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-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.