Abstract-Achromatic Dielectric Metasurface with Linear Phase Gradient in the Terahertz Domain

 

Ridong Jia,   Yufei Gao,   Quan Xu,   Xi Feng,   Qingwei Wang,  Jianqiang Gu,   Zhen Tian, Chunmei Ouyang,   Jiaguang Han,   Weili Zhang

https://onlinelibrary.wiley.com/doi/abs/10.1002/adom.202001403

The notion of metasurface has inspired the innovation of various functional devices in the terahertz band, but the intrisinc dispersion restricts their application in broadband scenarios. Here, two terahertz achromatic linear‐phase‐gradient metasurface devices are demonstrated, which are a beam deflector and a beam splitter, respectively. The phase and dispersion of the metasurfaces are simultaneously engineered by changing the geometric parameters of the unit cells made of silicon gratings and pillars. The simulated and experimental results demonstrate the achromatic feasibility of the beam deflector from 0.6 to 1.2 THz and of the beam splitter from 0.6 to 1.1 THz. The transmittances and the splitting ratios of the achromatic beam splitter are also analyzed. The metasurface based achromatic beam deflector and splitter presented here not only enrich the terahertz functional devices, but the methods and structures may also promote the research of broadband terahertz metasurfaces.

Abstract-All-dielectric nanograting for increasing terahertz radiation power of photoconductive antennas

Kemeng Wang, Jianqiang Gu, Wenqiao Shi, Youwen An, Yanfeng Li, Zhen Tian, Chunmei Ouyang, Jiaguang Han, and Weili Zhang

. (a) Schematic diagram of the nanograting-assisted PCA. The width of the coplanar line is 20 µm. Schematic diagram of the unit cells of (b) the Gy and (c) the Gx with the corresponding incidence polarizations, respectively, in which p = 350 nm and the height of the unit cell is 200 nm. The areas of the nanograting in (b) and (c) are 80×80 µm2 and 50×80 µm2 respectively. Electric field distribution of (d) the substrate and (e) an optimized unit cell at the working wavelength (λ∼780 nm) when w = 185 nm. The green dashed line represents the envelops of the bare substrate and the nanograting unit cells, respectively. (f) Simulated anti-reflection effect of the nanogratings with varied w = 245, 230, 215, 200, and 185 nm, respectively. The y-axis is the ratio of the reflectance of the nanograting to its reference.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-28-13-19144

Photoconductive antenna (PCA) is a widely used terahertz (THz) radiation source, but its low radiated power limits the signal-to-noise ratio and bandwidth in THz imaging and spectroscopy applications. Here, we achieved significant PCA power enhancement through etching nanograting directly on the surface of the PCA substrate. The integrated nanograting not only maximizes the generation of photocarriers, but also benefits the bias electric field loaded on the photocarriers. Comparing with the conventional PCA, our PCA realizes a frequency independent THz power enhancement of 3.92 times in the range of 0.05-1.6 THz. Our results reported here not only provide a new method for increasing the THz power of PCAs, but also reveal another way that artificial nanostructures affect the PCAs, which paves the way for the subsequent researches of next-generation PCAs.

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

Abstract-Terahertz surface plasmonic waves: a review

Xueqian Zhang,  Quan Xu,  Lingbo Xia,  Yanfeng Li, Jianqiang Gu,  Zhen Tian,  Chunmei Ouyang,  Jiaguang Han,  Weili Zhang

https://www.spiedigitallibrary.org/journals/advanced-photonics/volume-2/issue-01/014001/Terahertz-surface-plasmonic-waves-a-review/10.1117/1.AP.2.1.014001.full

Terahertz science and technology promise many cutting-edge applications. Terahertz surface plasmonic waves that propagate at metal–dielectric interfaces deliver a potentially effective way to realize integrated terahertz devices and systems. Previous concerns regarding terahertz surface plasmonic waves have been based on their highly delocalized feature. However, recent advances in plasmonics indicate that the confinement of terahertz surface plasmonic waves, as well as their propagating behaviors, can be engineered by designing the surface environments, shapes, structures, materials, etc., enabling a unique and fascinating regime of plasmonic waves. Together with the essential spectral property of terahertz radiation, as well as the increasingly developed materials, microfabrication, and time-domain spectroscopy technologies, devices and systems based on terahertz surface plasmonic waves may pave the way toward highly integrated platforms for multifunctional operation, implementation, and processing of terahertz waves in both fundamental science and practical applications. We present a review on terahertz surface plasmonic waves on various types of supports in a sequence of properties, excitation and detection, and applications. The current research trend and outlook of possible research directions for terahertz surface plasmonic waves are also outlined.

Abstract-Label-Free Measurements on the Solution of Monomeric and Dimeric Insulin Using a Periodical Terahertz Split Ring Resonator

Pengfei Wang, Liyuan Liu,  Xiangchao Wang, Jianbing Zhang, Wei Qi, Hongwei Zhao, Zhen Tian, Mingxia He



https://www.sciencedirect.com/science/article/abs/pii/S0301010419300035

A silicon-based sensor, consisting of periodical split ring resonators was demonstrated to monitor monomeric and dimeric insulin solutions in terahertz time-domain spectroscopy (THz-TDS). As concentration increases, the resonant frequencies of sensor show red shifts with a rapid trend in dilute solutions and a slow trend in concentrated solutions, consistent with CST simulations. For monomeric insulin, the turning concentration is 11.6mg/ml and the total frequency shift is larger than that of dimeric insulin which shows a turning concentration at 17.4mg/ml. The deviation in frequency shift and turning concentration can be attributed to the structural changes of insulin during dimerization process.

Abstract-Self-Assembled Chiral Nanoribbons Studied by Terahertz Time-Domain Spectroscopy and Other Biological Methods

Pengfei Wang, Yuefei Wang, Liyuan Liu, Jinwu Zhao, Zhen Tian, Wei Qi, Jianbing Zhang, Hongwei Zhao, Mingxia He



https://www.sciencedirect.com/science/article/abs/pii/S0009261419300363

We report the use of terahertz time-domain spectroscopy (THz-TDS) to probe chiral self-assembled twisted nanoribbons from a ferrocene-modified dipeptide consisting of two L-amino acids (Fc-FF). Supported by molecular dynamics simulation, the THz absorption peaks at 0.9 THz and 2.7 THz are attributed to distinct characteristic vibrational modes of Fc-FF single crystal. The relative absorption intensity is introduced to characterize the discrepancy in THz peak absorption which originates from symmetry breaking. Combined with the results of scanning electron microscopy, circular dichroism, Fourier transform infrared spectroscopy and X-ray diffraction, THz-TDS unravels great potentials to distinguish subtle structural changes within chiral self-assembled materials.

Abstract-Anisotropic plasmonic response of black phosphorus nanostrips in terahertz metamaterials

Qingqing Fo,  Ling Pan, Xieyu Chen, Quan Xu, Chunmei Ouyang, Xueqian Zhang,   Zhen Tian, Jianqiang Gu, Liyuan Liu,   Jiaguang Han,  Weili Zhang

https://ieeexplore.ieee.org/document/8369157/

Two-dimensional black phosphorus (BP) recently emerged as an outstanding material for optoelectronics and nanophotonics applications. In contrast to graphene, BP has a sufficiently large electronic bandgap and its high carrier mobility allows for efficient free-carrier absorption in the infrared and terahertz regimes. Here, we present a reflective structure to enhance the response of nanostructured monolayer BP at terahertz frequencies and investigate localized surface plasmon resonances in BP nanostrip arrays. Anisotropic absorption is observed in the proposed BP metamaterials due to the puckered crystal structure of monolayer BP, and further investigations show that the plasmonic resonances are strongly depending on the geometric parameters of the nanostrips and the coupling between the adjacent nanostrips. We expect that the monolayer BP is an outstanding candidate of highly anisotropic plasmonic material for ultra-scaled optoelectronic integration.

Abstract-Polarization-independent all-silicon dielectric metasurfaces in the terahertz regime

Huifang Zhang, Xueqian Zhang, Quan Xu, Qiu Wang, Yuehong Xu, Minggui Wei, Yanfeng Li, Jianqiang Gu, Zhen Tian, Chunmei Ouyang, Xixiang Zhang, Cong Hu, Jiaguang Han, and Weili Zhang

https://www.osapublishing.org/prj/abstract.cfm?uri=prj-6-1-24&origin=search

Dielectric metasurfaces have achieved great success in realizing high-efficiency wavefront control in the optical and infrared ranges. Here, we experimentally demonstrate several efficient, polarization-independent, all-silicon dielectric metasurfaces in the terahertz regime. The metasurfaces are composed of cylindrical silicon pillars on a silicon substrate, which can be easily fabricated using etching technology for semiconductors. By locally tailoring the diameter of the pillars, full control over abrupt phase changes can be achieved. To show the controlling ability of the metasurfaces, an anomalous deflector, three Bessel beam generators, and three vortex beam generators are fabricated and characterized. We also show that the proposed metasurfaces can be easily combined to form composite devices with extended functionalities. The proposed controlling method has promising applications in developing low-loss, ultra-compact spatial terahertz modulation devices.

© 2017 Chinese Laser Press

Abstract-Transmission and plasmonic resonances on quasicrystal metasurfaces

Quanlong Yang, Jianqiang Gu, Yuehong Xu, Yanfeng Li, Xueqian Zhang, Zhen Tian, Chunmei Ouyang, Jiaguang Han, and Weili Zhang

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-20-24173&origin=search

The control of light-matter interaction in metasurfaces offers an unexplored potential for the excitation and manipulation of light. Here, we combine experimental terahertz time-domain spectroscopy and near-field scanning terahertz microscopy to demonstrate the role of reciprocal vectors in the transmission and plasmonic resonances of quasicrystal metasurfaces. An investigation of two-dimensional metasurface structures with different rotationally symmetric quasicrystal arrangements demonstrates that the transmission minima resulting from Wood’s anomaly are directly related to the surface plasmon resonances. We also find that the surface plasmon resonances of the quasicrystal metasurface were determined by the reciprocal vectors, which could be well explained by the coupling condition of the resonances, and the characteristic frequencies remain un-shifted under various slit sizes. Our findings demonstrate a new potential in developing novel plasmonic metasurfaces.

© 2017 Optical Society of America

Abstract-Polarization-controlled asymmetric excitation of surface plasmons

Quan Xu, Xueqian Zhang, Quanlong Yang, Chunxiu Tian, Yuehong Xu, Jianbing Zhang, Hongwei Zhao, Yanfeng Li, Chunmei Ouyang, Zhen Tian, Jianqiang Gu, Xixiang Zhang, Jiaguang Han, and Weili Zhang

https://www.osapublishing.org/optica/abstract.cfm?uri=optica-4-9-1044&origin=search

Free-space light can be coupled into propagating surface waves at a metal–dielectric interface, known as surface plasmons (SPs). This process has traditionally faced challenges in preserving the incident polarization information and controlling the directionality of the excited SPs. The recently reported polarization-controlled asymmetric excitation of SPs in metasurfaces has attracted much attention for its promise in developing innovative plasmonic devices. However, the unit elements in these works were purposely designed in certain orthogonal polarizations, i.e., linear or circular polarizations, resulting in limited two-level polarization controllability. Here, we introduce a coupled-mode theory to overcome this limit. We demonstrated theoretically and experimentally that, by utilizing the coupling effect between a pair of split-ring-shaped slit resonators, exotic asymmetric excitation of SPs can be obtained under the 𝑥$x$-, 𝑦$y$-, left-handed circular, and right-handed circular polarization incidences, while the polarization information of the incident light can be preserved in the excited SPs. The versatility of the presented design scheme would offer opportunities for polarization sensing and polarization-controlled plasmonic devices.

© 2017 Optical Society of America

Abstract-Polarization-dependent electromagnetic responses in an A-shape metasurface

Ning Zhang, Quan Xu, Shaoxian Li, Chunmei Ouyang, Xueqian Zhang, Yanfeng Li, Jianqiang Gu, Zhen Tian, Jiaguang Han, and Weili Zhang

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-17-20689&origin=search

We numerically and experimentally demonstrate polarization-dependent terahertz responses in a proposed metasurface of A-shape resonators. With the horizontal polarization incidence, the observed transmission window is formed by two resonance dips, corresponding to the inductive-capacitive resonance at the lower frequency and the high-order antisymmetric resonance at a higher frequency, respectively. When the incident wave is perpendicularly polarized, the transmission window arises from the plasmon-induced transparency spectral response. The origin of the polarization-sensitive resonance properties is revealed by mapping the electric field and terahertz-induced surface current in the proposed metamaterials. Moreover, the influence of the geometry of the A-shape microstructures on the transmission spectra is analyzed. These polarization-dependent metamaterials may provide more degrees of freedom in tuning the electromagnetic responses, thus offering a path toward robust metamaterials design.

© 2017 Optical Society of America

Abstract-Temperature-dependent birefringence of lithium triborate, LBO in the THz regime

• Kechao Song, 
• Zhen Tian, 
• Weili Zhang & 
• Mingwei Wang

https://www.nature.com/articles/s41598-017-08626-2?WT.feed_name=subjects_materials-science

Optical properties of lithium triborate (LBO) in the terahertz regime (0.2–2 THz) were characterized using broadband terahertz time-domain spectroscopy. The frequency dependence of refractive index and absorption coefficient of the LBO crystal was experimentally investigated over the temperature range of 77–297 K, which the experimental results indicated that LBO has very low optical absorption coefficient at terahertz frequencies especially for the beam polarization along the crystal’s principal dielectric axis X. Moreover, a giant birefringence was observed, and the refractive index difference between the axis X and Z gradually decreased with decreasing temperature, which is attributed to the behavior of the TO phonon modes of B1 and B2 symmetries at low frequencies at different temperatures. As potential applications, LBO can be exemplarily used as terahertz wave shapers, beam splitters, terahertz wave plates, circular polarizers and other polarization devices.

Abstract-Tailoring the plasmon-induced transparency resonances in terahertz metamaterials

Meng Liu, Zhen Tian, Xueqian Zhang, Jianqiang Gu, Chunmei Ouyang, Jiaguang Han, and Weili Zhang

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-17-19844


We experimentally demonstrate that a coupled metamaterial composed of sub-wavelength split-ring-resonators (SRRs) and closed-ring-resonators (CRRs) can tailor the plasmon-induced-transparency (PIT) resonances when the external electric field is parallel to the gaps of SRRs. Rotating or moving SRRs in vertical direction plays a critical role in the EIT functionality, while an excellent robust performance can be acquired via moving SRRs in the horizontal direction. Based on the results, a polarization-independent and polarization-dependent planar metamaterial are designed, fabricated and measured. In contrast to the spectral property of the polarization-independent medium, the polarization-dependent one is featured by isolated PIT phenomena in the frequency-domain, with respect to the horizontal and vertical polarized incident beam. Transmission responses of the PIT metamaterial are characterized with terahertz time-domain spectroscopy, showing a good agreement with the rigorous numerical simulation results. The presented work delivers a unique way to excite and modulate the PIT response, toward developing polarization-independent and polarization-dependent slow-light building blocks, ultrasensitive sensors and narrow-band filters functioning in the THz regime.

© 2017 Optical Society of America

Abstract-All-Dielectric Meta-lens Designed for Photoconductive Terahertz Antennas

 Qing Yu,  Jianqiang Gu, Quanlong Yang,  Ying Zhang,   Yanfeng Li, Zhen Tian, Chunmei Ouyang,   Jiaguang Han, John F. O'Hara, Weili Zhang

Impact Statement:

In this numerical study, we present a metasurface based lens directly integrated to a terahertz PCA transmitter which is rarely reported. Because its all-dielectric nature, the meta-lens not only offers an excellent collimation function, but also has a better transmittance efficiency than the traditional Si hyper-semispheric lens and most metal based terahertz meta-lenses. The meta-lens proposed here have promising applications in next-generation terahertz imaging and spectroscopy techniques.

Abstract:

We present an all-dielectric meta-lens designed to collimate terahertz waves emitted from a terahertz antenna. The meta-lens is not only thinner than a conventional bulk silicon lens, but also promises to eliminate the use of parabolic mirrors in a terahertz time-domain spectroscopy system. A systematic numerical study reveals that the meta-lens exhibits excellent performance in both the emitter and detector modules, converting between the spherical wave of the antennas and the collimated beam. The frequency and alignment dependences of the meta-lens are also investigated to comprehensively map its response characteristics. The all-dielectric meta-lens presented here may pave a way in developing high-performance integrated photoconductive terahertz antenna components.

Abstract-Terahertz surface plasmon polariton waveguiding with periodic metallic cylinders

Ying Zhang, Shaoxian Li, Quan Xu, Chunxiu Tian, Jianqiang Gu, Yanfeng Li, Zhen Tian, Chunmei Ouyang, Jiaguang Han, and Weili Zhang

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-13-14397

We demonstrated a structure with periodic cylinders arranged bilaterally and a thin dielectric layer covered inside that supports bound modes of surface plasmon polaritons at terahertz frequencies. This structure can confine the surface plasmon polaritons in the lateral direction, and at the same time reduce the field expansion into space. We examined and explored the characteristics of several different structures using scanning near-field terahertz microscopy. The proposed designs pave a novel way to terahertz waveguiding and may have important applications in the development of flexible, wideband and compact photonic circuits operating at terahertz frequencies.

© 2017 Optical Society of America

Abstract- Broadband and Robust Metalens with Nonlinear Phase Profiles for Efficient Terahertz Wave Control

Quanlong Yang, Jianqiang Gu, Yuehong Xu, Xueqian Zhang, Yanfeng Li, Chunmei Ouyang, Zhen Tian, Jiaguang Han, Weili Zhang,

http://onlinelibrary.wiley.com/doi/10.1002/adom.201601084/abstract

Metasurfaces, 2D artificial electromagnetic media, open up a new frontier of functional device design ranging from radio waves to the visible region. Particularly, metasurface-based lenses are indispensable in various practical terahertz applications. The authors aim at achieving flexible and robust metalenses for efficient terahertz wave control. In general, resolution and efficiency are two inevitable parameters in determining the focusing and imaging abilities of lenses, which however are rarely experimentally demonstrated in the terahertz band. In this Communication, three broadband and robust metalenses with nonlinear phase profiles are proposed, all of which are experimentally investigated by using near-field scanning terahertz microscopy (NSTM) with a spatial resolution of 50 µm. The measurement shows that the metalens can focus a 0.95 THz wave to a spot size of 580 µm and achieve a transmittance efficiency of 45%. In addition, the NSTM system facilitates an experimental investigation of the incidence angle dependence of the terahertz metalens, which proves the robust focusing feature of the proposed device. This demonstration delivers a promising metasurface design for potential applications in imaging and information processing that may be of interest for the entire electromagnetic spectrum.

Abstract-Frequency-Dependent Dual-Functional Coding Metasurfaces at Terahertz Frequencies

http://onlinelibrary.wiley.com/doi/10.1002/adom.201600471/full

A frequency-dependent dual-functional coding metasurface is proposed at terahertz frequencies using two layers of metamaterial structures, each of which is responsible for the independent control of reflection phases at two distinct frequencies. The zero interference between the functionalities at the lower and higher frequencies are promising for possible applications in multicolor holography for color displays or a frequency beam splitter.