Metasurfaces for manipulating terahertz waves

 

The associated THz responses include focusing, holograms, polarization modulation, special beams and active controlling. Credit: Xiaofei Zang, Bingshuang Yao, Lin Chen, Jingya Xie, Xuguang Guo, Alexei V. Balakin, Alexander P. Shkurinov, and Songlin Zhuang

https://phys.org/news/2021-03-metasurfaces-terahertz.html

by Chinese Academy of Sciences

THz waves have a plethora of applications ranging from biomedical and medical examinations, imaging, environmental monitoring, to wireless communications, because of abundant spectral information, low photon energy, strong penetrability, and shorter wavelength. THz waves with technological advances not only determined by high-efficiency sources and detectors but also decided by a variety of high-quality THz components/functional devices. However, traditional THz devices should be thick enough to realize the desired wave-manipulating functions, hindering the development of THz integrated systems and applications. Although metamaterials have shown groundbreaking discoveries due to tunable electric permittivity and magnetic permeability of a meta-atom, they are limited by technical challenges of fabrication and high loss of the metal-based unit cell.

In a new paper published in Light: Advanced Manufacturing, a team of scientists, led by Professor Songlin Zhuang from Terahertz Technology Innovation Research Institute, University of shanghai for Science and Technology, and co-workers have summarized recent advancements of metasurfaces for the manipulation of THz waves. These ultra-compact devices with unusual functionalities render metasurface devices very attractive for applications such as imaging, encryption, information modulation and THz communications.

Actually, metasurfaces typically consist of planar antennas that enable predesigned EM responses. The antennas are made of metals or traditional high-refractive index dielectrics that can be easily fabricated based on standard fabrication processes. In addition, metasurfaces with functionality in manipulating EM waves are dependent on abrupt phase changes at planar antenna interfaces, and thus the thickness of metasurfaces is much thinner than the incident wavelength. Metasurfaces can locally control the wavefront of EM waves at subwavelength resolution, leading to various practical applications such as metalenses, waveplates, vortex beam generators, beam steering and holograms. The ultrathin nature of metasurfaces, ease of fabrication, and subwavelength resolution in manipulation of EM waves make metasurfaces ideal candidates for THz device miniaturization (ultra-compact THz devices) and system integration.

The metasurface-based approach for manipulatig THz waves enables remarkable contributions in designing ultra-thin/ultra-compact and tunable THz components. The main advantages/contributions of THz metasurfaces can be concluded as follows: (1) THz components with reduced size: The functionalities of focusing, OAM, and polarization conversion realized by metasurfaces can be traditionally obtained by using a THz lens, helical phase plate, and half-wave (or quarter-wave) plate, respectively; (2) THz components with multiple functions: The traditional THz devices, e.g. THz lenses, waveplates, etc, always show a single function. Metasurfaces not only provide a flexible platform to realize ultra-thin/ultra-compact THz devices with single function, but also enable the unprecedented capability in designing multifunctional THz devices. (3) THz components with tunable function: Metasurfaces combined with VO2, graphene, etc, open a new avenue for designing THz components with active functions.

In conclusion, metasurfaces with planar structures can locally modify the wavefront of THz waves at subwavelength resolution. Metasurfaces not only provide an ultra-compact platform for manipulating the wavefront of THz waves, but also generate a plethora of applications that are difficult to achieve with conventional functional devices. As an overview, the recent developments of metasurfaces for manipulating THz waves were presented in this paper, and this progress report may open a new avenue to design ultra-thin or ultra-compact THz functional devices and systems.

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Metasurfaces for manipulating terahertz waves

                                              
LIGHT PUBLISHING CENTER, CHANGCHUN INSTITUTE OF OPTICS, FINE MECHANICS AND PHYSICS, CAS

The associated THz responses include focusing, holograms, polarization modulation, special beams and active controlling.

CREDIT

by Xiaofei Zang, Bingshuang Yao, Lin Chen, Jingya Xie, Xuguang Guo, Alexei V. Balakin, Alexander P. Shkurinov, and Songlin Zhuang

https://www.eurekalert.org/pub_releases/2021-03/lpcc-mfm032221.php

THz waves have a plethora of applications ranging from biomedical and medical examinations, imaging, environment monitoring, to wireless communications, because of the abundant spectral information, low photon energy, strong penetrability, and shorter wavelength. THz waves with technological advances not only determined by the high-efficiency sources and detectors but also decided by a variety of the high-quality THz components/functional devices. However, traditional THz devices should be thick enough to realize the desired wave-manipulating functions, hindering the development of THz integrated systems and applications. Although metamaterials have been shown groundbreaking discoveries due to the tunable electric permittivity and magnetic permeability of a meta-atom, they are limited to technical challenges of fabrication and high loss of the metal-based unit cell.

In a new paper published in Light: Advanced Manufacturing, a team of scientists, led by Professor Songlin Zhuang from Terahertz Technology Innovation Research Institute, University of Shanghai for Science and Technology, and co-workers have summarized the recent advancements of metasurfaces for the manipulation of THz waves. These ultra-compact devices with unusual functionalities render metasurface devices very attractive for applications such as imaging, encryption, information modulation and THz communications.

Actually, metasurfaces typically consist of planar antennas that enable predesigned EM responses. The antennas are made by metals or traditional high-refractive index dielectrics that can be easily fabricated based on the standard fabrication process. In addition, metasurfaces with the functionality in manipulating EM waves are dependent on the abrupt phase changes at planar antenna interfaces, and thus the thickness of metasurfaces is much thinner than the incident wavelength. Metasurfaces can locally control the wavefront of EM waves at subwavelength resolution, leading to various practical applications such as metalens, waveplates, vortex beam generators, beam steering and holograms. The ultrathin nature of metasurfaces, the ease of fabrication, and the subwavelength resolution in manipulating of EM waves make metasurfaces ideal candidates for THz device miniaturization (ultra-compact THz devices) and system integration.

The metasurface-based approach for manipulatig THz waves enables remarkable contributions in designing ultra-thin/ultra-compact and tunable THz components. The main advantages/contributions of THz metasurfaces can be concluded as follows: (1) THz components with reduced size: The functionalities of focusing, OAM, and polarization conversion realized by metasurfaces can be traditionally obtained by using a THz lens, helical phase plate, and half-wave (or quarter-wave) plate, respectively; (2) THz components with multiple functions: The traditional THz devices, e.g. THz lenses, waveplates, etc..., are always show a single function. Metasurfaces not only provide a flexible platform to realize ultra-thin/ultra-compact THz devices with single function, but also enable the unprecedented capability in designing multifunctional THz devices. (3) THz components with tunable function: Metasurfaces combined with VO2, graphene, etc, open a new avenue for designing THz components with active functions.

In conclusion, metasurfaces with planar structures can locally modify the wavefront of THz waves at subwavelength resolution. Metasurfaces not only provide an ultra-compact platform for manipulating the wavefront of THz waves, but also generate a plethora of applications that are difficult to achieve with conventional functional devices. As an overview, the recent developments of metasurfaces for manipulating THz waves were presented in this paper, and this progress report may open a new avenue to design ultra-thin or ultra-compact THz functional devices and systems.

Abstract-Quasibound states in the continuum in terahertz free-standing metal complementary periodic cross-shaped resonators

Dejun Liu, Feng Wu, Lin Chen, Feng Liu

https://arxiv.org/abs/2007.08104

We numerically and experimentally achieve quasi-bound states in the continuums (BICs) with high-Q factors in the free-standing metal complementary periodic cross-shaped resonators (CPCRs) at terahertz (THz) frequencies. Such induced quasi-BICs arises from the breaking of the mirror symmetry of CPCRs. By properly tuning the asymmetric factor, the measured Q factor of quasi-BIC can reach 102, which is lower than the simulated Q factor of 166 due to the limited system resolutions. We also simulate the electric field magnitude and vector distributions at the quasi-BICs, where the out-phase alignment between the electric dipoles is found. The sharp quasi-BICs realized in this thin free-standing metal structure may immediately boost the performance of filters and sensors in terahertz wave manipulation or biomolecular sensing.

Abstract-Terahertz composite plasmonic slabs based on double-layer metallic gratings

Dejun Liu, Lin Chen, Xiaohu Wu, and Feng Liu

Sketch of the composite plasmonic slab (CPS). The slab consists of double-layer metal gratings and a dielectric film. (a) The side view of the CPS. (b) The 3D model of the CPS. (c) The image of the experimental sample of a single-layer metal (copper) grating.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-28-12-18212

One composite plasmonic slab with a broad bandgap (40%) is experimentally and numerically demonstrated in the terahertz (THz) region. The composite slab consists of double-layer metallic gratings and a dielectric film, which supports two resonant modes. Electric field vectors and charge distributions proved that the low-frequency resonant mode originates from the symmetric plasmonic mode, while the high-frequency resonant mode is induced by the hybrid mode of plasmonic and dielectric modes. Compared with the double-layer metallic grating, the inserted dielectric film significantly enhances the transmission of the transverse magnetic (TM) waves and induces Fano resonances. The near-field coupling between metal gratings and dielectric film can be manipulated by changing the thickness and the refractive index of dielectric films. We further demonstrated that the plasmonic bandgap can be manipulated by tuning the grating width. These results suggest that this composite plasmonic slab is promising in terahertz integrated components development such as a filter, polarizer, or sensor.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Terahertz composite plasmonic slabs based on double-layer metallic gratings

Dejun Liu, Lin Chen, Feng Liu

https://arxiv.org/abs/2003.04113

A composite plasmonic slab based on double-layer metallic gratings and a dielectric film is experimentally and numerically demonstrated in terahertz (THz) regions, which can support two resonance modes and then form a broad bandgap (40%). As compared to the double-layer metal grating, the dielectric film in composite THz slabs significantly enhances the transmission of the transverse magnetic (TM) mode. Electric field vector proved that the low-frequency resonance mode originates from the symmetric plasmonic mode and the high-frequency resonance mode is induced by the hybrid mode of plasmonic and dielectric modes. The inherently near field coupling between metal gratings and dielectric film has been analyzed by changing the structural parameters. We further demonstrate that by tuning the metallic grating width, the plasmonic bandgap can be manipulated. These results suggest that this composite plasmonic slab has great potential for use as a filter, polarizer, and sensor in THz regions.

Abstract-Terahertz-frequency temporal differentiator enabled by a high-Q resonator

Jingya Xie, Xi Zhu, Hongxiang Zhang, Xiaofei Zang, Lin Chen, Alexey V. Balakin, Alexander P. Shkurinov, and Yiming Zhu

Schematic of the experimental setup for measurements of THz integrated temporal differentiator. AWG: arbitrary waveform generators. RF&MSG: radio frequency microwave source. SGX Module: signal generator extension module. HA: horn antenna. DD: direct detector. The insets (a) and (b) are the schematic drawing of the TRR and cross-sectional view of waveguides, respectively.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-28-6-7898

Terahertz (THz) fundamental “building blocks” equivalent to those used in multi-functional electronic circuits are very helpful for actual applications in THz data-processing technology and communication. Here, we theoretically and experimentally demonstrate a THz temporal differentiator based on an on-chip high-quality (Q) factor resonator. The resonator is made of low-loss high-resistivity silicon material in a monolithic, integrated platform, which is carefully designed to operate near the critical coupling region. The experiment demonstrates that the device can perform the first-order time derivative of the input signal electric field complex envelope at 214.72 GHz. Our investigation provides an effective approach for terahertz pulse re-shaping and real-time differential computing units.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Predict sample’s line positions of absorption peaks in terahertz band with the forced radiation intensity of molecular electric dipoles

Zhongwei Zhang, Zhi Zhu, Minghui Yuan, Minghui Li, Guanjun You, Lin Chen, Yiming Zhu


https://www.sciencedirect.com/science/article/abs/pii/S0030401819309782

Directly predicting the line positions of samples in the terahertz (THz) band is of significant importance for their THz identification. However, it is really a challenge to gain accurately the line positions by means of theoretical calculation, because the calculation typically involves various parameters, such as level energy and transition moment, which usually we hardly get directly. Based on the classical forced vibration model of dipoles, we propose a quantitative expression, i.e. the forced radiation intensity of molecular electric dipoles, which intend to predict the line positions of absorption peaks in the THz fingerprint spectra of a sample. We verified our expression by 9 recognized frequencies selected from the fingerprint spectra of water vapor in the THz band. Both the line positions and intensities of the absorption peaks of water vapor we calculated by the expression are well consistent with the experimental measurements. The line positions we calculated are also more accurate and comprehensive than that of water clusters simulated from Density Functional Theory (DFT). Our findings further support the theory of coherent superposition to advance a new method to exactly analyze the generation mechanism of molecular THz-fingerprint spectroscopy of a sample.

Abstract-A Multi‐Foci Metalens with Polarization‐Rotated Focal Points

Xiaofei Zang, Hongzhen Ding,   Yuttana Intaravanne,   Lin Chen,   Yan Peng,   Jingya Xie,   Qinghong Ke,   Alexey V. Balakin,   Alexander P. Shkurinov,   Xianzhong Chen, Yiming Zhu,   Songlin Zhuang,

https://onlinelibrary.wiley.com/doi/abs/10.1002/lpor.201900182

Benefiting from the unprecedented capability of metasurfaces in the manipulation of light propagation, metalenses can provide novel functions that are very challenging or impossible to achieve with conventional lenses. Here, an approach to realizing multi‐foci metalenses is proposed and experimentally demonstrated with polarization‐rotated focal points based on geometric metasurfaces. Multi‐foci metalenses with various polarization rotation directions are developed using silicon pillars with spatially variant orientations. The focusing characteristic and longitudinal polarization‐dependent imaging capability are demonstrated upon the illumination of a linearly polarized light beam. The uniqueness of this multi‐foci metalens with polarization‐rotated focal points may open a new avenue for imaging, sensing, and information processing.

Abstract-Metamaterial-enhanced terahertz vibrational spectroscopy for thin film detection

Jingya Xie, Xi Zhu, Xiaofei Zang, Qingqing Cheng, Lin Chen, Yiming Zhu,

Fig. 1 Schematic illustration of (a) the experiment setup, (b) optical image of the metamaterial with L-tartaric acid deposited on it, and (c) the SRR unit cell.

https://www.osapublishing.org/ome/abstract.cfm?uri=ome-8-1-128&origin=search

We present metamaterial-enhanced terahertz vibrational spectroscopy to solve the low sensitivity problem of the THz ray absorption detection in molecular and biomolecular thin film. In a proof-of-principle experiment, we demonstrate the system in split ring resonators (SRRs) metamaterial that is strongly coupled to L-tartaric acid molecular under a low-temperature condition. The experimental results show that the extinction ratio of the detected signal can be significantly improved from 1.75 dB to 4.5 dB. The numerical calculations confirm and explain the experimental observations. By detuning the resonance of metamaterial, the behavior of the spectral signal is modified. When the SRRs and molecular vibrational resonance frequencies are closely aligned, a clear mode splitting is observed resulting in a transparency transmission with enhanced extinction ratio. This method shows great potential for application in thin film sensing by detecting molecular vibrations in the lower-energy terahertz region.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Metasurface for multi-channel terahertz beam splitters and polarization rotators

XiaoFei Zang, HanHong Gong, Zhen Li, JingYa Xie, QingQing Cheng,   Lin Chen,   Alexander P. Shkurinov, YiMing Zhu, SongLin Zhuang,

https://aip.scitation.org/doi/abs/10.1063/1.5028401

Terahertz beam splitters and polarization rotators are two typical devices with wide applications ranging from terahertz communication to system integration. However, they are faced with severe challenges in manipulating THz waves in multiple channels, which is desirable for system integration and device miniaturization. Here, we propose a method to design ultra-thin multi-channel THz beam splitters and polarization rotators simultaneously. The reflected beams are divided into four beams with nearly the same density under illumination of linear-polarized THz waves, while the polarization of reflected beams in each channel is modulated with a rotation angle or invariable with respect to the incident THz waves, leading to the multi-channel polarization rotator (multiple polarization rotation in the reflective channels) and beam splitter, respectively. Reflective metasurfaces, created by patterning metal-rods with different orientations on a polyimide film, were fabricated and measured to demonstrate these characteristics. The proposed approach provides an efficient way of controlling polarization of THz waves in various channels, which significantly simplifies THz functional devices and the experimental system.

Abstract-Characterization of Thin Metal Films Using Terahertz Spectroscopy

 Zhiyong Wang,  Yunhui Han, Xu, Lin Chen,  Chuanwei Li,  Liang Wu, Weili Zhang,

http://ieeexplore.ieee.org/document/8254401/

A theoretical model is presented to simulate the transmission of thin metal films. Using terahertz spectroscopy, the two key characteristic parameters, conductivity and thickness, are determined simultaneously by fitting the experimental data to the theoretical model. Three different metal films, aluminum, copper, and silver, are investigated. The measured conductivities are significantly less than the bulk values. The sample thicknesses obtained are consistent with those estimated by the quartz thickness monitor.

Abstract-Terahertz integrated device: high-Q silicon dielectric resonators

Jingya Xie, Xi Zhu, Xiaofei Zang, Qingqing Cheng, Lin Chen, and Yiming Zhu

https://www.osapublishing.org/ome/abstract.cfm?uri=ome-8-1-50

We design, fabricate, and characterize the terahertz integrated resonators on the silicon platform. Based on mode analysis and selection, the high-Q feature of resonators made of low-loss high-resistivity Si material is achieved due to the excitation of the whispering gallery mode on waveguide-coupled single-mode racetrack rings and disk cavities. The experimental results demonstrate that the Q-factor can reach up to 2839 at 218.345 GHz, which is significantly improved compared with conventional THz cavities. These high Q-factor integrated resonators can be used as on-chip terahertz ultrasensitive sensors and as terahertz functional integrated circuits.

© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Double-stacked hyperbolic metamaterial waveguide arrays for efficient and broadband terahertz quarter-wave plates

• 
  
  
• Xianmin Ke, 
• Hua Zhu, 
• Junhao Li, 
• Lin Chen & 
• Xun Li

http://www.nature.com/articles/s41598-017-00726-3

We demonstrate how it is possible to achieve weak dispersion in the phase delay between two orthogonal polarization states by using double-stacked hyperbolic metamaterial (HMM) waveguide arrays. The weak dispersion in the phase delay originates from the different signs of phase delay from the two different HMM waveguide arrays. The condition of dispersion-free phase delay for the transmitted waves has been theoretically derived from the transmission matrix as the propagation characteristic of the HMM waveguide is involved. We further reveal that the designed double-stacked HMM waveguide array can function as an efficient quarter-wave plate that enables the conversion of linearly polarized light to circularly polarized light within a broad frequency band. In addition, the bandwidth over which the degree of linear polarization is nearly unity and over which the angle of linear polarization is kept at approximately 45° is basically consistent with the phase bandwidth. This offers a promising approach for developing a practical polarization converter in the terahertz domain.

Abstract-Defect-Induced Fano Resonances in Corrugated Plasmonic Metamaterials

http://onlinelibrary.wiley.com/doi/10.1002/adom.201600960/abstract

A novel defect-induced planar meta-atom that supports multiple Fano resonances in a defective corrugated metallic disk (CMD) structure is proposed. Numerical and experimental results reveal that multiple Fano resonances can be excited at terahertz frequencies when the symmetry of the CMD is broken by introducing a small angular defect. These multiple Fano resonances result from mutual coupling between the bright dipolar mode evoked by the edge of the wedge-shaped slice and dark multipole spoof localized surface plasmon modes. Furthermore, the influence of the angle of defect on the Q-factor and the resonance intensity of the quadrupolar resonance peak is investigated. Large values of figure of merit are obtained due to higher Fano resonance intensity and Q-factor. Results from two defective slices in the CMD structure validate the mechanism of the observed phenomenon. The findings of this work would enable a defect-induced Fano resonance platform for biosensing, terahertz domain filtering, and strong light–matter interactions.

Abstract-Excitation of dark multipolar plasmonic resonances at terahertz frequencies

• 
  • 
    
• Lin Chen
• , YuMing Wei
• , XiaoFei Zang
• , YiMing Zhu
•  & SongLin Zhuang

http://www.nature.com/articles/srep22027

We experimentally observe the excitation of dark multipolar spoof localized surface plasmon resonances in a hybrid structure consisting of a corrugated metallic disk coupled with a C-shaped dipole resonator. The uncoupled corrugated metallic disk only supports a dipolar resonance in the transmission spectrum due to perfect symmetry of the structure. However, the dark multipolar spoof localized surface plasmon resonances emerge when coupled with a bright C-shaped resonator which is placed in the vicinity of the corrugated metallic disk. These excited multipolar resonances show minimum influence on the coupling distance between the C-shaped resonator and corrugated metallic disk. The resonance frequencies of the radiative modes are controlled by varying the angle of the C-shaped resonator and the inner disk radius, both of which play dominant roles in the excitation of the spoof localized surface plasmons. Observation of such a transition from the dark to radiative nature of multipolar spoof localized plasmon resonances would find potential applications in terahertz based resonant plasmonic and metamaterial devices.

Abstract-Ultrabroad terahertz bandpass filter by hyperbolic metamaterial waveguide

Ultrabroad terahertz bandpass filter by hyperbolic metamaterial waveguide Xuetong Zhou, Xiang Yin, Tian Zhang, Lin Chen, and Xun Li  »View Author Affiliations

Optics Express, Vol. 23, Issue 9, pp. 11657-11664 (2015)
http://dx.doi.org/10.1364/OE.23.011657

View Full Text Article

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http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-23-9-11657

We propose and demonstrate an ultrabroad terahertz (THz) bandpass filter (BPF) by integrating two different-sized tapered hyperbolic metamaterial (HMM) waveguides, each of which has wide but different absorption and transmission bands, into a unit cell. With proper structural design of each HMM waveguide to control the absorption and transmission bands, we numerically demonstrate the designed BPF is capable of operating with a broad passband in the THz domain. A typical TM-polarized HMM BPF has a peak transmission of 37% at 3.3 THz with the passband bandwidth of 2.2 THz ranging from 2.97 to 5.17 THz. The co-designed three-dimensional HMM BPF also shows the capability of operating with independence to the polarization of incident light because of the structural symmetry and has sharp bandedge transitions of 22.6 and 17.6 dB/THz to the stop bands, respectively. The presented results here hold great promise for developing practical THz BPF with various applications in THz field.

© 2015 Optical Society of America