Abstract-Frequency-tunable continuous-wave random lasers at terahertz frequencies

Simone Biasco, Harvey E. Beere, David A. Ritchie, Lianhe Li, A. G. Davies, Edmund Linfield,  Miriam S. Vitiello

Fig. 1: Device schematics and electromagnetic simulations

https://www.nature.com/articles/s41377-019-0152-z

Random lasers are a class of devices in which feedback arises from multiple elastic scattering in a highly disordered structure, providing an almost ideal light source for artefact-free imaging due to achievable low spatial coherence. However, for many applications ranging from sensing and spectroscopy to speckle-free imaging, it is essential to have high-radiance sources operating in continuous-wave (CW). In this paper, we demonstrate CW operation of a random laser using an electrically pumped quantum-cascade laser gain medium in which a bi-dimensional (2D) random distribution of air holes is patterned into the top metal waveguide. We obtain a highly collimated vertical emission at ~3 THz, with a 430 GHz bandwidth, device operation up to 110 K, peak (pulsed) power of 21 mW, and CW emission of 1.7 mW. Furthermore, we show that an external cavity formed with a movable mirror can be used to tune a random laser, obtaining continuous frequency tuning over 11 GHz.

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-Optical side-band generation in THz Fabry-Perot laser cavities

Owen P. Marshall, Md. Khairuzzaman, Harvey E. Beere, David A. Ritchie,  Subhasish Chakrabort,

http://aip.scitation.org/doi/abs/10.1063/1.5001334

Optical nonlinearities in semiconductor laser cavities can be exploited to characterize the properties of laser radiation or perform high speed frequency conversion operations. For example, nonlinear up-conversion inside the cavity of quantum cascade lasers allows the use of near infrared optical components to measure high-speed terahertz or mid-infrared optical effects. This letter investigates two aspects of cavity up-conversion which control both the bandwidth and up-converted power: waveguide dispersion and cavity feedback. Specifically, we up-convert multi-mode Fabry Perot terahertz laser emission and detect each THz mode as a sideband signal on an optical carrier in the near infrared. Analysis of these results shows that a single frequency near infrared laser can up-convert terahertz modes spanning a bandwidth of approximately 220 GHz, limited by the group index mismatch between the near infrared and terahertz waves. Second, transfer matrix techniques are used to study strong cavity feedback on all three waves, which produces etalon-like resonances in the sideband power. This can significantly enhance the efficiency of the conversion process, in agreement with experiments. It is thus possible to achieve high up-conversion efficiency in quantum cascade lasers for both characterizing broadband laser sources and performing frequency conversion in the near infrared.

Abstract-Terahertz s-SNOM with > λ/1000 resolution based on self-mixing in quantum cascade lasers

 Binbin Wei, Robert Wallis,  Stephen Kindness,  Oleg Mitrofanov,   Harvey E. Beere,  David A. Ritchie,   Riccardo Degl'Innocenti

http://ieeexplore.ieee.org/document/8086433/


Near-field imaging techniques have great potential in many applications, ranging from the investigation of the optical properties of solid state and 2D materials to the excitation and direct retrieval of plasmonic resonant modes, to the mapping of carrier concentrations in semiconductor devices. Further to this, the capability of performing imaging with non-ionizing terahertz (THz) radiation on a subwavelength scale is of fundamental importance in biological applications and healthcare. The implementation of stable, compact solid state sources such as quantum cascade lasers (QCLs) in apertureless scanning near field optical microscopes (s-SNOM), instead of bulkier gas lasers, has been already reported with a resolution ≥ 1 μm [1] based on metallic tips. Here we report on the realization of an s-SNOM, based on tuning fork sensors [2], to maintain a constant sample/tip distance in tapping mode, and using quantum cascade lasers emitting around 3 THz as both source and detector in a self-mixing scheme [3]. The implementation of a fast and efficient feedback mechanism allowed the achievement of a spatial resolution lower than 100 nm, as shown in Fig. 1, thus achieving the record resolution with a QCL better than λ/1000. The self-mixing approach allows an extremely sensitive and fast detection scheme, which overcomes the slow response of traditional THz detectors, by monitoring the scattered signal fed back into the QCL cavity, modulating the power or the bias. In order to enhance the sensitivity of the whole apparatus, as well as the collection of the scattered light, silicon lenses have been attached to the QCLs with an antireflection parylene coating which was thick enough to strongly reduce the laser emission, but still allowed enough power for alignment. Figure 1 reports the topography a) and the THz voltage signal on the QCL b) of Au square features (top-left square corner) over a Si substrate, exhibiting an enhanced scattering. As the reference voltage used for subtraction from the QCL voltage was placed lower than the QCL voltage, the THz signal dropped on the Au square.

Abstract-Coherent detection of THz laser signals in optical fiber systems

Thomas G. Folland, Owen P. Marshall, Harvey E. Beere, David A. Ritchie, and Subhasish Chakraborty


https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-21-25566&origin=search

Terahertz (THz) coherent detectors are crucial for the stabilization and measurement of the properties of quantum cascade lasers (QCLs). This paper describes the exploitation of intra-cavity sum frequency generation to up-convert the emission of a THz QCL to the near infrared for detection with fiber optic coupled components alone. Specifically, a low cost infrared photodiode is used to detect a radio frequency (RF) signal with a signal-to-noise ratio of approximately 20dB, generated by beating the up-converted THz wave and a near infrared local oscillator. This RF beat note allows direct analysis of the THz QCL emission in time and frequency domains. The application of this technique for QCL characterization is demonstrated by analyzing the continuous tuning of the RF signal over 2 GHz, which arises from mode tuning across the QCL’s operational current range.

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-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-Improved Tuning Fork for Terahertz Quartz-Enhanced Photoacoustic Spectroscopy

Angelo Sampaolo 1,2

, Pietro Patimisco 1,2

, Marilena Giglio 1

, Miriam S. Vitiello 3

, Harvey E. Beere 4

,David A. Ritchie 4

, Gaetano Scamarcio 1

, Frank K. Tittel 2

 and Vincenzo Spagnolo 1,* 

1 Dipartimento Interateneo di Fisica, Università degli studi di Bari Aldo Moro e Politecnico di Bari, Via Amendola 173, Bari I-70126, Italy2 Department of Electrical and Computer Engineering, Rice University, 6100 Main Street, Houston, TX 77005, USA3 NEST, CNR-Istituto Nanoscienze and Scuola Normale Superiore, Piazza San Silvestro 12, Pisa I-56127, Italy4 Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3 0HE, UK

* Author to whom correspondence should be addressed.

Academic Editor: Markus W. Sigrist

Received: 8 February 2016 / Revised: 14 March 2016 / Accepted: 23 March 2016 / Published: 25 March 2016

http://www.mdpi.com/1424-8220/16/4/439

We report on a quartz-enhanced photoacoustic (QEPAS) sensor for methanol (CH3OH) detection employing a novel quartz tuning fork (QTF), specifically designed to enhance the QEPAS sensing performance in the terahertz (THz) spectral range. A discussion of the QTF properties in terms of resonance frequency, quality factor and acousto-electric transduction efficiency as a function of prong sizes and spacing between the QTF prongs is presented. The QTF was employed in a QEPAS sensor system using a 3.93 THz quantum cascade laser as the excitation source in resonance with a CH3OH rotational absorption line located at 131.054 cm−1. A minimum detection limit of 160 ppb in 30 s integration time, corresponding to a normalized noise equivalent absorption NNEA = 3.75 × 10−11 cm−1W/Hz½, was achieved, representing a nearly one-order-of-magnitude improvement with respect to previous reports.

Abstract-Phase-locked arrays of surface-emitting graded-photonic-heterostructure terahertz semiconductor lasers

Phase-locked arrays of surface-emitting graded-photonic-heterostructure terahertz semiconductor lasers Yacine Halioua, Gangyi Xu, Souad Moumdji, Lianhe Li, Jingxuan Zhu, Edmund H. Linfield, A.Giles Davies, Harvey E. Beere, David A. Ritchie, and Raffaele Colombelli  »View Author Affiliations

http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-23-5-6915

Optics Express, Vol. 23, Issue 5, pp. 6915-6923 (2015)
http://dx.doi.org/10.1364/OE.23.006915

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We have demonstrated that a hybrid laser array, combining graded-photonic-heterostructure terahertz semiconductor lasers with a ring resonator, allows the relative phase (either symmetric or anti-symmetric) between the sources to be fixed by design. We have successfully phase-locked up to five separate lasers. Compared with a single device, we achieved a clear narrowing of the output beam profile.

© 2015 Optical Society of America

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.

© 2013 AIP Publishing LLC

Abstract-Y coupled terahertz quantum cascade lasers

http://arxiv.org/abs/1206.0972

Owen P. Marshall, Subhasish Chakraborty, Md. Khairuzzaman, Harvey E. Beere, David A. Ritchie

(Submitted on 5 Jun 2012)

Abstract: Here we demonstrate a Y coupled terahertz (THz) quantum cascade laser (QCL) system. The two THz QCLs working around 2.85 THz are driven by independent electrical pulsers. Total peak THz output power of the Y system, with both arms being driven synchronously, is found to be more than the linear sum of the peak powers from the individual arms; 10.4 mW compared with 9.6 mW (4.7 mW + 4.9 mW). Furthermore, we demonstrate that the emission spectra of this coupled system are significantly different to that of either arm alone, or to the linear combination of their individual spectra.

Comments:	9 pages, 3 figures
Subjects:	Optics (physics.optics); Other Condensed Matter (cond-mat.other)
Cite as:	arXiv:1206.0972v1 [physics.optics]

Submission history

From: Subhasish Chakraborty [view email]
[v1] Tue, 5 Jun 2012 15:53:34 GMT (1026kb