Abstract-Tunable broadband terahertz polarizer using graphene-metal hybrid metasurface

K. Meng, S. J. Park, L. H. Li, D. R. Bacon, L. Chen, K. Chae, J. Y. Park, A. D. Burnett, E. H. Linfield, A. G. Davies, and J. E. Cunningham

 (a) Schematic diagram of the graphene-metal hybrid wire grid structure. Upper figure: cross-section of the array, lower figure: top view of the array. (b) Schematic diagram of the THz transmission experiment: the lower electrode was used for applying gate voltage and the upper two electrodes were connected to a source meter for measuring the conductivity (indicated by G) (c) DC conductivities of graphene in each device as a function of gate voltage. (d) SEM image of the graphene-metal hybrid wire grids with 𝛬𝑀/𝐺=30𝜇𝑚${\Lambda }_{M/G}=30\mu m$. (e) Raman spectrum of the graphene in a typical device.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-23-33768

We demonstrate an electrically tunable polarizer for terahertz (THz) frequency electromagnetic waves formed from a hybrid graphene-metal metasurface. Broadband (>3 THz) polarization-dependent modulation of THz transmission is demonstrated as a function of the graphene conductivity for various wire grid geometries, each tuned by gating using an overlaid ion gel. We show a strong enhancement of modulation (up to ∼17 times) compared to graphene wire grids in the frequency range of 0.2–2.5 THz upon introduction of the metallic elements. Theoretical calculations, considering both plasmonic coupling and Drude absorption, are in good agreement with our experimental findings.

Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Abstract-Diffraction-limited ultrabroadband terahertz spectroscopy

• M. Baillergeau
• , K. Maussang
• , T. Nirrengarten
• , J. Palomo
• , L. H. Li
• , E. H. Linfield
• , A. G. Davies
• , S. Dhillon
• ,J. Tignon
•  & J. Mangeney
• 
  

http://www.nature.com/articles/srep24811

Diffraction is the ultimate limit at which details of objects can be resolved in conventional optical spectroscopy and imaging systems. In the THz spectral range, spectroscopy systems increasingly rely on ultra-broadband radiation (extending over more 5 octaves) making a great challenge to reach resolution limited by diffraction. Here, we propose an original easy-to-implement wavefront manipulation concept to achieve ultrabroadband THz spectroscopy system with diffraction-limited resolution. Applying this concept to a large-area photoconductive emitter, we demonstrate diffraction-limited ultra-broadband spectroscopy system up to 14.5 THz with a dynamic range of 103. The strong focusing of ultrabroadband THz radiation provided by our approach is essential for investigating single micrometer-scale objects such as graphene flakes or living cells, and besides for achieving intense ultra-broadband THz electric fields. 

Figure 1: THz radiation properties under plane wave-front optical excitation of the emitter at selected frequencies higher than c/(2πwTHz).

Abstract-Engineered far-fields of metal-metal terahertz quantum cascade lasers with integrated planar horn structures

F. Wang, I. Kundu, L. Chen, L. Li, E. H. Linfield, A. G. Davies, S. Moumdji, R. Colombelli, J. Mangeney, J. Tignon, and S. S. Dhillon

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-24-3-2174

The far-field emission profile of terahertz quantum cascade lasers (QCLs) in metal-metal waveguides is controlled in directionality and form through planar horn-type shape structures, whilst conserving a broad spectral response. The structures produce a gradual change in the high modal confinement of the waveguides and permit an improved far-field emission profile and resulting in a four-fold increase in the emitted output power. The two-dimensional far-field patterns are measured at 77 K and are agreement in with 3D modal simulations. The influence of parasitic high-order transverse modes is shown to be controlled by engineering the horn structure (ridge and horn widths), allowing only the fundamental mode to be coupled out.

© 2016 Optical Society of America

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Absrtact-The MBE growth and optimization of high performance terahertz frequency quantum cascade lasers

The MBE growth and optimization of high performance terahertz frequency quantum cascade lasers L. H. Li, J. X. Zhu, L. Chen, A. G. Davies, and E. H. Linfield  »View Author Affiliations

http://www.opticsinfobase.org/oe/fulltext.cfm?uri=oe-23-3-2720&id=310967

Optics Express, Vol. 23, Issue 3, pp. 2720-2729 (2015)
http://dx.doi.org/10.1364/OE.23.002720

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The technique of molecular beam epitaxy has recently been used to demonstrate the growth of terahertz frequency GaAs/AlGaAs quantum cascade lasers (QCL) with Watt-level optical output powers. In this paper, we discuss the critical importance of achieving accurate layer thicknesses and alloy compositions during growth, and demonstrate that precise growth control as well as run-to-run growth reproducibility is possible. We also discuss the importance of minimizing background doping level in maximizing QCL performance. By selecting high-performance active region designs, and optimizing the injection doping level and device fabrication, we demonstrate total optical (two-facet) output powers as high as 1.56 W.

© 2015 Optical Society of America

Abstract-Coupled-cavity terahertz quantum cascade lasers for single mode operation

H. Li^1,a), J. M. Manceau^1,b), A. Andronico¹, V. Jagtap¹, C. Sirtori¹, L. H. Li²,E. H. Linfield², A. G. ² and S. Barbieri^1,c)

 HIDE AFFILIATIONS
1 Laboratoire Matériaux et Phénomènes Quantiques, Université Paris Diderot and CNRS, UMR 7162, 10 rue A. Domont et L. Duquet, 75205 Paris, France
2 School of Electronic and Electrical Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom
a) Electronic mail: hua.li@univ-paris-diderot.fr
b) Present address: Institut d'Electronique Fondamentale, Université Paris Sud and CNRS, UMR 8622, 91405 Orsay, France
c) Electronic mail: stefano.barbieri@univ-paris-diderot.fr
Appl. Phys. Lett. 104, 241102 (2014); http://dx.doi.org/10.1063/1.4884056
http://scitation.aip.org/content/aip/journal/apl/104/24/10.1063/1.4884056
We demonstrate the operation of coupled-cavity terahertz frequency quantum-cascade lasers composed of two sub-cavities separated by an air gap realized by optical lithography and dryetching. This geometry allows stable, single mode operation with typical side mode suppression ratios in the 30–40 dB range. We employ a transfer matrix method to model the mode selection mechanism. The obtained results are in good agreement with the measurements and allow prediction of the operating frequency.

Abstract-Terahertz inverse synthetic aperture radar imaging using self-mixing interferometry with a quantum cascade laser

H. S. Lui, T. Taimre, K. Bertling, Y. L. Lim, P. Dean, S. P. Khanna, M. Lachab, A. Valavanis, D. Indjin, E. H. Linfield, A. G. Davies, and A. D. Rakić  »View Author Affiliations
http://www.opticsinfobase.org/ol/abstract.cfm?uri=ol-39-9-2629

We propose a terahertz (THz)-frequency synthetic aperture radar imaging technique based on self-mixing (SM) interferometry, using a quantum cascade laser. A signal processing method is employed which extracts and exploits the radar-related information contained in the SM signals, enabling the creation of THz images with improved spatial resolution. We demonstrate this by imaging a standard resolution test target, achieving resolution beyond the diffraction limit.

© 2014 Optical Society of America

Abstract-Terahertz inverse synthetic aperture radar imaging using self-mixing interferometry with a quantum cascade laser

H. S. Lui, T. Taimre, K. Bertling, Y. L. Lim, P. Dean, S. P. Khanna, M. Lachab, A. Valavanis, D. Indjin, E. H. Linfield, A. G. Davies, and A. D. Rakić  »View Author Affiliations

http://www.opticsinfobase.org/ol/abstract.cfm?URI=ol-39-9-2629

Optics Letters, Vol. 39, Issue 9, pp. 2629-2632 (2014)
http://dx.doi.org/10.1364/OL.39.002629

We propose a terahertz (THz)-frequency synthetic aperture radar imaging technique based on self-mixing (SM) interferometry, using a quantum cascade laser. A signal processing method is employed which extracts and exploits the radar-related information contained in the SM signals, enabling the creation of THz images with improved spatial resolution. We demonstrate this by imaging a standard resolution test target, achieving resolution beyond the diffraction limit.

© 2014 Optical Society of America