Abstract-Accelerating terahertz all-optical modulation by hot carriers effects of silver nanorods in PVA film

AIP Advances

Jian-Ping Yu, Sai Chen,  Fei Fan, Shi-Tong Xu, Jie-Rong Cheng,  Xiang-Fan Chen, Lin Xiao, Sheng-Jiang Chang

(a) SEM image of the hybrid plasmonic PVA film; (b) Scheme of the all-optical modulation setup.

https://aip.scitation.org/doi/full/10.1063/1.5098386

While the terahertz regime has proved to be a promising solution for wireless big-data transmission, the lack of available functional devices working in this band seriously constrain the wide engineering applications. In this paper, the silver nanorods in polyvinyl alcohol (PVA) film are developed for enhancing and accelerating THz all-optical modulation. The modulation depth can reach 80% under 0.6W/cm2, and the speed can reach 3kHz which is much faster than our previous research. It has been attributed to the plasmonic hot carriers’ effect of the silver nanorods, which makes the photoexcited electrons more easily jump out of the barrier and assemble in the heterostructure between PVA and silicon. Moreover, the photoelectronic response is proved to be related to the plasmonic absorption of the pumping light, which means the modulation speed is decided by the color of the pump light. This experiment phenomenon provides a way to build color-selective terahertz all-optical modulators. Considering its capability for broadband, efficient, and fast modulation of THz waves, this low-cost and conveniently fabricated device could be broadly applied in polymer related THz devices.

Abstract-Mechanical modulation of terahertz wave via buckled carbon nanotube sheets

Shi-Tong Xu, Lin-Lin Mou, Fei Fan, Sai Chen, Zhikai Zhao, Dong Xiang, Mônica Jung de Andrade, Zunfeng Liu, Sheng-Jiang Chang,

Fig. 2 (a) Schematic diagram of THz-TDS, insets: experimental tensile setup. (b) The stress-strain curve of bare rubber, BCNTS20/rubber, and BCNTS60/rubber. (c) Time-domain THz spectra of bare rubber substrate at different strain 0%, 30%, 60%, 90%, 120% and 150%.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-26-22-28738

Manipulation of terahertz (THz) wave plays an important role in THz imaging, communication, and detection. The difficulty in manipulating the THz wave includes single function, untunable, and inconvenient integration. Here, we present a mechanically tunable THz polarizer by using stretchable buckled carbon nanotube sheets on natural rubber substrate (BCNTS/rubber). The transmittance and degree of polarization of THz wave can be modulated by stretching the BCNTS/rubber. The experiments showed that the degree of polarization increased from 17% to 97%, and the modulation depth reached 365% in the range of 0.2-1.2 THz, as the BCNTS/rubber was stretched from 0% to 150% strain. These changes can be also used for high strain sensing up to 150% strain, with a maximum sensitivity of 2.5 M/S. A spatial modulation of THz imaging was also realized by stretching and rotating BCNTS/rubber. The theoretical analysis and numerical modeling further confirm the BCNTS/rubber changes from weak anisotropic to highly anisotropic structure, which play key roles in THz wave modulation. This approach for active THz wave manipulation can be widely used in polarization imaging, wearable material for security, and highly sensitive strain sensing.

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

Abstract-Carbon nanotube attached subwavelength grating for broadband terahertz polarization conversion and dispersion control

Shi-Tong Xu, Sai Chen, Lin-LinMou, Fei Fan, Zun-FengLiu,  Sheng-Jiang Chang


https://www.sciencedirect.com/science/article/pii/S0008622318307000

Carbon nanotube (CNT), as a recently emerged newfangled nanomaterial, has been applied in terahertz (THz) polarizer due to its anisotropic hyper-ordered orientation. Herein, we simply adhere two orthogonal CNT sheets to the both sides of dielectric subwavelength grating to form a compound structure (i.e. CNT@Grating). Due to the subwavelength integration and cavity modes, the CNT@Grating presents local resonances between two CNT sheets, which greatly enhances the polarization rotation and expands the bandwidth. Compared with the large phase shift dispersion of single subwavelength grating, its phase shift dispersion and impedance can be manipulated by adjusting the layer number of CNT, and finally a broadband close zero dispersion from 0.4 to 0.95 THz has been obtained, which leads to a broadband THz polarization conversion. This work provides a new design idea towards practical applications for THz broadband polarization conversion and dispersion control.

Abstract-Tunable terahertz wave-plate based on dual-frequency liquid crystal controlled by alternating electric field

Jian-Ping Yu, Sai Chen, Fei Fan, Jie-Rong Cheng, Shi-Tong Xu, Xiang-Hui Wang, and Sheng-Jiang Chang

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-26-2-663

In this work, the optically anisotropic property of dual-frequency liquid crystals (DFLC) in terahertz (THz) regime has been experimentally investigated, which indicates that the refractive index and birefringence of DFLC can be continuously modulated by both the alternating frequency and intensity of the alternating electric field. This tunability originates from the rotation of DFLC molecules induced by alternating electric fields. The results show that by modulating the alternating frequency from 1 kHz to 100 kHz under 30 kV/m electric field, the 600 μm thickness DFLC cell can play as a tunable quarter-wave plate above 0.68 THz, or a half-wave plate above 1.33 THz. Besides, it can be viewed as a tunable THz phase shifter from 0 to π. Therefore, due to its novel tuning mechanism, DFLC will be of great significance in dynamic manipulating on THz phase and polarization.

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

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 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-State conversion based on terahertz plasmonics with vanadium dioxide coating controlled by optical pumping

Fei Fan, Wen-Hao Gu, Sai Chen, Xiang-Hui Wang, and Sheng-Jiang Chang »View Author Affiliations

http://www.opticsinfobase.org/ol/abstract.cfm?uri=ol-38-9-1582

The state conversion and terahertz (THz) wave modulation based on a plasmonic device composed of silicon column arrays with vanadium dioxide (VO 2 ) coating were experimentally demonstrated. For double 45° tilted optical pumping, a state conversion from dielectric photonic crystal (PC) to metallic PC was demonstrated due to the insulator–metal transition (IMT) of VO 2 with the pump power increasing. In this process, a broadband intensity modulation with 70% modulation depth was achieved. Furthermore, for normally incident optical pumping, another state conversion from dielectric PC to plasmonic device was also demonstrated due to the partial IMT of VO 2 , and the out of plane PC resonance gradually changed to be plasmonic resonances. This device and its modulation scheme will be of great significance for potential THz applications.

© 2013 Optical Society of America

Abstract-Tunable nonreciprocal terahertz transmission and enhancement based on metal/magneto-optic plasmonic lens

Fei Fan, Sai Chen, Xiang-Hui Wang, and Sheng-Jiang Chang

»View Author Affiliations

http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-21-7-8614


A tunable metal/magneto-optic plasmonic lens for terahertz isolator is demonstrated. Based on the magneto-optical effect of the semiconductor material and non-symmetrical structure, this plasmonic lens has not only the focusing feature but also nonreciprocal transmission property. Moreover, a transmission enhancement through this device greatly larger than that of the ordinary metallic slit arrays is contributed by the extraordinary optical transmission effect of the magneto surface plasmon polaritons. The results show that the proposed isolator has an isolation bandwidth of larger than 0.4THz and the maximum isolation of higher than 110dB, and its operating frequency also can be broadly tuned by changing the external magnetic field or temperature. This low-loss, high isolation, broadband tunable nonreciprocal terahertz transmission mechanism has a great potential for terahertz application systems.
© 2013 OSA