Abstract-Cavity-based photoconductive sources for real-time terahertz imaging

J. Hawecker, V. Pistore, A. Minasyan, K. Maussang, J. Palomo, I. Sagnes, J.-M. Manceau, R. Colombelli, J. Tignon, J. Mangeney, and S. S. Dhillon

(a) Optical image (front) of the object. (b) Optical image (back of object) of hidden text. (c) Real-time image from the THz camera using the high average power from the quasi-cavity PC switch showing the hidden text. 

https://www.osapublishing.org/prj/abstract.cfm?uri=prj-8-6-858

Optically driven photoconductive switches are one of the predominant sources currently used in terahertz imaging systems. However, owing to their low average powers, only raster-based images can be taken, resulting in slow acquisition. In this work, we show that by placing a photoconductive switch within a cavity, we are able to generate absolute average THz powers of 181 μW with the frequency of the THz emission centered at 1.5 THz—specifications ideally adapted to applications such as non-destructive imaging. The cavity is based on a metal–insulator–metal structure that permits an enhancement of the average power by almost 1 order of magnitude compared to a standard structure, while conserving a broadband spectral response. We demonstrate proof-of-principle real-time imaging using this source, with the broadband spectrum permitting to eliminate strong diffraction artifacts.

© 2020 Chinese Laser Press

Abstract-Extraction-controlled terahertz frequency quantum cascade lasers with a diagonal LO-phonon extraction and injection stage

Y. J. Han, L. H. Li, A. Grier, L. Chen, A. Valavanis, J. Zhu, J. R. Freeman, N. Isac, R. Colombelli, P. Dean, A. G. Davies, and E. H. Linfield

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-24-25-28583

We report an extraction-controlled terahertz (THz)-frequency quantum cascade laser design in which a diagonal LO-phonon scattering process is used to achieve efficient current injection into the upper laser level of each period and simultaneously extract electrons from the adjacent period. The effects of the diagonality of the radiative transition are investigated, and a design with a scaled oscillator strength of 0.45 is shown experimentally to provide the highest temperature performance. A 3.3 THz device processed into a double-metal waveguide configuration operated up to 123 K in pulsed mode, with a threshold current density of 1.3 kA/cm2 at 10 K. The QCL structures are modeled using an extended density matrix approach, and the large threshold current is attributed to parasitic current paths associated with the upper laser levels. The simplicity of this design makes it an ideal platform to investigate the scattering injection process.

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.

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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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Abstract-Subwavelength metallic waveguides as a tool for extreme confinement of THz surface waves

D. Gacemi, J. Mangeney, R. Colombelli, A. Degiron, 
http://www.nature.com/srep/2013/130306/srep01369/full/srep01369.html
Research on surface waves supported by metals at THz frequencies is experiencing a tremendous growth due to their potential for imaging, biological sensing and high-speed electronic circuits. Harnessing their properties is, however, challenging because these waves are typically poorly confined and weakly bound to the metal surface. Many design strategies have been introduced to overcome these limitations and achieve increased modal confinement, including patterned surfaces, coated waveguides and a variety of sub-wavelength geometries. Here we provide evidence, using a combination of numerical simulations and time-resolved experiments, that shrinking the transverse size of a generic metallic structure always leads to solutions with extreme field confinement. The existence of such a general behavior offers a new perspective on energy confinement and should benefit future developments in THz science and technology.

Abstract-Hybrid electronic-photonic subwavelength cavities operating at terahertz frequencies

http://prb.aps.org/abstract/PRB/v87/i4/e041408

E. Strupiechonski¹, G. Xu¹, P. Cavalié², N. Isac¹, S. Dhillon², J. Tignon², G. Beaudoin³, I. Sagnes³, A. Degiron^1,*, and R. Colombelli^1,† 
1Institut d’Electronique Fondamentale, Université Paris Sud and CNRS, UMR8622, 91405 Orsay, France
²Laboratoire Pierre Aigrain, Ecole Normale Supérieure, CNRS (UMR 8551), Université P. et M. Curie, Université D. Diderot, 75231 Paris Cedex 05, France
³Laboratoire de Photonique et Nanostructures, CNRS UPR20, 91460 Marcoussis, France

We report on the concept and realization of terahertz (THz) metallic/semiconductor cavities characterized by dimensions as small as λ_eff/9 in all directions of space (or λ/30 with the vacuum wavelength). We experimentally prove that the capacitance and inductance of these devices are not interdependent, as in purely photonic cavities, but that they can be adjusted almost independently, as in an electronic circuit. This functionality proves that the dimensions of these hybrid electronic-photonic devices are intrinsically not limited by diffraction as in conventional photonic resonators. Using arguments from transmission line theory, we show that it is necessary to include at least one metallic loop in the cavity design to access this regime and we note, as a corollary, that some recent proposals to miniaturize THz resonators and devices beyond the diffraction limit do not meet this fundamental requirement. Our results shed a light on the ability to describe THz/metallic cavities in terms of circuit elements. Furthermore, upon insertion of electrical contacts, these extremely subwavelength resonators can potentially lead to advances in THz detectors and phased array antennas.

©2013 American Physical Society