Abstract-Asymmetric gap resonator based near-field coupling in terahertz metamaterials (Conference Presentation)

S. Jagan Mohan Rao, Yogesh Kumar Srivastava, Gagan Kumar,  Dibakar Roy Chowdhury

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10983/109830Z/Asymmetric-gap-resonator-based-near-field-coupling-in-terahertz-metamaterials/10.1117/12.2519820.short?SSO=1

The metamaterial is an arrangement of artificial structural elements designed to achieve advantageous and unusual electromagnetic properties. In the unit cell level, metamaterials are composed of an array of small structured elements called split ring resonators (SRRs). Recently, a lot of emphases has been given to the realization of terahertz metamaterials owing to its significance in the construction of terahertz photonic components. In this context, near-field coupling in terahertz metamaterials is extremely crucial. The short-range coupling in metamaterials occurs via the electric and magnetic fields due to the close proximity of the neighboring resonators. The electric field mainly couples through the gaps of SRRs, while the magnetic field couples through the circumference. In this work, we experimentally investigate near-field gap to gap capacitive coupling between a pair of single split gap ring resonators (SRRs) in a terahertz metamaterial. This has been achieved by manipulating the near field electric interactions via changing one resonator split gap with respect to the other resonator split gap for several inter resonator separations. Introducing asymmetry by changing the split gap in one resonator with respect to the other resonator, results in the split in the fundamental resonance mode when operated in the strong near-field coupled regime. The split occurs because of the strong near field capacitive/electric interactions between the resonators. We have further calculated Q factor for the lower and higher resonance modes for different inter resonator separations. The modulation of resonances in capacitive coupled planar terahertz metamaterial systems studied through this work has great potential in manipulating and controlling electromagnetic waves which can ultimately result in novel applications for terahertz frequency domain.

Abstract-Plasmon-induced transparency in graphene-based terahertz metamaterials

Koijam Monika Devi, Maidul Islam, Dibakar Roy Chowdhury, Amarendra K. Sarma,  Gagan Kumar,


http://iopscience.iop.org/article/10.1209/0295-5075/120/27005/meta


Plasmon-induced transparency (PIT) effect in a terahertz graphene metamaterial is numerically and theoretically analyzed. The proposed metamaterial comprises of a pair of graphene split ring resonators placed alternately on both sides of a graphene strip of nanometer scale. The PIT effect in the graphene metamaterial is studied for different vertical and horizontal configurations. We have shown that the PIT effect can be tuned by varying the Fermi energy of the graphene layer. A theoretical model using the three-level plasmonic system is established in order to validate the numerical results. Our studies could be significant in designing graphene-based frequency agile ultra-thin devices for terahertz applications.

Abstract-Terahertz Plasmonic Waveguide Based Thin Film Sensor

Maidul Islam, Dibakar Roy Chowdhury, Amir Ahmad, and Gagan Kumar

https://www.osapublishing.org/jlt/abstract.cfm?uri=jlt-35-23-5215

In this paper, we investigate sensing capabilities of a planar plasmonic terahertz waveguide. The waveguide is comprised of one-dimensional array of periodically arranged subwavelength scale corrugations in the form of rectangular dimples in order to ensure the plasmonic response. The terahertz waveguide transmission is observed for polyimide (as thin film) substance filling the dimples. The refractive index of the polyimide film is varied to examine various sensing parameters such as frequency shift, sensitivity, and figure of merit of the fundamental plasmonic resonance supported by the waveguide. We employ a semianalytical transmission line 𝐿𝐶$LC$ circuit model to elucidate our numerical observations. In efforts to improve sensing characteristics, we also examine sensing capabilities of a plasmonic waveguide having V-shaped corrugations and compare results with that of rectangular dimples. The proposed study could be significant in developing new terahertz sensors with improved sensitivity utilizing the plasmonic waveguides.

Abstract-Plasmon induced transparency in graphene based terahertz metamaterials

Koijam Monika Devi, M. Islam, Dibakar R. Chowdhury, Amarendra K. Sarma, Gagan Kumar

https://arxiv.org/abs/1711.03775

Plasmon induced transparency (PIT) effect in a terahertz graphene metamaterial is numerically and theoretically analyzed. The proposed metamaterial comprises of a pair of graphene split ring resonators placed alternately on both sides of a graphene strip of nanometer scale. The PIT effect in the graphene metamaterial is studied for different vertical and horizontal configurations. Our results reveal that there is no PIT effect in the graphene metamaterial when the centers of both the split ring resonators and the graphene strip are collinear to each other. This is a noteworthy feature, as the PIT effect does not vanish for similar configuration in a metal-based metamaterial structure. We have further shown that the PIT effect can be tuned by varying the Fermi energy of graphene layer. A theoretical model using the three level plasmonic system is established in order to validate the numerical results. Our studies could be significant in designing graphene based frequency agile ultra-thin devices for terahertz applications.

Abstract-Terahertz guided mode propagation in a planar plasmonic waveguide and slow light properties

Maidul Islam, Dibakar Roy Chowdhury, and Gagan Kumar

https://www.osapublishing.org/abstract.cfm?URI=photonics-2016-P1A.21

We examine terahertz guided mode properties of a plasmonic waveguide comprising periodic corrugations along the direction of propagation as well as within the structures. We also analyse the slow light properties of the guided modes.

© 2016 OSA

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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

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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-Application of nanoporous silicon substrates for terahertz spectroscopy

Shu-Zee A. Lo, Gagan Kumar, Thomas E. Murphy, and Edwin J. Heilweil  »View Author Affiliations

Mid to far-infrared (terahertz) spectroscopy is a valuable tool for probing and characterizing macromolecular structures and motions of complex molecules, including low frequency vibrational and phonon modes in condensed phases. We describe here an improved and readily implemented method for performing terahertz spectroscopic measurements by using a nanoporous silicon substrate to capture and concentrate the substance to be analyzed. We compare the results to conventional sampling methods, including dissolution and crystallization on a flat silicon surface and dispersing crystallites in compressed polyethylene pellets, and show that the use of a transparent, nanoporous substrate provides both increased sensitivity and yields sharper spectral features than conventional solid-state sampling approaches. FTIR measurements are reported over the spectral range from 50–2000 cm⁻¹ (1.5–60 THz), for salicylic acid, dicyanobenzene, glycine, and aspartame.