Abstract-Tunable terahertz phase shifter based on dielectric artificial birefringence grating filled with polymer dispersed liquid crystal

Xin Zhang, Fei Fan, Chun-Yue Zhang, Yun-Yun Ji, Xiang-Hui Wang, and Sheng-Jiang Chang

Schematic diagram in the experiment and experimental data. (a) The structure of the dielectric gradient grating filled with PDLCs in the experiment; (b) The schematic diagram of geometry configuration in the measurement; (c) The time domain signals of + 45° and -45° LP components at 0 V and (d) 80 V measured by THz-TDS system, respectively.

https://www.osapublishing.org/ome/abstract.cfm?uri=ome-10-2-282

An active terahertz (THz) anisotropic manipulation is based on a structure combined polymer dispersed liquid crystal (PDLC) with sub-wavelength dielectric gradient grating. In this structure, the PDLC works as an adjustable anisotropic material due to the change of the optical axis direction induced by applying a biased electric field, while the dielectric grating serves as an artificial high birefringence material. By using an appropriate design, the THz birefringence of this structure can be enhanced or offset under different biased voltages, and the phase shift curve of this structure becomes flatter than that of the pure PDLC cell due to the dispersion manipulation of the grating. Moreover, the experimental results fit with the simulative designing, demonstrating that the phase shift of the structure can vary from π to 0 near 0.8 THz when the electric field increases from 0 to 80V, and this device realizes the function of polarization conversion as a tunable THz half-wave plate. This work exhibits potential applications in THz functional devices, such as actively controlled phase shifters and polarization convertors combined LC with artificial microstructure.

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

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-Terahertz dielectric anisotropy enhancement in dual-frequency liquid crystal induced by carbon nanotubes

Yun-Yun Ji, Fei Fana, Shi-Tong Xu, Jian-Ping Yu, Yan Liu, Xiang-Hui Wang, Sheng-Jiang Chang,



https://www.sciencedirect.com/science/article/pii/S0008622319306608

Dual-frequency liquid crystals (DFLCs), the mixture of positive and negative liquid crystal (LC) molecules, exhibit unique alternating (AC) frequency dependent anisotropic properties. Carbon nanotube (CNT), as a novel nanomaterial with strong anisotropy, has attracted much attention in recent years. Herein, we investigate the tunable terahertz birefringence and phase shift characteristics of the DFLC doped with CNT (CNT-LC) by using the terahertz time-domain polarization spectroscopy. The results show that the CNT-LC (1.5 wt%) can reach 0.5π at 0.793 THz, which can be used as a tunable THz phase shifter that is 0.12π higher than the pure DFLCs. Furthermore, through measuring the output polarization state, it is confirmed that the dielectric anisotropy enhancement mechanism of CNT-LC originates from the surface interaction between CNTs and LC molecules. Therefore, a tunable quarter-wave plate with the active polarization conversion from linearly polarized (LP) to LP or LP to circularly polarized (CP) can be realized at 0.925 THz in the CNT-LC, while the pure DFLC cannot be achieved at the same frequency. The dielectric anisotropy enhancement of CNT-LC shows its utility in the improvement of various tunable terahertz LC phase shifter and wave plate.

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-Broadband controllable terahertz quarter-wave plate based on graphene gratings with liquid crystals

Yun-Yun Ji, Fei Fan, Xiang-Hui Wang, and Sheng-Jiang Chang

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-26-10-12852

Developing the broadband controllable or tunable terahertz (THz) polarization and phase devices are in an urgent need. In this paper, we demonstrate a broadband controllable THz quarter-wave plate (QWP) with double layers of graphene grating and a layer of liquid crystals. The double layer graphene gratings can achieve a switchable QWP to switch between linear-to-linear and linear-to-circular polarization states with over 0.35THz bandwidth in the ON or OFF state by applying biased electric field on the graphene grating or not. Moreover, this QWP based on the structure of periodic gradient grating can significantly enhance the phase difference between two orthogonally polarized components compared to that based on equal-periodic grating structure because of the additional phase distribution of the gradient structures. Furthermore, we incorporate liquid crystals into the graphene grating to form a tunable QWP, of which operating frequency can be continuously tuned in a wide frequency range by electrically controlling the molecular director of the liquid crystals. The results show that the graphene periodic gradient grating with LCs not only broadens the operating bandwidth, but also reduces the external electric field. This device offers a further step in the development of THz polarization and phase devices for potential applications in THz polarized imaging, spectroscopy, and communication.

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

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-Terahertz artificial birefringence and tunable phase shifter based on dielectric metasurface with compound lattice

Yun-Yun Ji, Fei Fan, Meng Chen, Lei Yang, and Sheng-Jiang Chang

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-10-11405

A dielectric metasurface with line-square compound lattice structure has been fabricated and demonstrated in the terahertz (THz) regime by the THz time-domain spectroscopy and numerical simulation. A polarization dependent electromagnetically induced transparency (EIT) effect is achieved in this metasurface due to the mode coupling and interference between the resonance modes in line and square subunits of the metasurface. Accompany with the EIT effect, a large artificial birefringence effect between two orthogonal polarization states is also observed in this compound metasurface, of which birefringence is over 0.6. Furthermore, the liquid crystals are filled on the surface of this dielectric metasurface to fabricate an electrically tunable THz LC phase shifter. The experimental results show that its tunable phase shift under the biased electric field reaches 0.33π, 1.8 times higher than the bare silicon, which confirms the enhancement role of THz microstructure on the LC phase shift in the THz regime. The large birefringence phase shift of this compound metasurface and its LC tunable phase shifter will be of great significance for potential applications in THz polarization and phase devices.

© 2017 Optical Society of America

• Meng Chen
• , Fei Fan
• , Shi-Tong Xu
•  & Sheng-Jiang Chang

http://www.nature.com/articles/srep38562

Subwavelength dielectric gratings are widely applied in the phase and polarization manipulation of light. However, the dispersion of the normal dielectric gratings is not flat while their birefringences are not enough in the THz regime. In this paper, we have fabricated two all-dielectric gratings with gradient grids in the THz regime, of which artificial birefringence is much larger than that of the equal-grid dielectric grating demonstrated by both experiments and simulations. The transmission and dispersion characteristics are also improved since the gradient grids break the periodicity of grating lattices as a chirp feature. From 0.6–1.4 THz, a broadband birefringence reaches 0.35 with a low dispersion and good linearity of phase shift, and the maximum phase shift is 1.4π. Furthermore, these gradient gratings are applied as half-wave plates and realize a linear polarization conversion with a conversion rate over 99%, also much higher than the equal-grid gratings. These gradient gratings show great advantages compared to the periodic gratings and provide a new way in the designing of artificial birefringence material.

Abstract-Terahertz polarization converter and one-way transmission based on double-layer magneto-plasmonics of magnetized InSb

Fei Fan, Shi-Tong Xu, Xiang-Hui Wang, and Sheng-Jiang Chang

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-24-23-26431

In this work, we investigate the nonreciprocal circular dichroism for terahertz (THz) waves in magnetized InSb by the theoretical calculation and numerical simulation, which indicates that longitudinally magnetized InSb can be applied to the circular polarizer and nonreciprocal one-way transmission for the circular polarization THz waves. Furthermore, we propose a double-layer magnetoplasmonics based on the longitudinally magnetized InSb, and find two MO enhancement mechanisms in this device: the magneto surface plasmon resonance on the InSb-metal surface and Fabry–Pérot resonances between two orthogonal metallic gratings. These two resonance mechanisms enlarge the MO polarization rotation and greatly reduce the external magnetic field below 0.1T. The one-way transmission and perfect linear polarization conversion can be realized over 70dB, of which the transmittance can be modulated from 0 to 80% when the weak magnetic field changes from 0 to 0.1T under the low temperature around 200K. This magnetoplasmonic device has broad potential as a THz isolator, modulator, polarization convertor, and filter in the THz application systems.

© 2016 Optical Society of America

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Abstract-Study on a tunable narrow-band filter based on magnetic defects in photonic crystal in the terahertz region

Hui Zhang ; Xian-feng Xu ; Fei Fan ; Sheng-jiang Chang

http://opticalengineering.spiedigitallibrary.org/article.aspx?articleid=2280033

Hui Zhang, Xian-feng Xu

China University of Petroleum (East China), College of Science, Department of Physics and Optoelectronic Engineering, Changjiang West Road 66, Qingdao 266580, China

Fei Fan, Sheng-jiang Chang

Nankai University, Institute of Modern Optics, Weijin Road 94, Tianjin 300071, China

Opt. Eng. 54(4), 047104 (Apr 23, 2015). doi:10.1117/1.OE.54.4.047104

Abstract.  A narrow-band filter with a broad tuning range is designed based on a magnetic field controlled ferrite defect in photonic crystal for a terahertz (THz) wave. The resonance defect modes of a ferrite defect in photonic crystal in the THz region are studied by using the finite difference time domain method. Detailed calculations on the shifts of the defect mode frequency and transmission properties reveal that the peak frequency of transmission spectra can continuously vary from 0.77 to 0.95 THz under the external magnetic field and the bandwidth of the filter is about 0.015 THz.

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

Abstract-Terahertz modulator based on insulator–metal transition in photonic crystal waveguide

http://www.opticsinfobase.org/ao/abstract.cfm?uri=ao-51-20-4589
Fei Fan, Yu Hou, Zi-Wei Jiang, Xiang-Hui Wang, and Sheng-Jiang Chang
A terahertz modulator based on the insulator–metal transition (IMT) in a photonic crystal waveguide (PCW) coated by vanadium dioxide (VO2) film is proposed. The numerical simulations show that a dielectric state and a metallic state with quite different photonic band structures and transmission properties in the proposed PCW are reciprocally converted because of the IMT of VO2, and the pass-bands of this PCW are greatly shifted from 0.68 to 0.8 and 1.02 to 1.25 THz to 0.8–1.45 THz. This PCW significantly enhances the modulation depth and sensitivity compared with bare VO2 film. Extensive investigation demonstrates that the thickness of VO2 film greatly affects the IMT process in the PCW, and limits the ultimate modulation depth of the device. The proposed modulation scheme will be of great significance for potential THz applications.