimization algorithm to optimize the phase required by holograms and the multiplexing technology of on-chip directional, we achieved a quad-fold multiplexed far-field holographic display with independent encoding capability. Eventually, to verify the on-chip quad-fold multiplexed holography performance, we fabricated an on-chip metasurface sample by the conventional electron-beam lithography technique and the reactive ion etching processing. Through end-fire coupling from the laser source at λ = 650 nm into the on-chip metasurface sample along ±x/±y - directions, the far-field holographic images of the four letters (“A”, “B”, “C”, and “D”) multiplexing are successfully observed at their corresponding areas. The method proposed here opens up new prospects for the multifunctional integration of on-chip metasurfaces and provides an alternative approach for integrated optical communication with high information storage capacity.The on-chip metasurface is introduced into integrated optical waveguides to achieve arbitrary modulation of guided waves, which provides a convenient and versatile platform for the conversion between guided waves and free-space functions. Despite previous explorations in on-chip holography demonstration, it still faces critical challenges to expand the encoding freedom and multiplexing. Here, we propose and experimentally demonstrate a quad-fold multiplexed holographic display optics device based on an on-chip metasurface. By mixing the detour phase and Pancharatnam-Berry (PB) phase, the on-chip metasurface couples the guided waves into free space in circular polarization, destroying the phase degeneracy that exists in the wavevector directions with only the detour phase. Moreover, by utilizing simulated annealing phase optimization algorithm and multiplexing, we achieved a quad-fold multiplexed far-field holographic display with independent encoding capability. The proposed method in this paper opens up a new prospect for multifunctional integration of on-chip metasurfaces and provides an alternative approach for integrated optical communication with high information storage capacity.

Overview: As an ideal 3D display technology, holography can reconstruct the wavefront of the whole light wave, and can provide all the 3D depth cues required by the human eyes, including binocular parallax, motion parallax, accommodation, occlusion, etc. Due to the limitation of the modulation principle, DMD and most SLM cannot optically reconstruct the complex amplitude of a wavefield, resulting in partial information loss and complex wavefront calculation. At the same time, the two devices have a pixel size larger than 6 μm, which is much larger than the wavelength of visible light. The limitation of large pixel size and modulation principle brings many disadvantages, such as narrow field of view, twin-image, narrow band, and multi-order diffraction, which greatly restrict the development of CGH. As a new class of light field modulators, metasurface can control the amplitude, phase, polarization and dispersion of the light simultaneously by optimizing the design and arrangement of the elements. Thanks to the previous exploration of micro-nano manufacturing technology and materials for metasurface, the size of the unit cell can be reduced to the order of sub-wavelength. According to the grating equation, the smaller the pixel size is, the larger the diffraction angle is. Therefore, metasurface can provide a diffraction angle close to 90°. As the loading medium of holograms, metasurface meets the requirements of holograms for high-precision and complex light field modulation and has the advantages of high design freedom, high spatial resolution, low noise, broadband and so on, providing a solution to some problems currently faced by CGH. In this paper, the basic process of designing meta-holography devices is discussed. Furthermore, the basic concepts and development of static meta-holography are introduced based on the principles of metasurfaces, including phase modulation, amplitude modulation, complex-amplitude modulation, and nonlinear modulation. However, such static meta-holography devices cannot change the display patterns after design and manufacture, which is inconsistent with the rapidly changing real world and requirements of diverse functions, limiting its applications. Therefore, the two methods of realizing dynamic meta-holography are introduced in detail. Finally, the micro-nano fabrication technologies for metasurface are discussed. In conclusion, this paper presents the design, principle, development, and manufacturing implementation of meta-holographic devices in an all-around way, and puts forward problems and possible solutions for the development of meta-holography at present.The pursuit of real-time, full-color, three-dimension (3D), and dynamic display has inspired a rich body of industrial and academic research. With the introduction of "Metaverse", there is an increasing demand for high-performance 3D display devices and technologies. Holographic technology is an ideal approach for future naked-eye 3D display. However, traditional dynamic holographic devices have brought many shortcomings such as small field of view (FOV) and limited information capacity, which hinder the practical applications. As a new class of light field modulator, metasurface is expected to achieve remarkable breakthroughs in the field of holographic display with the advantages of their small pixel size and the emerging ability to manipulate light. This paper gives an overview of the development of meta-holography from four aspects: the design strategy, the modulation principle, the methods for realizing dynamic display and the micro-nano fabrication technologies for optical metasurface. We finally include a brief discussion of the future direction in this field.