Abstract-Terahertz Emission from Compensated Magnetic Heterostructures

Mengji Chen, Rahul Mishra, Yang Wu, Kyusup Lee, Hyunsoo Yang

https://arxiv.org/abs/1806.02517

Terahertz emission spectroscopy (TES) has recently played an important role in unveiling the spin dynamics at a terahertz (THz) frequency range. So far, ferromagnetic (FM)/nonmagnetic (NM) heterostructures have been intensively studied as THz sources. Compensated magnets such as a ferrimagnet (FIM) and antiferromagnet (AFM) are other types of magnetic materials with interesting spin dynamics. In this work, we study TES from compensated magnetic heterostructures including CoGd FIM alloy or IrMn AFM layers. Systematic measurements on composition and temperature dependences of THz emission from CoGd/Pt bilayer structures are conducted. It is found that the emitted THz field is determined by the net spin polarization of the laser induced spin current rather than the net magnetization. The temperature robustness of the FIM based THz emitter is also demonstrated. On the other hand, an AFM plays a different role in THz emission. The IrMn/Pt bilayer shows negligible THz signals, whereas Co/IrMn induces sizable THz outputs, indicating that IrMn is not a good spin current generator, but a good detector. Our results not only suggest that a compensated magnet can be utilized for robust THz emission, but also provide a new approach to study the magnetization dynamics especially near the magnetization compensation point

Abstact-Tunable terahertz reflection of graphene via ionic liquid gating

Yang Wu, Xuepeng Qiu, Hongwei Liu, Jingbo Liu, Yuanfu Chen, Lin Ke, Hyunsoo Yang

(Submitted on 1 Feb 2017)

https://arxiv.org/abs/1702.00114

We report a highly efficient tunable THz reflector in graphene. By applying a small gate voltage (up to 3 V), the reflectance of graphene is modulated from a minimum of 0.79% to a maximum of 33.4% using graphene/ionic liquid structures at room temperature, and the reflection tuning is uniform within a wide spectral range (0.1 - 1.5 THz). Our observation is explained by the Drude model, which describes the THz wave-induced intraband transition in graphene. This tunable reflectance of graphene may contribute to broadband THz mirrors, deformable THz mirrors, variable THz beam splitters and other optical components.

Abstract-High-Performance THz Emitters Based on Ferromagnetic/Nonmagnetic Heterostructures

http://onlinelibrary.wiley.com/doi/10.1002/adma.201603031/full

A low-cost, intense, broadband, noise resistive, magnetic field controllable, flexible, and low power driven THz emitter based on thin nonmagnetic/ferromagnetic metallic heterostructures is demonstrated. The THz emission origins from the inverse spin Hall Effect. The proposed devices are not only promising for a wide range of THz equipment, but also offer an alternative approach to characterize the spin-orbit interaction in nonmagnetic/ferromagnetic bilayers.

Abstract-Tunable terahertz reflection of graphene via ionic liquid gating

Yang Wu1,2,3, Xuepeng Qiu1, Hongwei Liu3, Jingbo Liu4, Yuanfu Chen4, Lin Ke3 and 
Hyunsoo Yang1,2

http://iopscience.iop.org/article/10.1088/1361-6528/aa57ad

We report a highly efficient tunable THz reflector in graphene. By applying a small gate voltage (up to ±3 V), the reflectance of graphene is modulated from a minimum of 0.79% to a maximum of 33.4% using graphene/ionic liquid structures at room temperature, and the reflection tuning is uniform within a wide spectral range (0.1–1.5 THz). Our observation is explained by the Drude model, which describes the THz wave-induced intraband transition in graphene. This tunable reflectance of graphene may contribute to broadband THz mirrors, deformable THz mirrors, variable THz beam splitters and other optical components.

Abstract-Flexible terahertz modulator based on coplanar-gate graphene field-effect transistor structure

Jingbo Liu, Pingjian Li, Yuanfu Chen, Xinbo Song, Qi Mao, Yang Wu, Fei Qi, Binjie Zheng, Jiarui He, Hyunsoo Yang, Qiye Wen, and Wanli Zhang

https://www.osapublishing.org/ol/abstract.cfm?uri=ol-41-4-816

The terahertz (THz) modulators, as an essential component of the THz system, have been developed by many efforts until now. However, the development of flexible THz modulators is hindered due to the lack of flexible THz modulating materials. Herein, for the first time to the best of our knowledge, we demonstrated the feasibility of flexible THz modulators based on the coplanar-gate field-effect transistor (FET) structure of ion-gel/graphene/polyethylene terephthalate. The THz transmittance through this THz graphene modulator can be well controlled with a modulation depth up to 22% by tuning the carrier concentration of graphene via electrical gating. Furthermore, because of the integration of high flexibilities of graphene, ion-gel, and polyethylene terephthalate (PET), the proposed THz graphene modulator shows superior flexible performance, where the modulation properties can be maintained almost unchanged, not only under bending deformations, but also before and after bending 1000 times. In addition, due to the unique structure of ion-gel/graphene/PET, the flexible THz graphene modulator has a low insertion loss (1.2 dB). Therefore, this Letter is expected to be beneficial for the potential applications, ranging from the traditional compact THz system to a new flexible THz technology.

© 2016 Optical Society of America

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Abstract-Graphene terahertz modulators by ionic liquid gating

Yang Wu, Chan La-o-vorakiat, Xuepeng Qiu, Jingbo Liu, Praveen Deorani, Karan Banerjee, Jaesung Son, Yuanfu Chen, Elbert E. M. Chia, Hyunsoo Yang

http://xxx.tau.ac.il/abs/1506.02363

(Submitted on 8 Jun 2015)

Graphene based THz modulators are promising due to the conical band structure and high carrier mobility of graphene. Here, we tune the Fermi level of graphene via electrical gating with the help of ionic liquid to control the THz transmittance. It is found that, in the THz range, both the absorbance and reflectance of the device increase proportionately to the available density of states due to intraband transitions. Compact, stable, and repeatable THz transmittance modulation up to 93% (or 99%) for a single (or stacked) device has been demonstrated in a broad frequency range from 0.1 to 2.5 THz, with an applied voltage of only 3 V at room temperature.

Abstract-Graphene Terahertz Modulators by Ionic Liquid Gating

1. Yang Wu^1,2, 
2. Chan La-o-vorakiat^3,4,
3. Xuepeng Qiu⁵, 
4. Jingbo Liu⁶, 
5. Praveen Deorani⁵, 
6. Karan Banerjee⁵, 
7. Jaesung Son⁵, 
8. Yuanfu Chen⁶, 
9. Elbert E. M. Chia^4,*and
10. Hyunsoo Yang^1,2,*

Article first published online: 3 FEB 2015

http://onlinelibrary.wiley.com/doi/10.1002/adma.201405251/abstract;jsessionid=4D3B95BDF171BDBF815E1504D6336819.f03t03?deniedAccessCustomisedMessage=&userIsAuthenticated=false

DOI: 10.1002/adma.201405251

© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Excellent-performance terahertz (THz) modulators based on graphene/ionic liquid/graphene sandwich structures are demonstrated. The modulation covers a broadband frequency range from 0.1 to 2.5 THz with a modulation depth of up to 99% by applying a small gate voltage of 3 V. The outstanding performance of the proposed devices is due to the conical band structure of the graphene and the powerful gating effect of the ionic liquid in proximity to the graphene.

Abstract-Graphene/liquid crystal based terahertz phase shifters

 

Yang Wu, Xuezhong Ruan, Chih-Hsin Chen, Young Jun Shin, Youngbin Lee, Jing Niu, Jingbo Liu, Yuanfu Chen, Kun-Lin Yang, Xinhai Zhang, Jong-Hyun Ahn, and Hyunsoo Yang  »View Author Affiliations
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-18-21395&origin=search

Due to its high electrical conductivity and excellent transmittance at terahertz frequencies, graphene is a promising candidate as transparent electrodes for terahertz devices. We demonstrate a liquid crystal based terahertz phase shifter with the graphene films as transparent electrodes. The maximum phase shift is 10.8 degree and the saturation voltage is 5 V with a 50 µm liquid crystal cell. The transmittance at terahertz frequencies and electrical conductivity depending on the number of graphene layer are also investigated. The proposed phase shifter provides a continuous tunability, fully electrical controllability, and low DC voltage operation.

Abstract-Graphene/liquid crystal based terahertz phase shifters

Yang Wu, Xuezhong Ruan, Chih-Hsin Chen, Young Jun Shin, Youngbin Lee, Jing Niu, Jingbo Liu,Yuanfu Chen, Kun-Lin Yang, Xinhai Zhang, Jong-Hyun Ahn, Hyunsoo Yang

http://arxiv.org/abs/1309.1595

Due to its high electrical conductivity and excellent transmittance at terahertz frequencies, graphene is a promising candidate as transparent electrodes for terahertz devices. We demonstrate a liquid crystal based terahertz phase shifter with the graphene films as transparent electrodes. The maximum phase shift is 10.8 degree and the saturation voltage is 5 V with a 50 um liquid crystal cell. The transmittance at terahertz frequencies and electrical conductivity depending on the number of graphene layer are also investigated. The proposed phase shifter provides a continuous tunability, fully electrical controllability, and low DC voltage operation.