Abstract-Graphene-based waveguide-integrated terahertz modulator

Martin Mittendorff, Shanshan Li, and Thomas E. Murphy

ACS Photonics, Just Accepted Manuscript

DOI: 10.1021/acsphotonics.6b00751

Publication Date (Web): January 5, 2017

Copyright © 2017 American Chemical Society

http://pubs.acs.org/doi/abs/10.1021/acsphotonics.6b00751?journalCode=apchd5

One of the major difficulties in the development of optoelectronic THz modulators is finding an active material that allows for large modulation depth. Graphene is a promising candidate because in the terahertz regime it behaves as a Drude metal with conductivity that can be electrostatically tuned through the application of a gate voltage. However, the maximum absorption incurred when a terahertz signal passes through a monolayer of graphene is still only of order 10-20%, even for the highest practically achievable carrier concentrations. We demonstrate here a THz modulator that overcomes this fundamental limitation by incorporating a graphene sheet on the surface of a passive silicon dielectric waveguide, in which the evanescent field penetrates the graphene sheet. By applying a gate voltage to the graphene sheet, a modulation depth of up to 50% was achieved. The performance of the modulator is confirmed through electromagnetic simulations, which give further insights into the spatial structure of the guided mode and polarization-dependence of the modulation. We show, both theoretically and experimentally that the modulation depth can be increased to over 90% by integrating the graphene sheet at the center of the waveguide.

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 nonlinear conduction and absorption saturation in silicon waveguides

Shanshan Li, Gagan Kumar, and Thomas E. Murphy
https://www.osapublishing.org/optica/abstract.cfm?uri=optica-2-6-553

The interaction of terahertz waves with silicon is usually explained using a linear model of conduction in which free carriers respond to the oscillating electric field, leading to absorption. Here we employ a silicon dielectric waveguide to confine and concentrate terahertz pulses, and observe that the absorption saturates under strong terahertz fields. By comparing the response between lightly-doped and intrinsic silicon waveguides, we confirm the role of hot carriers in this saturable absorption. We introduce a nonlinear dynamical model of Drude conductivity that, when incorporated into a wave propagation equation, predicts a comparable field-induced transparency and elucidates the physical mechanism underlying this nonlinear effect: velocity saturation—an effect that fundamentally limits the speed of most semiconductor devices. The results are numerically confirmed by Monte Carlo simulations of the Boltzmann transport equation, coupled with split-step nonlinear wave propagation. The results reported here could have significance in understanding and designing a variety of emerging and future terahertz devices, such as waveguides, mixers, detectors, and oscillators.

© 2015 Optical Society of America

Full Article  |  PDF Article

Abstract-Terahertz nonlinear conduction and absorption saturation in silicon waveguides

Shanshan Li, Gagan Kumar, Thomas E. Murphy
http://xxx.tau.ac.il/abs/1503.05639

We employ a silicon dielectric waveguide to confine and concentrate terahertz pulses, and observe that the absorption saturates under strong terahertz fields. By comparing the response between lightly-doped and intrinsic silicon waveguides, we confirm the role of hot carriers in this saturable absorption. We introduce a nonlinear dynamical model of Drude conductivity that, when incorporated into a wave propagation equation, accurately reproduces the observations and elucidates the physical mechanisms underlying this nonlinear effect. The results are numerically confirmed by Monte Carlo simulations of the Boltzmann transport equation, coupled with split-step nonlinear wave propagation.

Abstract-Terahertz surface plasmon waveguide based on a one-dimensional array of silicon pillars

Gagan Kumar1,3, Shanshan Li2, Mohammad M Jadidi2 and

Thomas E Murphy1,2

1 Institute for Research in Electronics and Applied Physics, University of

Maryland, College Park, MD 20742, USA

2 Department of Electrical and Computer Engineering, University of Maryland,

College Park, MD 20742, USA

E-mail: gkm2010@umd.edu

New Journal of Physics 15 (2013) 085031 (11pp)

Received 24 April 2013

Published 28 August 2013

Online at http://www.njp.org/

doi:10.1088/1367-2630/15/8/085031

http://m.iopscience.iop.org/1367-2630/15/8/085031/pdf/1367-2630_15_8_085031.pdf

Abstract. We experimentally demonstrate a three-dimensional plasmonic

terahertz waveguide by lithographically patterning an array of sub-wavelength

pillars on a silicon substrate. Doped silicon can exhibit conductive properties

at terahertz frequencies, making it a convenient substitute for conventional

metals in plasmonic devices. However, the surface wave solution at a doped

silicon surface is usually poorly confined and lossy. Here we demonstrate that

by patterning the silicon surface with an array of sub-wavelength pillars, the

resulting structure can support a terahertz surface mode that is tightly confined

in both transverse directions. Further, we observe that the resonant behavior

associated with the surface modes depends on the dimensions of the pillars, and

can be tailored through control of the structural parameters. We experimentally

fabricated devices with different geometries, and characterized the performance

using terahertz time-domain spectroscopy. The resulting waveguide characteristics
 are confirmed using finite element numerical simulations, and we further show that
 a simple one-dimensional analytical theory adequately predicts the observed dispersion
 relation.

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