Research | PDI | 18-01-2021

The stairway to heaven – and other destinations: Quantum-cascade lasers for high-resolution terahertz spectroscopy

Terahertz (THz) quantum-cascade lasers (QCLs) are powerful, narrow-band, and frequency-tunable sources of radiation for high-resolution spectroscopy in the far-infrared or THz spectral range. The PDI develops and produces this type of lasers for different applications.

Top view of a terahertz quantum-cascade laser taken in an optical microscope. The laser ridge with the emitting facet on the left side corresponds to the line with dots, but without visible wires. The wires above and below the laser ridge connect the bottom contact. The visible length of the ridge in the image is 2.6 mm. Image: K. Biermann / PDI

As a first real-world application, the local oscillator of the heterodyne spectrometer on board the Stratospheric Observatory for Infrared Astronomy (SOFIA) operated by NASA and the German Aerospace Center (DLR) – the QCL’s stairway to heaven – has been equipped with our QCLs. For fundamental research and industrial applications, high-resolution absorption spectroscopy based on fine-structure transitions in different atoms and ions in the range from 2.3 to 4.7 THz will allow for the quantitative determination of the densities of several species in plasma processes.

Heterodyne spectrometers are based on the mixing of the signal frequency with the local-oscillator frequency. In contrast, high-resolution absorption spectra are acquired by tuning the frequency of the QCL across the absorption line. The linewidth of the QCL radiation has to be significantly smaller than the absorption feature to be measured. After the successful implementation of the QCL-based local oscillator in the sky, THz absorption spectrometers are developed for the quantitative determination of the density of atoms and ions in plasma processes and may be useful for industrial applications. This stairway from heaven may open the path to down-to-earth applications.

QCLs were originally invented by F. Capasso and his coworkers more than 25 years ago for the mid-infrared region. The radiation is emitted by transitions between subbands within the conduction band rather than by transitions across the energy gap as in conventional semiconductor lasers. The optically active regions, which contain the laser levels, are connected by extractor/injector stages forming a cascade structure – a stairway for electrons. Both, the electron transport through the heterostructure and the emission wavelengths, can be tailored by varying the thickness of the individual layers.


The rather complex heterostructures with a total thickness of typically about 10 μm are grown by molecular beam epitaxy with a high stability of the growth parameters over up to 20 hours. In order to achieve the necessary long-term stability, in-situ growth rate control methods are employed. To form a laser waveguide, the wafers are processed using wet chemical or dry etching. Edge emitting Fabry-Pérot ridge lasers (a) represent a straight-forward and robust approach. For single-mode operation, distributed-feedback lasers (b), two-section cavity lasers (c), or very short cavities are used. For practical applications, the THz QCLs can be operated in mechanical cryocoolers (d) – or even in miniature cryocoolers – to provide the necessary cryogenic operating temperatures.



Paul-Drude-Institut für Festkörperelektronik, Leibniz-Institut im Forschungsverbund Berlin e. V.

Prof. Dr. Holger Grahn
Head of Department Semiconductor Spectroscopy
Phone +49 30 20377-318