Abstract-Through-substrate terahertz time-domain reflection spectroscopy for environmental graphene conductivity mapping

Applied Physics Letters

 Hungyen Lin,  Oliver J. Burton, Sebastian Engelbrecht, Kai-Henning Tybussek,  Bernd M. Fischer,  Stephan Hofmann

Schematic of the reflection-based THz-TDS to measure the complex conductivity of graphene through the substrate in a nitrogen purge environment.

https://aip.scitation.org/doi/abs/10.1063/1.5135644

We demonstrate how terahertz time-domain spectroscopy (THz-TDS) operating in reflection geometry can be used for quantitative conductivity mapping of large area chemical vapor deposited graphene films through silicon support. We validate the technique against measurements performed using the established transmission based THz-TDS. Our through-substrate approach allows unhindered access to the graphene top surface and thus, as we discuss, opens up pathways to perform in situ and in-operando THz-TDS using environmental cells.

H.L. acknowledges financial support from the EPSRC (Grant No. EP/R019460/1). S.H. acknowledges funding from the EPSRC (Grant No. EP/K016636/1, GRAPHTED). We also thank Dr. Philipp Braeuninger-Weimer for useful discussion. Additional data for this article are available at https://doi.org/10.17635/lancaster/researchdata/336.

Abstract-Terahertz Nanoscopy of Plasmonic Resonances with a Quantum Cascade Laser

Riccardo Degl’Innocenti , Robert Wallis, Binbin Wei, Long Xiao, Stephen J. Kindness, Oleg Mitrofanov, Philipp Braeuninger-Weimer, Stephan Hofmann, Harvey E. Beere, David A. Ritchie

https://www.blogger.com/blogger.g?blogID=124073320791841682#editor/target=post;postID=2366204696351202320

We present a terahertz (THz) scattering near-field optical microscope (s-SNOM) based on a quantum cascade laser implemented as both source and detector in a self-mixing scheme utilizing resonant quartz tuning forks as a sensitive nanopositioning element. The homemade s-SNOM, based on a resonant tuning fork and metallic tip, operates in tapping mode with a spatial resolution of ∼78 nm. The quantum cascade laser is realized from a bound-to-continuum active region design with a central emission of ∼2.85 THz, which has been lens-coupled in order to maximize the feedback into the laser cavity. Accordingly, the spatial resolution corresponds to >λ/1000. The s-SNOM has been used to investigate a bidimensional plasmonic photonic crystal and to observe the optical resonant modes supported by coupled plasmonic planar antennas, showing remarkable agreement with the theoretical predictions. The compactness, unique sensitivity, and fast acquisition capability of this approach make the proposed s-SNOM a unique tool for solid-state investigations and biomedical imaging.

Abstract-Contactless graphene conductivity mapping on a wide range of substrates with terahertz time-domain reflection spectroscopy

Hungyen Lin, Philipp Braeuninger-Weimer, Varun S. Kamboj, David S. Jessop, Riccardo Degl’Innocenti, Harvey E. Beere, David A. Ritchie, J. Axel Zeitler,  Stephan Hofmann

https://www.nature.com/articles/s41598-017-09809-7?WT.feed_name=subjects_materials-science

We demonstrate how terahertz time-domain spectroscopy (THz-TDS) operating in reflection geometry can be used for quantitative conductivity mapping of large area chemical vapour deposited graphene films on sapphire, silicon dioxide/silicon and germanium. We validate the technique against measurements performed with previously established conventional transmission based THz-TDS and are able to resolve conductivity changes in response to induced back-gate voltages. Compared to the transmission geometry, measurement in reflection mode requires careful alignment and complex analysis, but circumvents the need of a terahertz transparent substrate, potentially enabling fast, contactless, in-line characterisation of graphene films on non-insulating substrates such as germanium.

Abstract-Bolometric detection of terahertz quantum cascade laser radiation with graphene-plasmonic antenna arrays

Riccardo Degl'Innocenti1,3, Long Xiao1,2, Stephen J Kindness1, Varun S Kamboj1, Binbin Wei1, Philipp Braeuninger-Weimer2, Kenichi Nakanishi2, Adrianus I Aria2, Stephan Hofmann2, 

 David A Ritchie1 , Harvey E Beere1

Published 27 March 2017 • © 2017 IOP Publishing Ltd 

http://iopscience.iop.org/article/10.1088/1361-6463/aa64bf/meta

We present a fast room temperature terahertz detector based on graphene loaded plasmonic antenna arrays. The antenna elements, which are arranged in series and are shorted by graphene, are contacting source and drain metallic pads, thus providing both the optical resonant element and the electrodes. The distance between the antenna's arms of approximately 300 nm allows a strong field enhancement in the graphene region, when the incident radiation is resonant with the antennas. The current passing through the source and drain is dependent on the graphene's conductivity, which is modified by the power impinging onto the detector as well as from the biasing back-gate voltage. The incident radiation power is thus translated into a current modification, with the main detection mechanism being attributed to the bolometric effect. The device has been characterized and tested with two bound to continuum terahertz quantum cascade lasers emitting at a single frequency around 2 THz and 2.7 THz yielding a maximum responsivity of ~2 mA W−1.

Abstract-Robust mapping of electrical properties of graphene from terahertz time-domain spectroscopy with timing jitter correction

Patrick R. Whelan, Krzysztof Iwaszczuk, Ruizhi Wang, Stephan Hofmann, Peter Bøggild, and Peter Uhd Jepsen
obust mapping of electrical properties of graphene from terahertz time-domain spectroscopy with timing jitter correction

We demonstrate a method for reliably determining the electrical properties of graphene including the carrier scattering time and carrier drift mobility from terahertz time- domain spectroscopy measurements (THz-TDS). By comparing transients originating from directly transmitted pulses and the echoes from internal reflections in a substrate, we are able to extract electrical properties irrespective of random time delays between pulses emitted in a THz-TDS setup. If such time delays are not accounted for they can significantly influence the extracted properties of the material. The technique is useful for a robust determination of electrical properties from THz-TDS measurements and is compatible with substrate materials where transients from internal reflections are well-separated in time.

© 2017 Optical Society of America

Full Article  |  PDF Article

Abstract-Fast room temperature detection of terahertz quantum cascade lasers with graphene loaded bow-tie plasmonic antenna arrays

Riccardo Degl'Innocenti, Long Xiao, David S. Jessop, Stephen J Kindness, Yuan Ren, Hungyen Lin, J. Axel Zeitler, Jack A. Alexander-Webber, Hannah J Joyce, Philipp Braeuninger-Weimer, Stephan Hofmann, Harvey E Beere, and David A. Ritchie

ACS Photonics, Just Accepted Manuscript

DOI: 10.1021/acsphotonics.6b00405

Publication Date (Web): September 20, 2016

Copyright © 2016 American Chemical Society

https://www.blogger.com/blogger.g?blogID=124073320791841682#editor/target=post;postID=4022381804460754923

We present a fast room temperature terahertz detector based on interdigitated bow-tie antennas contacting graphene. Highly efficient photodetection was achieved by using two metals with different work functions as the arms of a bow-tie antenna contacting graphene. Arrays of the bow-ties were fabricated in order to enhance the responsivity and coupling of the incoming light to the detector realizing an efficient imaging system. The device has been characterized and tested with a terahertz quantum cascade laser emitting in single frequency around 2 THz yielding a responsivity of ~ 34 μA/W and a noise-equivalent-power of ~1.5E-7W/Hz1/2

Abstract-Fast modulation of terahertz quantum cascade lasers using graphene loaded plasmonic antennas

Riccardo Degl'Innocenti, David S. Jessop, Christian W. O. Sol, Long Xiao, Stephen J. Kindness, Hungyen Lin, J. Axel Zeitler, Philipp Braeuninger-Weimer, Stephan Hofmann, Yuan Ren, Varun S. Kamboj, Jonathan P. Griffiths, Harvey E. Beere, and David A. Ritchie

ACS Photonics, Just Accepted Manuscript

DOI: 10.1021/acsphotonics.5b00672

Publication Date (Web): February 23, 2016

Copyright © 2016 American Chemical Society

http://pubs.acs.org/doi/abs/10.1021/acsphotonics.5b00672?journalCode=apchd5

We report the fast amplitude modulation of a quantum cascade laser emitting in single mode operation in the terahertz frequency range by employing compact, integrated devices based on the interplay between plasmonic antenna arrays and monolayer graphene. By acting on the carrier concentration of graphene the optical response of these plasmonic resonances was modified. The modulators characteristics have been studied by using both time domain spectroscopic laser systems, yielding the broad frequency response of these resonant arrays, and quantum cascade lasers, providing us with a narrow and stable laser source, a mandatory prerequisite for the determination of the modulation speed of these devices. The measured modulation speed exhibits a cut-off frequency of 5.5 MHz ± 1.1 MHz. These results represent the first step toward the realization of fast integrated circuitry for communications in the terahertz frequency range.

Abstract-Intrinsic terahertz plasmon signatures in chemical vapour deposited graphene

Shruti Badhwar¹, Juraj Sibik², Piran R. Kidambi³, Harvey E. Beere¹, J. Axel Zeitler³, Stephan Hofmann³, and David A. Ritchie¹

¹Department of Physics, University of Cambridge, Cambridge CB30HE, United Kingdom
²Department of Chemical Engineering and Biotechnology, University of Cambridge, Cambridge CB23RA, United Kingdom
³Department of Engineering, University of Cambridge, Cambridge CB30FA, United Kingdom 

http://apl.aip.org/resource/1/applab/v103/i12/p121110_s1?isAuthorized=no

Plasmonic resonance at terahertz (THz) frequencies can be achieved by gating graphene grown via chemical vapour deposition (CVD) to a high carrier concentration. THz time domain spectroscopy of such gated monolayer graphene shows resonance features around 1.6 THz, which appear as absorption peaks when the graphene is electrostatically p-doped and change to enhanced transmission when the graphene is n-doped. Superimposed on the Drude-like frequency response of graphene, these resonance features are related to the inherent poly-crystallinity of CVD graphene. An understanding of these features is necessary for the development of future THz optical elements based on CVD graphene.

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