Abstract-Ultrathin freestanding terahertz vector beam generators with free phase modulation

 

Huixian Zhou, Jierong Cheng, Fei Fan, Xianghui Wang, Shengjiang Chang

Schematic of the bilayer metasurface for VBs’ generation. (b) and (c) show the structure in the top and bottom layers, respectively, where the discretized sectors and their symmetry axes are marked. (d) Schematic of the polarization conversion in each sector and the orientation of the metaatoms.

https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-29-2-1384&id=446364

Simultaneous control of phase and polarization offers a large degree of freedom to tailor the beam properties, for instance, enabling generation of structured beams such as vector beams and vector vortex beams. Here, we propose an ultrathin freestanding metasurface operating at the terahertz frequency for efficient generation of vector vortex beam with an arbitrarily defined topological charge from linearly polarized excitation. The metasurface is composed of bilayer metallic patterns separated by a thin quartz slab, with one layer determining the transmission polarization and the other controlling the transmission phase. The tightly cascaded two layers form a Fabry-Perot cavity to maximize the efficiency of the polarization and phase control. Two metasurfaces for generation of radially polarized vector beam with uniform phase and vortex phase are fabricated and tested at 0.14 THz. The experimental results successfully demonstrate the generation of high-quality vector beams with the desired phase. In the experiment, the ultrathin and freestanding properties allow the metasurface to be easily combined with other components, which shows great potential for the development of various compact terahertz systems.

© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Graphene metalenses with diverse electrical tunabilities at different terahertz frequencies

Huixian Zhou, Jierong Cheng, Fei Fan, Xianghui Wang, Shengjiang Chang,

https://www.spiedigitallibrary.org/journals/optical-engineering/volume-59/issue-12/127106/Graphene-metalenses-with-diverse-electrical-tunabilities-at-different-terahertz-frequencies/10.1117/1.OE.59.12.127106.short?SSO=1

Graphene has attracted widespread attention in dynamic optoelectronic devices due to its tunable electrical and optical properties. But different modulation capabilities of the graphene-based designs at different frequencies are less studied. We study the electrical tunability of transmissive metalenses based on graphene when working at three frequencies 0.3, 1.25, and 2 THz, respectively. The constitutive meta-atoms are composed of graphene patches and metallic gratings for efficient phase shift in the orthogonal polarization. Although the conductivity of graphene is tunable at all the frequencies, responses of meta-atoms show weak and strong dependence on the Fermi level at the low and high frequencies, respectively. Therefore, the focal length of the metalens is not electrically tunable at 0.3 THz. In contrast, the metalenses designed at 1.25 and 2 THz show electrically adjustable focal lengths, and the tuning range of the focal length increases with frequency. The research here provides clear guidance for the design of graphene metalenses with different electrical tunabilities for a variety of application scenarios.

© 2020 Society of Photo-Optical Instrumentation Engineers (SPIE) 0091-3286/2020/$28.00 © 2020 SPIE

Abstract-Efficient Wide-Band Large-Angle Refraction and Splitting of a Terahertz Beam by Low-Index 3D-Printed Bilayer Metagratings

Xipu Dong, Jierong Cheng, Fei Fan, Xianghui Wang, and Shengjiang Chang

https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.14.014064

Near-perfect anomalous reflection and refraction have been demonstrated using passive lossless metasurfaces and metagratings operating at microwave, infrared, and visible frequencies, while related studies at terahertz frequencies are lacking. Here we propose low-index (with a refractive index of 1.57) 3D-printed dielectric metagratings for efficient wide-band diffraction engineering at low terahertz frequencies. A simplified analytical model reveals that the number of propagating waveguide modes inside the grating is a key factor in diffraction engineering, and is insufficient in a low-index design regardless of the detailed dimensions in a period. Additional waveguide modes are introduced in asymmetric bilayer and trilayer metagratings, providing sufficient degrees of freedom for efficient large-angle anomalous refraction and beam splitting. These metagratings are inherently wide-band, benefiting from low dispersion of the waveguide modes. Three metagratings are designed, 3D printed, and tested experimentally at 0.14 THz for ${70}^{\circ }$ refraction, ${80}^{\circ }$ refraction, and $\pm {70}^{\circ }$ beam splitting. The measured efficiency shows good agreement with the design. The proposed metagratings, with simple structures and large feature sizes, can be easily scaled to applications at higher terahertz frequencies.

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Abstract-Nonreciprocal terahertz beam steering based on magneto-optic metagratings

Zhiyu Tan, 

Fei Fan,  

Xipu Dong, 

Jierong Cheng & 

Shengjiang Chang, 

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• https://www.nature.com/articles/s41598-019-56789-x

In this work, an active nonreciprocal THz beam steering has been proposed based on a transversely magnetized metal/InSb metagrating. The nonreciprocal dispersion relation and phase shift characteristics of the metal/InSb waveguide are investigated in details. A metagrating structure with gradient phase shift has been designed based on the metal/InSb waveguide. Under the external magnetic field (EMF), the THz beam can be changed among 0, +1st, and −1st order of the metagrating. Due to the nonreciprocity of the metal/InSb metagrating, the deflection angle can be controlled by changing the positive and negative directions of the EMF, to realize bilateral symmetric scanning from −67.8° to 67.8° with over 70% diffraction efficiency, and this device also exhibits the nonreciprocal one-way transmission as an isolator with the isolation of 13 dB. This low-loss, large deflection degree, nonreciprocal beam scanner has a great potential application in the THz regime.

Abstract-Active Terahertz Shielding and Absorption Based on Graphene Foam Modulated by Electric and Optical Field Excitation

Shi‐Tong Xu, Fei Fan, Jierong Cheng,  Honghui Chen, Wenle Ma, Yi Huang, Shengjiang Chang,

https://onlinelibrary.wiley.com/doi/abs/10.1002/adom.201900555

Ultralight materials for broadband terahertz (THz) shielding and absorption are promising in practical THz applications. Here, active THz shielding and absorption properties of 3D graphene foam (GF) controlled by both laser pumping and biased electric field are investigated. The GF can be tuned from OFF‐shielding state to ON‐shielding state when the external field excitations are applied, and 10 dB shielding bandwidth expands from 0 to a broad band of 0.2–1.6 THz. Further researches show that the GF always keeps very low THz reflection either with or without external fields, but its absorption characteristics can be remarkably controlled from 13% to 95.4% at 0.3 THz by the power of the external excitations, and its specific average terahertz absorption performance increases from 3.9 × 103 to 1.95 × 104 dB cm3 g−1. This modulation mechanism reveals that the carrier density in GF increases one order of magnitude from 2.6 × 1014 cm−3 to 3.15 × 1015 cm−3. Finally, the tunable THz shielding and absorption characteristics of this GF device are demonstrated by THz transmission imaging, which shows its great potential applications in active THz imaging, radar, and electromagnetic compatibility.

Abstract-Broadband phase shift engineering for terahertz waves based on dielectric metasurface

Qianyi Mu, Hengzhi Lin, Fei Fan, Jierong Cheng, Xianghui Wang, Sheng Jiang,




https://www.sciencedirect.com/science/article/abs/pii/S0030401818309143
Broadband terahertz (THz) phase shift engineering and zero-dispersion waveplates based on dielectric metasurface have been investigated, of which structure is a periodical rectangular scattering units on silicon substrates. By designing proper geometric parameters of metasurface structure, the value, dispersion and bandwidth of the phase shift curves can be effectively manipulated. Based on this, the broadband half waveplate (HWP) and quarter waveplate (QWP) have been designed and fabricated. The experimental results show that the HWP can work in the broad range of 0.7–1.35 THz with the polarization conversion ratio (PCR) of close to 100% and the transmission of over 70%. And the QWP can operate in the range of 0.7$\sim$0.85THz with the PCR of over 90% and the transmission of over 70%. The method of phase shift engineering based on dielectric metasurfaces and these broadband zero-dispersion waveplates have great potential in promoting the performance of THz application systems.

Abstract-Graphene-based near-field optical microscopy: high-resolution imaging using reconfigurable gratings

Sandeep Inampudi, Jierong Cheng, and Hossein Mosallaei

https://www.osapublishing.org/ao/abstract.cfm?uri=ao-56-11-3132&origin=search

High-resolution and fast-paced optical microscopy is a requirement for current trends in biotechnology and materials industry. The most reliable and adaptable technique so far to obtain higher resolution than conventional microscopy is near-field scanning optical microscopy (NSOM), which suffers from a slow-paced nature. Stemming from the principles of diffraction imaging, we present fast-paced graphene-based scanning-free wide-field optical microscopy that provides image resolution that competes with NSOM. Instead of spatial scanning of a sharp tip, we utilize the active reconfigurable nature of graphene’s surface conductivity to vary the diffraction properties of a planar digitized atomically thin graphene sheet placed in the near field of an object. Scattered light through various realizations of gratings is collected at the far-field distance and postprocessed using a transmission function of surface gratings developed on the principles of rigorous coupled wave analysis. We demonstrate image resolutions of the order of 𝜆0/16${\lambda }_0/16$ using computational measurements through binary graphene gratings and numerical postprocessing. We also present an optimization scheme based on the genetic algorithm to predesign the unit cell structure of the gratings to minimize the complexity of postprocessing methods. We present and compare the imaging performance and noise tolerance of both grating types. While the results presented in this article are at terahertz frequencies (𝜆0=10  μm${\lambda }_0=10\mathrm{\mu m}$), where graphene is highly plasmonic, the proposed microscopy principle can be readily extended to any frequency regime subject to the availability of tunable materials.

© 2017 Optical Society of America