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

Wei, B; Wallis, R; Kindness, S; Mitrofanov, O; Beere, HE; Ritchie, DA; Degl'Innocenti, R; (2017) Terahertz s-SNOM with > λ/1000 resolution based on self-mixing in quantum cascade lasers. In: Proceedings of the European Conference on Lasers and Electro-Optics 2017. Optical Society of America (OSA): Munich, Germany. Green open access

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Abstract

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.

Type: Proceedings paper
Title: Terahertz s-SNOM with > λ/1000 resolution based on self-mixing in quantum cascade lasers
Event: European Conference on Lasers and Electro-Optics 2017
Location: Munich, GERMANY
Dates: 25 June 2017 - 29 June 2017
ISBN-13: 978-1-5090-6736-7
Open access status: An open access version is available from UCL Discovery
Publisher version: https://www.osapublishing.org/abstract.cfm?uri=CLE...
Language: English
Additional information: This version is the author accepted manuscript. For information on re-use, please refer to the publisher’s terms and conditions.
UCL classification: UCL
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science > Dept of Electronic and Electrical Eng
URI: https://discovery.ucl.ac.uk/id/eprint/10114003
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