Abstract-The development and applications of terahertz quantum cascade lasers

http://www.dart-europe.eu/full.php?id=965095

Author	Alhathlool, Raed, Hussain S

Terahertz frequency quantum cascade lasers (THz QCLs) are compact, semiconductor sources of coherent THz radiation, and have numerous potential applications in chemical sensing and industrial inspection, as well as security and biomedical imaging. In this thesis, the development of QCLs as sources of THz radiation is explored, together with their application in self-mixing (SM) imaging systems. The effect of reducing the etch depth of the THz QCL active region was explored, and its influence on QCL performance evaluated. This was aimed of improving the thermal management in QCLs, as well as optimizing the structures for integration with electrical photonic components. The reliance of THz QCLs on slow and incoherent thermal detectors has limited their practical use in THz systems. This was addressed by using a THz QCL as both the radiation source and an interferometric detector. THz sensing and imaging through SM interferometry in a QCL was demonstrated, in which radiation is reflected from an object back into the QCL cavity, causing changes in the laser properties, depending on the amplitude and phase of the reflection. This allows simple, ‘detector-free’, sensing of displacement and reflectivity, with high-sensitivity owing to the coherent nature of the detection. The equivalence between SM-perturbations to the THz power and the laser voltage was shown. Owing to the high SM sensitivity, high-resolution stand-off imaging at round-trip distances of up to 21 m through air was demonstrated - the longest range interferometric sensing with a THz QCL to date. Coherent three-dimensional (3D) terahertz imaging through SM in a THz QCL was also performed, in which the surface height was extracted from the phase of the SM signal. To achieve tunable single mode THz QCL emission, which is highly beneficial for imaging and sensing applications, surface acoustic waves (SAWs) propagation across the sloped etched facets of a QCL mesa was demonstrated. The work also investigated the effect on device performance of SAW propagation over the QCL active region. This demonstration could pave the way for monolithic integration of QCLs into terahertz circuits.

Summary-Detection of terahertz frequency radiation via the photothermoelastic response of zincblende crystals

Published in JOSA B, Vol. 30 Issue 12, pp.3151-3160 (2013)
by Paul Dean, Aziati H. Awang, Iman Kundu, Raed Alhathlool, Suraj P. Khanna,Lianhe H. Li, Andrew Burnett, Edmund H. Linfield, and A. Giles Davies

http://www.opticsinfobase.org/spotlight/summary.cfm?URI=josab-30-12-3151

Spotlight summary: Terahertz (far-infrared) spectroscopy is becoming an increasingly valuable tool in science and industry. It is widely used to study conductive materials on the macroscopic and nano-scale, and to perform non-destructive imaging. The efficient detection of light at terahertz (THz) frequencies remains a challenge, as signals are often weak and can be swamped by background infrared light. Often, cumbersome and expensive liquid helium-cooled detectors are required to observe incoherent THz radiation. In this article, Dean et al. report a study of a novel way to detect incoherent terahertz radiation via a photothermoelastic effect in zinc-blende crystals. Importantly, all the detection components required operate at room temperature, and are robust and inexpensive.

In the technique of THz time-domain spectroscopy zinc-blende crystals, such as ZnTe, can be utilized to generate and detect broadband pulses of THz radiation using femtosecond infrared pulses. In an electro-optic crystal, which lacks an inversion centre, the electric field of the THz radiation creates a birefringence via the linear electro-optic (Pockels) effect. This can then be measured optically using an infrared gate pulse synchronized to the THz pulse. Recently, the detection of continuous-wave THz radiation with unsynchronized infrared pulses has been reported for the zinc-blende crystals CdTe and ZnTe. The mechanism for the detection of THz radiation was found to be a thermally-induced change in birefringence, rather than an electro-optic effect.

The paper by Dean et al. substantially extends upon earlier studies, by quantifying the magnitude of this effect for ZnTe and GaP detection crystals, and identifying the underlying mechanism. A temperature-dependent refractive index is ruled out as the origin of the observed birefringence change; rather, a photothermoelastic mechanism is proposed. In this scheme a focussed THz beam is absorbed in a zinc-blende crystal and heats it, creating a locally-stressed region. The photoelastic effect, in which a stress field creates a birefringence, then alters the polarization state of the near-infrared detection beam. A comprehensive and quantitative model is developed and reported by Dean et al. for the thermally-induced photoelastic detection of THz radiation. This model is validated experimentally by spatially-resolving the detected signal.

Excitingly, Dean et al. performed their measurements with a near-infrared (788nm) continuous-wave beam with 20mW power. These powers are readily accessible with cheap solid-state diode lasers, rather than the expensive ultrafast lasers used in the previous studies. Intriguingly, the photothermoelastic detection method does not place stringent constraints on the crystal structure of the detection material, suggesting that it may be witnessed in optical media other than zinc-blende crystals. The findings are an important step forward in the understanding of this class of THz detector, lower its cost and complexity, and provide a route to try to enhance its responsivity.

--James Lloyd-Hughes

Abstract-Detection of terahertz frequency radiation via the photothermoelastic response of zincblende crystals

Paul Dean, Aziati H. Awang, Iman Kundu, Raed Alhathlool, Suraj P. Khanna, Lianhe H. Li, Andrew Burnett, Edmund H. Linfield, and A. Giles Davies  

We present experimental evidence for a photothermoelastic response in zincblende crystals illuminated by quantum cascade laser sources in the frequency range 2.2–2.9 THz. Results obtained using an optically balanced sampling arrangement indicate a mechanism whereby the stress distribution established through localized heating of the crystal induces a change in optical birefringence via the photoelastic response of the crystal. A full mathematic model of this photothermoelastic mechanism in (110)-orientated crystals is presented, and shown to agree well with experimental measurements of the magnitude, and the orientational and spatial dependencies of the sampled signal in ZnTe and GaP crystals.

© 2013 Optical Society of America

Abstract-Coherent three-dimensional terahertz imaging through self-mixing in a quantum cascade laser

Paul Dean^1,a), Alex Valavanis¹, James Keeley¹, Karl Bertling², Yah Leng Lim²,Raed Alhathlool¹, Siddhant Chowdhury¹, Thomas Taimre³, Lianhe H. Li¹,Dragan Indjin¹, Stephen J. Wilson², Aleksandar D. Rakić², Edmund H. Linfield¹ and A. Giles Davies¹
^{http://scitation.aip.org/content/aip/journal/apl/103/18/10.1063/1.4827886}

1 School of Electronic and Electrical Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom
2 School of Information Technology and Electrical Engineering, The University of Queensland, St. Lucia, Queensland 4072, Australia
3 School of Mathematics and Physics, The University of Queensland, St. Lucia, Queensland 4072, Australia

We demonstrate coherent terahertz (THz) frequency imaging using the self-mixing effect in aquantum cascade laser (QCL). Self-mixing voltage waveforms are acquired at each pixel of a two-dimensional image of etched GaAs structures and fitted to a three-mirror laser model, enabling extraction of the amplitude and phase parameters of the reflected field. From the phase, wereconstruct the depth of the sample surface, and we show that the amplitude can be related to the sample reflectance. Our approach is experimentally simple and compact, and does not require frequency stabilization of the THz QCL.