Abstract-Wideband, high-resolution terahertz spectroscopy by light-induced frequency tuning of quantum-cascade lasers

T. Alam, M. Wienold, X. Lü, K. Biermann, L. Schrottke, H. T. Grahn, and H.-W. Hübers

Fig. 1 (a) Schematics of the experimental setup. The QCL (yellow box) is mounted in a He-flow cryostat. BS - dichroic beamsplitter; OL - objective lens; Ge:Ga - photoconductive Ge:Ga detector. (b) Microscope image of the illuminated QCL facet. The excitation spot with a diameter of approximately 90 μm originates from a multimode diode laser emitting at 809 nm and exhibits essentially a flat-top profile. (c) Calculated profile of the waveguide mode in the vertical (epitaxial-growth) direction for different frequencies (the mode propagates perpendicular to y along the waveguide ridge). The active region (a. r.) has a height of 10 μm and corresponds to the QCL ridge structure in (b).

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-4-5420

Near-infrared optical excitation enables wideband frequency tuning of terahertz quantum-cascade lasers. In this work, we demonstrate the feasibility of the approach for molecular laser absorption spectroscopy. We present a physical model which explains the observed frequency tuning characteristics by the optical excitation of an electron-hole plasma. Due to an improved excitation configuration as compared to previous work, we observe a single-mode continuous-wave frequency coverage of as much as 40 GHz for a laser at 3.1 THz. This represents, for the same device, a ten-fold improvement over the usually employed tuning by current. The method can be readily applied to a large class of devices.

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