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-Terahertz wave modulation enhanced by laser processed PVA film on Si substrate

Weimin Liu, Fei Fan, Shitong Xu, Meng Chen, Xianghui Wang, Shengjiang Chang,

https://www.nature.com/articles/s41598-018-26778-7

An optically pumped ultrasensitive broadband terahertz (THz) wave modulator based on polyvinyl alcohol (PVA) film on Si wafer was demonstrated in this work. The THz time domain spectroscopy experiments confirm that the PVA/Si can drastically enhance the photo-induced THz wave modulation on the Si surface, especially when the PVA film is heated by a high-power laser. A modulation depth of 72% can be achieved only under 0.55 W/cm2 modulated laser power, which is superior significantly to the bare Si. The numerical simulations indicate that the laser processed PVA (LP-PVA) film increases the photo-generated carrier concentration on the Si surface in two orders of magnitude higher than that of bare Si. Moreover, the modulation mechanism and the dynamic process of laser heating on the PVA/Si have been discussed. This highly efficient THz modulation mechanism and its simple fabrication method have great application potentials in THz modulators.

Abstract-Nanoparticles doped film sensing based on terahertz metamaterials

Weimin Liu, . Author links open the author workspace.Fei Fan, . Author links open the author workspace.Shengjiang Chang, Jiaqing Hou, . Author links open the author workspace.Meng Chen, . Author links open the author workspace.Xianghui  Wang, Jinjun Bai, 

http://www.sciencedirect.com/science/article/pii/S0030401817306739?via%3Dihub


. Author links open the author workspace.FeiFan
A nanoparticles concentration sensor based on doped film and terahertz (THz) metamaterial has been proposed. By coating the nanoparticles doped polyvinyl alcohol (PVA) film on the surface of THz metamaterial, the effects of nanoparticle concentration on the metamaterial resonances are investigated through experiments and numerical simulations. Results show that resonant frequency of the metamaterial linearly decreases with the increment of doping concentration. Furthermore, numerical simulations illustrate that the redshift of resonance results from the changes of refractive index of the doped film. The concentration sensitivity of this sensor is 3.12 GHz/0.1%, and the refractive index sensitivity reaches 53.33 GHz/RIU. This work provides a non-contact, nondestructive and sensitive method for the detection of nanoparticles concentration and brings out a new application on THz film metamaterial sensing.

Abstract-Ultrasensitive terahertz modulation by silicon-grown MoS2 nanosheets

Sai Chen,   Fei Fan,   Yinping Miao,   Xiaotong He,  Shengjiang Chang and   Kai Liang Zhang  

Nanoscale, 2016, Accepted Manuscript

DOI: 10.1039/C5NR08101G
Received 16 Nov 2015, Accepted 27 Jan 2016
First published online 28 Jan 2016
http://pubs.rsc.org/en/content/articlelanding/2016/nr/c5nr08101g#!divAbstract


Two dimensions (2D) materials play more and more important roles these days, due to its broad applications in many areas. Here, we proposed an optically pumped terahertz (THz) modulator, based on Si-grown MoS₂ nanosheets. The broadband modulation effect has been proved by THz time domain spectroscopy and numerical simulation. The modulation depth of this Si-grown MoS2 nanosheet can reach over 75% under the low pumping power of 0.24 W/cm2, much deeper than that of bare silicon. By theoretical model and simulation, it is proved that the broadband modulation effect can be described as a free carrier absorption for THz waves in the Drude form. Importantly, a catalyst mechanism in the Si-grown MoS₂ is concluded that MoS₂-Si heterostructure make the MoS₂ can catalyze more carriers generated on the Si surface. This novel 2D material has a high effective modulation on THz waves under a low pumping power density, which gives itself a bright potential in THz applications.

Abstract-Terahertz transmission and sensing properties of microstructured PMMA tube waveguide

Fei Fan, Xuanzhou Zhang, Shanshan Li, Decai Deng, Ning Wang, Hao Zhang, and Shengjiang Chang
https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-23-21-27204&id=330151

A terahertz (THz) tube waveguide with grating structure has been designed, fabricated and characterized as a microstructure waveguide sensor. The resonance and polarization properties of this microstructured tube have been experimentally and theoretically investigated, which indicates that the grating etched on the tube surface has a remarkable modulation effect on the tube resonance and polarization dependence for THz waves. Moreover, a real-time quantitative sensing has been realized based on this tube waveguide in the THz time-domain spectroscopy system. Compared with the bare tube without grating, the grating structure strongly enhances the interaction between THz evanescent field on the tube surface and analytes, improving the sensitivity. This microstructured PMMA THz tube reveals a high sensitivity of 50GHz/μl and precision of larger than 0.125μl with a good linear relationship for THz sensing applications.

© 2015 Optical Society of America

Abstract-Terahertz isolator based on nonreciprocal magneto-metasurface

Terahertz isolator based on nonreciprocal magneto-metasurface Sai Chen, Fei Fan, Xianghui Wang, Pengfei Wu, Hui Zhang, and Shengjiang Chang  »View Author Affiliations

Optics Express, Vol. 23, Issue 2, pp. 1015-1024 (2015)
http://dx.doi.org/10.1364/OE.23.001015

http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-23-2-1015

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A magneto-metasurface with nonreciprocal terahertz (THz) transmission has been proposed to form a THz isolator. Importantly, we have discussed the two necessary conditions for THz nonreciprocal transmission in the metasurface: (1) There should be magneto-optical responses for THz waves in the metasurface; (2) The transmission system of the metasurface needs to be asymmetric for forward and backward waves. These two conditions lead to the time reversal symmetry breaking of system, and the magnetoplasmon mode splitting and nonreciprocal resonance enhancement can be observed in the asymmetry magneto-metasurface. Moreover, the isolation dependences and tunability on the external magnetic field and temperature have also been investigated, which shows that the best operating state with a high isolation can be designed. The numerical simulations show a maximum isolation of 43 dB and a 10 dB operating bandwidth of 20 GHz under an external magnetic field of 0.3 T, and the insertion loss is smaller than 1.79 dB. This low-loss, high isolation, easy coupling THz isolator has broadly potentials for THz application systems.

© 2015 Optical Society of America

Abstract-Terahertz polarization splitter based on orthogonal microstructure dual-core photonic crystal fiber

Shanshan Li, Hao Zhang, Yu Hou, Jinjun Bai, Weiwei Liu, andShengjiang Chang  »View Author Affiliations

A broadband polarization splitter operating in the terahertz (THz) band is proposed based on dual-core photonic crystal fiber with orthogonal microstructure in the core regions. The Index Converse Matching Coupling method is presented to design the THz polarization splitter for the first time, which exhibits several advantages, such as short splitting length, high extinction ratio, low loss, and broad operation bandwidth. By numerical simulation, it has been found that the strong coupling occurs within a frequency range of 0.4–0.7 THz. The operation bandwidth is more than 0.15 THz (equal to 138 μm). The shortest splitting length is only 1.83 cm at 0.4 THz. The extinction ratios for both of x and y polarization are better than −15  dB when the frequency is larger than 0.51 THz. The lowest material absorption loss is only 0.34 dB at 0.4 THz. Moreover, this structure is simple to design and easy to fabricate over its counterparts in the communication band. Our research offers an effective method to design a broadband THz device and would be of significance for future relevant applications

© 2013 Optical Society of America

Abstract-Mechanically tunable terahertz metamaterials

Jining Li^1,2, Charan M. Shah³, Withawat Withayachumnankul¹, Benjamin S.-Y. Ung¹, Arnan Mitchell³, Sharath Sriram³, Madhu Bhaskaran³, Shengjiang Chang², and Derek Abbott¹

¹School of Electrical and Electronic Engineering, The University of Adelaide, Adelaide, SA 5005, Australia
²Institute of Modern Optics, Nankai University, Tianjin 300071, China
³Functional Materials and Microsystems Research Group, RMIT University, Melbourne, VIC 3001, Australia                        

http://apl.aip.org/resource/1/applab/v102/i12/p121101_s1?isAuthorized=no

Electromagnetic device design and flexible electronics fabrication are combined to demonstrate mechanically tunable metamaterials operating at terahertz frequencies. Each metamaterial comprises a planar array of resonators on a highly elastic polydimethylsiloxane substrate. The resonance of the metamaterials is controllable through substrate deformation. Applying a stretching force to the substrate changes the inter-cell capacitance and hence the resonance frequency of the resonators. In the experiment, greater than 8% of the tuning range is achieved with good repeatability over several stretching-relaxing cycles. This study promises applications in remote strain sensing and other controllable metamaterial-based devices.

© 2013 American Institute of Physics