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

Abstract-High-temperature, continuous-wave operation of terahertz quantum-cascade lasers with metal-metal waveguides and third-order distributed feedback

M. Wienold, B. Röben, L. Schrottke, R. Sharma, A. Tahraoui, K. Biermann, and H. T. Grahn  »View Author Affiliations

Optics Express, Vol. 22, Issue 3, pp. 3334-3348 (2014)
http://dx.doi.org/10.1364/OE.22.003334

http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-22-3-3334

Currently, different competing waveguide and resonator concepts exist for terahertz quantum-cascade lasers (THz QCLs). We examine the continuous-wave (cw) performance of THz QCLs with single-plasmon (SP) and metal-metal (MM) waveguides fabricated from the same wafer. While SP QCLs are superior in terms of output power, the maximum operating temperature for MM QCLs is typically much higher. For SP QCLs, we observed cw operation up to 73 K as compared to 129 K for narrow (≤ 15 μm) MM QCLs. In the latter case, single-mode operation and a narrow beam profile were achieved by applying third-order distributed-feedback gratings and contact pads which are optically insulated from the intended resonators. We present a quantitative analytic model for the beam profile, which is based on experimentally accessible parameters.

© 2014 Optical Society of America

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)