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DDC: 530 × Subject: terahertz ×

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Now showing 1 - 3 of 3
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    Terahertz magnetic field enhancement in an asymmetric spiral metamaterial
    (Bristol : IOP Publ., 2018-10-25) Polley, Debanjan; Hagström, Nanna Zhou; Schmising, Clemens von Korff; Eisebitt, Stefan; Bonetti, Stefano
    We use finite element simulations in both the frequency and the time-domain to study the terahertz resonance characteristics of a metamaterial (MM) comprising a spiral connected to a straight arm. The MM acts as a RLC circuit whose resonance frequency can be precisely tuned by varying the characteristic geometrical parameters of the spiral: inner and outer radius, width and number of turns. We provide a simple analytical model that uses these geometrical parameters as input to give accurate estimates of the resonance frequency. Finite element simulations show that linearly polarized terahertz radiation efficiently couples to the MM thanks to the straight arm, inducing a current in the spiral, which in turn induces a resonant magnetic field enhancement at the center of the spiral. We observe a large (approximately 40 times) and uniform (over an area of ∼10 μm2) enhancement of the magnetic field for narrowband terahertz radiation with frequency matching the resonance frequency of the MM. When a broadband, single-cycle terahertz pulse propagates towards the MM, the peak magnetic field of the resulting band-passed waveform still maintains a six-fold enhancement compared to the peak impinging field. Using existing laser-based terahertz sources, our MM design allows to generate magnetic fields of the order of 2 T over a time scale of several picoseconds, enabling the investigation of nonlinear ultrafast spin dynamics in table-top experiments. Furthermore, our MM can be implemented to generate intense near-field narrowband, multi-cycle electromagnetic fields to study generic ultrafast resonant terahertz dynamics in condensed matter.
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    Terahertz absorption spectroscopy for measuring atomic oxygen densities in plasmas
    (Bristol : IOP Publ., 2023) Wubs, J.R.; Macherius, U.; Weltmann, K.-D.; Lü, X.; Röben, B.; Biermann, K.; Schrottke, L.; Grahn, H.T.; van Helden, J.H.
    This paper describes the first implementation of terahertz (THz) quantum cascade lasers for high-resolution absorption spectroscopy on plasmas. Absolute densities of ground state atomic oxygen were directly obtained by using the fine structure transition at approximately 4.75 THz. Measurements were performed on a low-pressure capacitively coupled radio frequency oxygen discharge. The detection limit in this arrangement was found to be 2 × 10 13 cm−3, while the measurement accuracy was within 5%, as demonstrated by reference measurements of a well-defined ammonia transition. The results show that the presented method is well suited to measure atomic oxygen densities, and it closes the THz gap for quantitative atomic density measurements in harsh environments such as plasmas.
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    Terahertz quantum-cascade lasers for spectroscopic applications
    (Berlin : Technische Universität Berlin, 2018) Röben, Benjamin Malte; Grahn, Holger T.
    Terahertz (THz) quantum-cascade lasers (QCLs) are unipolar semiconductor heterostructure lasers that emit in the far-infrared spectral range. They are very attractive radiation sources for spectroscopy, since they are very compact and exhibit typical output powers of severalmWas well as linewidths in the MHz to kHz range. This thesis presents the development of methods to tailor the emission characteristics of THz QCLs and employ them for spectroscopy with highest resolution and sensitivity. In many cases, these spectroscopic applications require that the far-field distribution of the THz QCLs exhibits only a single lobe. However, multiple lobes in the far-field distribution of THz QCLs were experimentally observed, which were unambiguously attributed to the typically employed mounting geometry and to the cryogenic operation environment such as the optical window. Based on these results, a method to obtain a single-lobed far-field distribution is demonstrated. A critical requirement to employ a THz QCL for high-resolution spectroscopy of a single absorption or emission line is the precise control of its emission frequency. This long-standing problem is solved by a newly developed technique relying on the mechanical polishing of the front facet. A QCL fabricated in this manner allows for spectroscopy at a maximal resolution in the MHz to kHz range, but its accessible bandwidth is usually limited to a few GHz. In contrast, a newly developed method to utilize QCLs as sources for THz Fourier transform spectrometers enables highly sensitive spectroscopy over a significantly larger bandwidth of at least 72 GHz with a maximal resolution of typically 100 MHz. The application of QCLs as sources for THz Fourier transform spectroscopy leads to a signal-to-noise ratio and dynamic range that is substantially increased by a factor of 10 to 100 as compared to conventional sources. The results presented in this thesis pave the way to routinely employ THz QCLs for spectroscopic applications in the near future.