Abstract-Frequency and power stabilization of a terahertz quantum-cascade laser using near-infrared optical excitation

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

 The schematic diagram of the setup used to stabilize the frequency and output power. The QCL is operated in a mechanical cryocooler. The combination of the absorption cell and the Ge:Ga detector A is used to lock the frequency, and the second Ge:Ga detector B is used as a reference for the output power stabilization. (b) Rear-facet illumination with a low-NA single-mode fiber.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-25-36846

We demonstrate a technique to simultaneously stabilize the frequency and output power of a terahertz quantum-cascade laser (QCL). This technique exploits frequency and power variations upon near-infrared illumination of the QCL with a diode laser. It does not require an external terahertz optical modulator. By locking the frequency to a molecular absorption line, we obtain a long-term (one-hour) linewidth of 260 kHz (full width at half maximum) and a root-mean-square power stability below 0.03%. With respect to the free-running case, this stabilization scheme improves the frequency stability by nearly two orders of magnitude and the power stability by a factor of three.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

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.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Terahertz quantum-cascade lasers with lateral distributed-feedback resonators

http://www.pdi-berlin.de/research/departments/semicond.-spectroscopy/research/scientific-highlights/terahertz-quantum-cascade-lasers-with-lateral-distributed-feedback-resonators

Terahertz quantum-cascade lasers (THz QCLs) are promising far-infrared sources for various applications such as local oscillators in heterodyne detectors. This requires a combination of sufficient output power, continuous-wave (cw) operation in small-sized cryocoolers, and single-mode emission at the specified target frequency. Obtaining such a combined goal requires a suitable QCL heterostructure as well as an appropriate resonator.

We developed distributed-feedback (DFB) resonators based on single-plasmon waveguides and first-order lateral DFB (lDFB) gratings [Fig. 1(a)]. These lasers exhibit single-mode emission with output powers of a few mW at operating temperatures, which are accessible by compact Stirling coolers. Based on a rigorous solution of Maxwell’s equations for the DFB unit cell, we developed a general method to calculate the coupling coefficients of lDFB gratings. This allows for an efficient simulation of the resonator properties within the framework of the one-dimensional coupled-mode equations.

One goal for THz astronomy is the heterodyne spectroscopy of the neutral oxygen (OI) fine-structure transition at 4.745 THz, which requires a local oscillator just a few GHz beside this transition frequency. Based on an optimized heterostructure and an adjusted lDFB grating, we achieved single-mode, cw operation around the target frequency [Fig. 1(b)].

Fig. 1: (a) Geometry and initial finite-element grid of the lDFB unit cell as used for simulations (periodic in the z-direction). 1: top metallization, 2: highly doped bottom contact layer, 3: bottom metallization, 4: active region, 5: semiinsulating GaAs substrate. (b) Emission spectra for a 0.1×1.2 mm² lDFB laser (grating period: 8.46 µm). The solid vertical line indicates the OI frequency, while the dashed lines indicate the specified side-band frequencies for the local oscillator.

M. Wienold, A. Tahraoui, L. Schrottke, R. Sharma, X. Lü, K. Biermann, R. Hey, and H. T. Grahn,

Lateral distributed-feedback gratings for single-mode, high-power terahertz quantum-cascade lasers,

Opt. Express 20, 11207–11217 (2012)