Subwavelength Artificial Structures Research Articles

Terahertz Metasurfaces for Polarization Manipulation and Detection: Principles and Emerging Applications

AbstractTerahertz frequencies locating between microwave and infrared regions can record and process optical information for next‐generation information technology such as 6G communication. Metasurfaces, as emerging planar optical components built with micro‐ or nanostructures, enable precise control, and manipulation of electromagnetic waves from near fields to far fields. These subwavelength artificial structures can be engineered to exhibit specific polarization responses, thus holding significant potential for applications in polarization detection. In the terahertz (THz) region, polarization information is crucial for identifying and analyzing the properties of materials in the terahertz. In this review, we focus on recent advances in terahertz metasurfaces for polarization manipulation and detection, including principles and emerging applications. New polarization detection methods such as polarization state conversion, polarization‐selective absorption, polarization‐selective focusing, and vector beam construction are discussed. Finally, it is concluded with a few perspectives on emerging trends and existing challenges in the fields of terahertz polarization manipulation and detection.

Metasurface-empowered high-efficiency and broadband terahertz vortex beam plates

Metasurfaces have been continuously garnering attention in both scientific and industrial fields owing to their unprecedented wavefront manipulation capabilities using arranged subwavelength artificial structures. Terahertz vortex beams have become a focus of research in recent years due to their prominent role in many cutting-edge applications. However, traditional terahertz vortex beam plates are often faced with challenges including large size, low efficiency, and limited working bandwidth. Here, we propose and experimentally demonstrate highly efficient and broadband vortex beam plates based on metasurface in the terahertz region. The experimental results well verify that the designed metasurfaces can efficiently generate terahertz vortex beams with different orbital angular momentum topological charges in the range of 0.5–1 THz. Notably, the maximum efficiency can reach about 65% at 0.5 THz. The proposed devices may play a vital role in developing vortex beams-related terahertz applications.

All-dielectric terahertz metalens using 3D-printing

Terahertz (THz) electromagnetic waves offer a new way of understanding various phenomena, owing to their non-ionizing properties resulting from their low energy, and non-destructive inspection features in dielectrics. Although THz emitters and detectors have been developed with much interest, the performance of optics in the THz band is still limited by the properties of the available materials. Metamaterials, which are the artificial subwavelength structures for implementing optical properties that do not exist in intrinsic materials, proposed as a new approach. However, their fabrication requires a complex semiconductor manufacturing process for its elaborate structure. In this study, we demonstrated the fabrication of all-dielectric THz metamaterial optics using an accessible 3D printing technology. A meta-element consisting of a square pillar exhibited full-wave phase controllability. The metalens was designed for 0.2 THz. The metalens fabricated using a low-cost LCD-type 3D printer and photopolymer resin showed good focusing performance and polarization insensitivity.

Simultaneous and independent control of phase and polarization in terahertz band for functional integration of multiple devices

The control of phase and polarization in terahertz (THz) range is very valuable for wireless communication, biomedicine, and security inspection. Recently, metasurfaces composed of sub-wavelength artificial structures are proposed for the control of THz waves. However, it is still a challenge to realize simultaneous and independent control of the wavefront and polarization distribution in the THz band, although which has been achieved in the optical band by using several metasurfaces. Here, metallic waveguide arrays (MWAs) are proposed to manipulate both the wavefront of two orthogonal polarization components and the phase difference between them so as to attain independent spatial distributions of the phase and polarization direction. By using the compact MWAs, the functional integration of multiple THz devices is attained, including the integration of a lens with a quarter-wave plate, a half-wave plate, and vector beam generators, respectively. The corresponding control efficiency can even be comparable with that of a commercial lens just for THz focusing. The functional integration of multiple devices makes the MWAs very significant in promoting the miniaturization of THz systems and reducing the insertion loss of functional devices, owing to the remarkable decrease of the number and size of required devices.

Tutorial on metalenses for advanced flat optics: Design, fabrication, and critical considerations

Metalenses comprised of artificial subwavelength structures known as meta-atoms have demonstrated abilities beyond conventional bulky optical components by modulating the phase, amplitude, and polarization of light in an ultrathin planar form factor. In this Tutorial, we present the fundamental principles and practical design procedures to exploit the abilities of metalenses, including achromaticity, high numerical aperture, and tunability. The fundamental principles include both plasmonic and dielectric meta-atoms, which require different physics to describe their light–matter interactions. In the phase modulation section, we compare the methods of physically implementing phase via meta-atoms including both the propagation and geometric phase methods. Next, we cover the recent progress of nanofabrication procedures from the perspective of the metalenses using materials such as titanium dioxide, gallium nitride, and hydrogenated amorphous silicon. We further compare the various fabrication methods with regard to the resolution, size, cost, and optical properties of fabricated metalenses. Then, we describe the critical considerations of metalenses including aberration-correction, numerical aperture, and tunability for advanced flat optics. Herein, we provide a practical guide for the design, fabrication, and critical considerations of metalenses with examples of research from early works to more recent developments.

Active wavefronts control with graphene-functionalized terahertz Metasurfaces

Metasurfaces, a new kind of artificial subwavelength structures, have exhibited unprecedented capabilities in wavefront control of electromagnetic waves. Currently, a series of active metasurfaces have been explored for engineering applications in anomalous refraction, ultrathin flat lens, holograms, and vortex beam generation, However, to date, most reported metasurfaces are mainly concentrated on active control on the polarization conversion and wavefront of circular polarized waves. Here, graphene-functionalized metasurfaces consisting of metallic double split ring resonators integrated with graphene (G-DSRRs) are proposed to actively control polarization conversion and wavefronts of linearly polarized waves at the terahertz region. By carefully designing spatially phase profile, anomalous reflection of linearly cross-polarized terahertz waves is demonstrated in a broadband range of frequencies. Moreover, the amplitude of the anomalous reflection terahertz wave can be dynamically tuned by varying Fermi energy via electrically doping graphene. Based on the tunable graphene-functionalized metasurface, several polarization conversion metasurface devices including vortex beam generators, invisibility cloaks, and metalens, are successfully designed and numerical results show that their amplitudes can be actively tuned to a significant extent by doping graphene. Therefore, such graphene-functionalized metasurfaces may have potential applications in THz telecommunications, high-resolution terahertz displays, and advanced THz imaging devices.

Classical and generalized geometric phase in electromagnetic metasurfaces

The geometric phase concept has profound implications in many branches of physics, from condensed matter physics to quantum systems. Although geometric phase has a long research history, novel theories, devices, and applications are constantly emerging with developments going down to the subwavelength scale. Specifically, as one of the main approaches to implement gradient phase modulation along a thin interface, geometric phase metasurfaces composed of spatially rotated subwavelength artificial structures have been utilized to construct various thin and planar meta-devices. In this paper, we first give a simple overview of the development of geometric phase in optics. Then, we focus on recent advances in continuously shaped geometric phase metasurfaces, geometric–dynamic composite phase metasurfaces, and nonlinear and high-order linear Pancharatnam–Berry phase metasurfaces. Finally, conclusions and outlooks for future developments are presented.

Chiroptical Metasurfaces: Principles, Classification, and Applications.

Chiral materials, which show different optical behaviors when illuminated by left or right circularly polarized light due to broken mirror symmetry, have greatly impacted the field of optical sensing over the past decade. To improve the sensitivity of chiral sensing platforms, enhancing the chiroptical response is necessary. Metasurfaces, which are two-dimensional metamaterials consisting of periodic subwavelength artificial structures, have recently attracted significant attention because of their ability to enhance the chiroptical response by manipulating amplitude, phase, and polarization of electromagnetic fields. Here, we reviewed the fundamentals of chiroptical metasurfaces as well as categorized types of chiroptical metasurfaces by their intrinsic or extrinsic chirality. Finally, we introduced applications of chiral metasurfaces such as multiplexing metaholograms, metalenses, and sensors.

Enhancing third-harmonic generation by quasi bound states in continuum in silicon nanoparticle arrays

Subwavelength artificial structures of high refractive index dielectrics provide an effective way to control and manipulate light on a nanoscale by enhancing electric and magnetic fields. This kind of structure usually has low absorption loss, but its performance is also limited by radiation loss, which will reduce the efficiency of its nonlinear response. This problem can be solved by using bound states in the continuum (BICs). The BICs are a kind of unconventional state which is in continuous domain but remains localized. They exist within a light cone and have an infinite quality factor. By combining BICs with nonlinear optics, high-<i>Q</i> resonances from quasi-BICs are used to excite and enhance the nonlinear response. The simulation shows that when the symmetry of the unit cell of the silicon nanoparticle arrays is broken, the BIC become the quasi-BIC, and the transmission spectrum will produce a high-<i>Q</i> narrow resonance valley. The resonance has polarization dependence of electric field. With the change of pump wavelength, the third-harmonic generation (THG) intensity first increases and then decreases gradually. The pump wavelength changed by several nanometers can change THG intensity by at least one order of magnitude. When the pump wavelength is adjusted to the resonance wavelength, the nonlinearity is significantly enhanced as a result of the strong field localization. The THG intensity is highly sensitive to the variation of asymmetric parameters. Only a change of 75 nm will result in a decrease of THG intensity by at least one order of magnitude. There is a third-order relationship between pump power and THG power. For the proposed structure, the factors affecting the conversion efficiency of THG include pump power, pump wavelength, polarization angle of pump light, and asymmetry parameter. When the polarization direction of electric field is along the short axis of the structure and the pump light at resonance wavelength is vertically incident to the structure with an asymmetric parameter of 0.125, the conversion efficiency of THG can be increased to ～2.6 × 10<sup>–6</sup> and the intensity of THG is increased by six orders of magnitude. The results are expected to be applied to designing the silicon-based optical nonlinear devices.

Metasurfaces enabled by asymmetric photonic spin-orbit interactions

Photonic spin-orbit interaction is an important phenomenon ignored by classical optics. In recent years, studies have found that this phenomenon can be significantly enhanced by artificial subwavelength structures and adjusted on demand. Traditional metasurfaces only support symmetric photon spin-orbit interactions, and there are limitations in conjugate symmetry, which makes it difficult to use different spin states for multifunctional integration, complex optical field regulation, information encryption, and storage. The asymmetric photon spin-orbit interaction can decouple left and right circularly polarized light, which brings new opportunities for breaking the above-mentioned theoretical and application limitations. This article first introduces the principle and realization method of asymmetric photon spin-orbit interactions, secondly introduces the representative applications and characteristics of asymmetric photon-spin-orbit interactions, and finally outlines the challenges and prospects of asymmetric photon spin-orbit interactions for future research directions.

Realization of Terahertz Wavefront Manipulation Using Transmission-Type Dielectric Metasurfaces

Metasurfaces, composed of an array of subwavelength artificial structures, have attracted great interest, owing to their high ability in locally manipulating the wavefront of electromagnetic waves. Here, we propose a dielectric metasurface based on a fused silica resonator, consisting of a rectangular-shaped bar placed in the center of a cross net-shaped structure, to manipulate the wavefront of terahertz waves. As proof of concept, several transmission-type devices for spatial modulation are designed at the target frequency of 0.14 THz, including on-axis and off-axis focusing, generation of a non-diffracting Bessel beam, and multi-focus lens. The simulated efficiencies range from 45% to 62%. This novel approach for manipulating THz wavefronts can be also used for information storage and other phase-related techniques in the rapid development of THz applications.

Few-layer metasurfaces with arbitrary scattering properties

Metasurfaces, which are planar arrays of subwavelength artificial structures, have emerged as excellent platforms for the integration and miniaturization of electromagnetic devices and provided ample possibilities for single-dimensional and multidimensional manipulations of electromagnetic waves. However, owing to the limited interactions between planar thin metallic nanostructures and electromagnetic waves as well as intrinsic losses in metals, metasurfaces exhibit disadvantages in terms of efficiency, controllability, and functionality. Recent advances in this field show that few-layer metasurfaces can overcome these drawbacks. Few-layer metasurfaces composed of more than one functional layer enable more degrees of design freedom. Hence, they possess unprecedented capabilities for electromagnetic wave manipulation, which have considerable impact in the area of nanophotonics. This article reviews recent advances in few-layer metasurfaces from the viewpoint of their scattering properties. The scattering matrix theory is briefly introduced, and the advantages and drawbacks of few-layer metasurfaces for the realization of arbitrary scattering properties are discussed. Then, a detailed overview of typical few-layer metasurfaces with various scattering properties and their design principles is provided. Finally, an outlook on the future directions and challenges in this promising research area is presented.

Metasurface optics for imaging applications

Abstract

Experimental Demonstration of Multidimensional and Multifunctional Metalenses Based on Photonic Spin Hall Effect

Metalens is one kind of two-dimensional ultrathin lenses with subwavelength artificial structures that can focus light in a compact, flexible way. However, most strategies for designing metalenses ...

Surface transformation multi-physics for controlling electromagnetic and acoustic waves simultaneously.

A multi-physics null medium that performs as a perfect endoscope for both electromagnetic and acoustic waves is designed by transformation optics, which opens a new way to control electromagnetic and acoustic waves simultaneously. Surface transformation multi-physics, which is a novel graphical method to design multi-physics devices, is proposed based on the directional projecting feature of a multi-physics null medium. Many multi-physics devices, including beam shifters, scattering reduction, imaging devices and beam steering devices, for both electromagnetic and acoustic waves can be simply designed in a surface-corresponding manner. All devices designed by surface transformation multi-physics only need one homogeneous anisotropic medium (null medium) to realize, which can be approximately implemented by a brass plate array without any artificial sub-wavelength structures. Numerical simulations are given to verify the performances of the designed multi-physics devices made of brass plate array.

Metasurface for Bending the Reflected Wave Under Oblique Incidence

Metasurfaces enable a new degree of controlling electromagnetic (EM) wave by modulating subwavelength artificial structures. In this article, a reflection-type metasurface based on the modified Pancharatnam-Berry (P-B) phase elements is proposed to manipulate the reflected EM wave under oblique incidence. Based on the modified P-B phase, the desired linear phase gradient along the x-axis can be achieved at the interface of the metasurface by adjusting the rotation angle of the unit cells. When a circularly polarized EM wave illuminates on the metasurface obliquely, the anomalous reflected wave can be observed in a broad bandwidth with high efficiency, and the reflection angle can be predicted by the generalized snell's law. Two metasurfaces with different phase gradients are designed in this article. The simulated results agree well with the theoretical designs, showing outstanding ability to manipulate the propagation of the EM wave. With specific phase gradient of the proposed metasurface, the anomalous reflected wave can be bent to the same side of the normal with the incident wave, which means that negative reflection effect can be achieved. Our design provides a solution to manipulate reflected beam under oblique incidence with high conversion efficiency, which has great practical applications in medical imaging, communication technology, and energy harvesting.

Nonlinear Modulation of Plasmonic Resonances in Graphene-Integrated Triangular Dimers at Terahertz Frequencies.

Metamaterials made from artificial subwavelength structures hold great potential in designing functional devices at microwave, terahertz, infrared, and optical frequencies. In this work, we study the active switching effect of the plasmonic resonance modes in triangular dimer (DTD) structure using graphene in the terahertz regime. The sole DTD structure can only support a dipolar bonding dimer plasmonic (BDP) mode, whose field enhancement factor at the gap center can reach 67.4. However, with a metallic junction in the dimer, the BDP mode switches to a charge transfer plasmonic (CTP) mode. When changing the metallic junction to a graphene stripe, an active modulation effect of the CTP mode can be realized by altering the nonlinear conductivity of graphene through strong-field terahertz incidence. The proposed design is quite promising in terahertz sensing, amplitude switching and nonlinear effect enhancement, etc.

Broadband Absorption Tailoring of SiO2/Cu/ITO Arrays Based on Hybrid Coupled Resonance Mode.

Sub-wavelength artificial photonic structures can be introduced to tailor and modulate the spectrum of materials, thus expanding the optical applications of these materials. On the basis of SiO2/Cu/ITO arrays, a hybrid coupled resonance (HCR) mechanism, including the epsilon-near-zero (ENZ) mode of ITO, local surface plasmon resonance (LSPR) mode and the microstructural gap resonance (GR) mode, was proposed and researched by systematically regulating the array period and layer thickness. The optical absorptions of the arrays were simulated under different conditions by the finite-difference time-domain (FDTD) method. ITO films were prepared and characterized to verify the existence of ENZ mode and Mie theory was used to describe the LSPR mode. The cross-sectional electric field distribution was analyzed while SiO2/Cu/ITO multilayers were also fabricated, of which absorption was measured and calculated by Macleod simulation to prove the existence of GR and LSPR mode. Finally, the broad-band tailoring of optical absorption peaks from 673 nm to 1873 nm with the intensities from 1.8 to 0.41 was realized, which expands the applications of ITO-based plasmonic metamaterials in the near infrared (NIR) region.

Generation of high-order orbital angular momentum beams and split beams simultaneously by employing anisotropic coding metasurfaces

Metasurfaces provide the freedom to manipulate electromagnetic (EM) waves by controlling the peculiarity of subwavelength artificial structures. Many exciting devices have been developed using metasurfaces that can even perform multiple functionalities. In this paper, we propose to transform linearly polarized EM waves into high-order orbital angular momentum (OAM) beams and specifically reflected split beams for different polarizations based on anisotropic coding metasurfaces. By means of precisely adjusting the geometry of the element structure, the reflection phases of two perpendicular polarizations can be encoded without interference, which is conductive to encode the metasurfaces independently in two different directions. Under this benefit, two kinds of 2-bit phase-coded metasurfaces are constructed and verified by experiments to demonstrate their versatility of generating effective high-order OAM beams and specified reflected split beams, respectively. Beyond that, by adjusting the coding scheme, more features can be implemented under this merit.

A broadband planar acoustic metamaterial lens

Based on quasi-conformal mapping approach, we propose a two-dimensional broadband and low-loss planar lens in the acoustic regime, which can be realized using gradient-index sub-wavelength artificial structures with hard boundary. In this design, QCM is employed in the acoustic regime to convert a parabolic form into a planar one, which makes the resultant material approximately isotropic through minimizing the anisotropic factor. It is found that the planar lens can show the same performance as its parabolic counterparts over a broadband range of frequency. Moreover, the lens is fabricated via 3D printing technology and measured experimentally, which gives identical results with the theoretical prediction. The proposed technique could be extended to realize different acoustic devices and applied for a series of practical applications.