Abstract-Study of a high-order mode terahertz backward wave ocsillator driven by multiple sheet electron beams

G. X. Shu, C. Q. Zhou, H. Xiong,  L. Chen, Z. F. Qian,  G. Liu,

https://ieeexplore.ieee.org/document/9015763

The concept of achieving powerful terahertz radiation by the interaction between high-order mode backward wave and multiple sheet electron beams is proposed to increase the operating frequency of the backward wave oscillator (BWO) to a high level such as over 1 THz. For the high-order mode operation, an orthogonal grating waveguide slow wave structure is proposed. Particle-in-cell simulations show that the high-order mode BWO can generate over 0.84 W power in the frequency range of 1.20-1.32 THz. The proposed methodology provides a potential solution to develop compact terahertz radiation sources with high output power and broad tunable bandwidth.

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-Increasing the sensitivity of terahertz split ring resonator metamaterials for dielectric sensing by localized substrate etching

K. Meng, S. J. Park, A. D. Burnett, T. Gill, C. D. Wood, M. Rosamond, L. H. Li, L. Chen, D. R. Bacon, J. R. Freeman, P. Dean, Y. H. Ahn, E. H. Linfield, A. G. Davies,  J. E. Cunningham, 

Fig. 1. (a) Schematic of THz transmission experiment for dielectric sensing using the etched metamaterials. (b) Schematic of THz metamaterials arrays with etched trenches. The periodicity of the metamaterials unit cells is indicated (c) An SEM image of the metamaterial with a trench depth of 1.74 µm. Cross-section SEM images of the metamaterials with trench depths t of (d) 1.74 µm and (e) 130 nm in the gap area.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-16-23164

We demonstrate a significant enhancement in the sensitivity of split ring resonator terahertz metamaterial dielectric sensors by the introduction of etched trenches into their inductive-capacitive gap area, both through finite element simulations and in experiments performed using terahertz time-domain spectroscopy. The enhanced sensitivity is demonstrated by observation of an increased frequency shift in response to overlaid dielectric material of thicknesses up to 18 µm deposited on to the sensor surface. We show that sensitivity to the dielectric is enhanced by a factor of up to ∼2.7 times by the incorporation of locally etched trenches with a depth of ∼3.4 µm, for example, and discuss the effect of the etching on the electrical properties of the sensors. Our experimental findings are in good agreement with simulations of the sensors obtained using finite element methods.

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-Silver-based surface plasmon waveguide for terahertz quantum cascade lasers

Y. J. Han, L. H. Li, J. Zhu, A. Valavanis, J. R. Freeman, L. Chen, M. Rosamond, P. Dean, A. G. Davies, and E. H. Linfield

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-26-4-3814

Terahertz-frequency quantum cascade lasers (THz QCLs) based on ridge waveguides incorporating silver waveguide layers have been investigated theoretically and experimentally, and compared with traditional gold-based devices. The threshold gain associated with silver-, gold- and copper-based devices, and the effects of titanium adhesion layers and top contact layers, in both surface-plasmon and double-metal waveguide geometries, have been analysed. Our simulations show that silver-based waveguides yield lower losses for THz QCLs across all practical operating temperatures and frequencies. Experimentally, QCLs with silver-based surface-plasmon waveguides were found to exhibit higher operating temperatures and higher output powers compared to those with identical but gold-based waveguides. Specifically, for a three-well resonant phonon active region with a scaled oscillator strength of 0.43 and doping density of 6.83 × 1015 cm−3, an increase of 5 K in the maximum operating temperature and 40% increase in the output power were demonstrated. These effects were found to be dependent on the active region design, and greater improvements were observed for QCLs with a larger radiative diagonality. Our results indicate that silver-based waveguide structures could potentially enable THz QCLs to operate at high temperatures.

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-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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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