Abstract-Total Internal Reflection Geometry: Exploiting Total Internal Reflection Geometry for Terahertz Devices and Enhanced Sample Characterization

Qiushuo Sun, Xuequan Chen, Xudong Liu, Rayko I. Stantchev, Emma Pickwell‐MacPherson

https://onlinelibrary.wiley.com/doi/abs/10.1002/adom.201900535

To promote potential applications of terahertz (THz) technology, more advanced functional THz devices with high performance are needed, including modulators, polarizers, lenses, wave retarders, and antireflection coatings. This work summarizes recent progress in THz components built on functional materials including graphene, vanadium dioxide, and metamaterials. The key message is that, while the choice of materials used in such devices is important, the geometry in which they are employed also has a significant effect on the performance achieved. In particular, devices operating in total internal reflection geometry are reviewed, and it is explained how this geometry is able to be exploited to achieve a variety of THz devices with broadband operation.

Abstract-Highly Efficient Ultra‐Broadband Terahertz Modulation Using Bidirectional Switching of Liquid Crystals

Xuequan Chen, Kaidi Li,  Rui Zhang, Swadesh Kumar Gupta, Abhishek Kumar Srivastava, Emma Pickwell‐MacPherson,

https://onlinelibrary.wiley.com/doi/abs/10.1002/adom.201901321

Accurately manipulating field strength and polarization state are essential in various terahertz applications. Such manipulations are based on the efficient modulation of the amplitude and phase of electromagnetic waves. However, there is a lack of such terahertz modulators with sufficient efficiency and bandwidth. Herein, the Brewster–critical angle is exploited for modulation by using a nematic liquid crystal. Unlike liquid crystal phase shifters that only give a narrowband phase delay via a one‐directional switch, the presented device modulates both the amplitude and phase across an ultra‐broadbandwidth via a bidirectional active switch. An average intensity modulation depth over 99.6% is achieved for 0.2–1.6 THz. Furthermore, highly accurate polarization conversion between linear and circular states is also realized for 0.4–1.8 THz, with the average degree of linear and circular polarizations as high as 0.994 and 0.998, respectively. The superior accuracy, bandwidth, and active control achieved provide great potential for multifunctional terahertz modulation.

Abstract-Exploiting Total Internal Reflection Geometry for Terahertz Devices and Enhanced Sample Characterization

Qiushuo Sun, Xuequan Chen, Xudong Liu, Rayko I. Stantchev 

https://onlinelibrary.wiley.com/doi/abs/10.1002/adom.201900535

To promote potential applications of terahertz (THz) technology, more advanced functional THz devices with high performance are needed, including modulators, polarizers, lenses, wave retarders, and antireflection coatings. This work summarizes recent progress in THz components built on functional materials including graphene, vanadium dioxide, and metamaterials. The key message is that, while the choice of materials used in such devices is important, the geometry in which they are employed also has a significant effect on the performance achieved. In particular, devices operating in total internal reflection geometry are reviewed, and it is explained how this geometry is able to be exploited to achieve a variety of THz devices with broadband operation.

Abstract-Graphene controlled Brewster angle device for ultra broadband terahertz modulation

Zefeng Chen, Xuequan Chen, Li Tao, Kun Chen, Mingzhu Long, Xudong Liu, Keyou Yan, Rayko I. Stantchev, Emma Pickwell-MacPherson, Jian-Bin Xu

https://www.nature.com/articles/s41467-018-07367-8


Terahertz modulators with high tunability of both intensity and phase are essential for effective control of electromagnetic properties. Due to the underlying physics behind existing approaches there is still a lack of broadband devices able to achieve deep modulation. Here, we demonstrate the effect of tunable Brewster angle controlled by graphene, and develop a highly-tunable solid-state graphene/quartz modulator based on this mechanism. The Brewster angle of the device can be tuned by varying the conductivity of the graphene through an electrical gate. In this way, we achieve near perfect intensity modulation with spectrally flat modulation depth of 99.3 to 99.9 percent and phase tunability of up to 140 degree in the frequency range from 0.5 to 1.6 THz. Different from using electromagnetic resonance effects (for example, metamaterials), this principle ensures that our device can operate in ultra-broadband. Thus it is an effective principle for terahertz modulation.

Abstract-Robust and accurate terahertz time-domain spectroscopic ellipsometry

Xuequan Chen, Edward P. J. Parrott, Zhe Huang, Hau-Ping Chan, and Emma Pickwell-MacPherson

https://www.osapublishing.org/prj/abstract.cfm?uri=prj-6-8-768

In this work, we show how fiber-based terahertz systems can be robustly configured for accurate terahertz ellipsometry. To this end, we explain how our algorithms can be successfully applied to achieve accurate spectroscopic ellipsometry with a high tolerance on the imperfect polarizer extinction ratio and pulse shift errors. Highly accurate characterization of transparent, absorptive, and conductive samples comprehensively demonstrates the versatility of our algorithms. The improved accuracy we achieve is a fundamental breakthrough for reflection-based measurements and overcomes the hurdle of phase uncertainty.

© 2018 Chinese Laser Press

Abstract-Exploiting a metal wire grating in total internal reflection geometry to achieve achromatic polarization conversion

Xudong Liu, Xuequan Chen, Edward P. J. Parrott, and Emma Pickwell-MacPherson

https://www.osapublishing.org/prj/abstract.cfm?uri=prj-5-4-299&origin=search

We demonstrate how a metal wire grating can work as a 45° polarization converter, a quarter-wave retarder, and a half-wave retarder over a broadband terahertz range when set up in total internal reflection geometry. Classical electromagnetic theory is applied to understand the mechanism, and equations to calculate the polarization state of reflected light are derived. We use a metal grating with a period of 20 μm and width of 10 μm on a fused silica surface: linearly polarized terahertz light incident from fused silica with a supercritical incident angle of 52° is totally reflected by the metal grating and air. The polarization of the terahertz light is rotated by 45°, 90°, and circularly polarized by simply rotating the wire grating. The performance is achromatic over the measured range of 0.1–0.7 THz and comparable to commercial visible light wave retarders.

© 2017 Chinese Laser Press

Abstract-A Robust Baseline and Reference Modification and Acquisition Algorithm for Accurate THz Imaging

 Xuequan Chen,  Edward P. J. Parrott ,  Benjamin S.-Y. Ung, Emma Pickwell-MacPherson

http://ieeexplore.ieee.org/document/7987808/

This paper reports an accurate baseline and reference modification and acquisition algorithm for terahertz (THz) reflection measurements. The algorithm solves the spatial phase variation problem, which is a major accuracy limitation of THz reflection imaging. It also overcomes the sampling error problem without taking multiple measurements by utilizing two-dimensional data fitting. The algorithm records the spatial information and the laser power status as a database to precisely modify the baseline and reference in a measurement, providing a simple, efficient, and accurate real-time baseline and reference acquisition method. The experimental results show that the algorithm significantly improves the accuracy of the extracted optical properties of the sample with only half of the time required in a traditional THz imaging by avoiding additional baseline and reference measurements. The strong robustness of the algorithm is also proved by the experiment, giving a stable performance against laser power fluctuations.