We review the most recent technological and application advances of quantum cascade lasers, underlining the present milestones and future directions from the Mid-infrared to the Terahertz spectral range. Challenges and developments, which are the subject of the contributions to this focus issue, are also introduced.

Quantum cascade lasers: 20 years of challenges.

A detailed review on the development of quartz-enhanced photoacoustic sensors (QEPAS) for the sensitive and selective quantification of molecular trace gas species with resolved spectroscopic features is reported. The basis of the QEPAS technique, the technology available to support this field in terms of key components, such as light sources and quartz-tuning forks and the recent developments in detection methods and performance limitations will be discussed. Furthermore, different experimental QEPAS methods such as: on-beam and off-beam QEPAS, quartz-enhanced evanescent wave photoacoustic detection, modulation-cancellation approach and mid-IR single mode fiber-coupled sensor systems will be reviewed and analysed. A QEPAS sensor operating in the THz range, employing a custom-made quartz-tuning fork and a THz quantum cascade laser will be also described. Finally, we evaluated data reported during the past decade and draw relevant and useful conclusions from this analysis.

https://www.mdpi.com/1424-8220/14/4/6165/pdf

Quartz-Enhanced Photoacoustic Spectroscopy: A Review

We provide an overview of recent developments of quantum cascade lasers (QCLs), from the mid-infrared (mid-IR) to the far-IR (THz) range, with a special focus on their metrological-grade applications in a number of fields. A special emphasis on the physics of the QCLs allows underlining peculiar effects and device features recently unveiled that pave the way to novel demanding photonics applications.

https://iopscience.iop.org/article/10.1088/0957-0233/25/1/012001/pdf

Quantum cascade lasers: a versatile source for precise measurements in the mid/far-infrared range

Surface plasmons have found a broad range of applications in photonic devices at visible and near-infrared wavelengths. In contrast, longer-wavelength surface electromagnetic waves, known as Sommerfeld or Zenneck waves, are characterized by poor confinement to surfaces and are therefore difficult to control using conventional metallo-dielectric plasmonic structures. However, patterning the surface with subwavelength periodic features can markedly reduce the asymptotic surface plasmon frequency, leading to 'spoof' surface plasmons with subwavelength confinement at infrared wavelengths and beyond, which mimic surface plasmons at much shorter wavelengths. We demonstrate that by directly sculpting designer spoof surface plasmon structures that tailor the dispersion of terahertz surface plasmon polaritons on the highly doped semiconductor facets of terahertz quantum cascade lasers, the performance of the lasers can be markedly enhanced. Using a simple one-dimensional grating design, the beam divergence of the lasers was reduced from approximately 180 degrees to approximately 10 degrees, the directivity was improved by over 10 decibels and the power collection efficiency was increased by a factor of about six compared with the original unpatterned devices. We achieve these improvements without compromising high-temperature performance of the lasers.

Designer spoof surface plasmon structures collimate terahertz laser beams

In this paper we review recent progress in obtain- ing laser action from semiconductor quantum cascade struc- tures covering the low THz region of the electromagnetic spec- trum, from 2THz (! ! 155µm) down to the sub-THz region (! > 300µm) Particularly, laser active region designs based on bound-to-continuum transition and magnetically assisted intra-well transition are presented The wide scalability of active region designs is discussed and illustrated with experimental data Latest results including the demonstration of laser action from quantum heterostructure at 950GHz are presented

THz and sub-THz quantum cascade lasers

The quantum cascade laser is a powerful, narrow linewidth, and continuous wave source of terahertz radiation. The authors have implemented a distributed feedback device in a spectrometer for high-resolution gas phase spectroscopy. Amplitude as well as frequency modulation schemes have been realized. The absolute frequency was determined by mixing the radiation from the quantum cascade laser with that from a gas laser. The pressure broadening and the pressure shift of a rotational transition of methanol at 2.519THz were measured in order to demonstrate the performance of the spectrometer.

/pdf/high-resolution-gas-phase-spectroscopy-with-a-distributed-13dctqoxuk.pdf

High-resolution gas phase spectroscopy with a distributed feedback terahertz quantum cascade laser

Radiation coupling efficiency and directive properties of integrated lens antennas with log-spiral, log-periodic, and double-slot planar feeds coupled to a hot-electron bolometer were experimentally studied at frequencies from 1 to 6 THz and compared with simulations based on the method of moments and physical-optics ray tracing. For all studied antennas, the modeled spectral dependence of the coupling efficiency fits to the experimental data obtained with both Fourier transform spectroscopy and noise temperature measurements only if the complex impedance of the bolometer is explicitly taken into account. At high frequencies, the radiation pattern of integrated antennas exhibits sidelobes, which are higher than those predicted by the antenna model

Terahertz Performance of Integrated Lens Antennas With a Hot-Electron Bolometer

We have implemented quantum cascade lasers (QCLs) operating at about 2.5 THz in a spectrometer for high resolution molecular spectroscopy. One QCL has a Fabry-Perot resonator while the other is a distributed feedback laser. Linewidth and frequency tunability of both QCLs were investigated by mixing the radiation from the QCL with that from a 2.5 THz gas laser. Both were found sufficient for Doppler limited spectroscopy. Rotational tranistions of methanol were detected in absorption using a QCL as radiation source. Amplitude as well as frequency modulation of the output power of the QCL were used. The absolute frequency was determined simultaneously with the absorption signal by mixing a small part of the radiation from the QCL with that from a gas laser. The pressure broadening and the pressure shift of a rotational transition of methanol at 2.519 THz were measured in order to demonstrate the performance of the spectrometer.

Molecular Spectroscopy with TeraHertz Quantum Cascade Lasers

We present terahertz performance of integrated lens antennas consisting of a complementary log-periodic planar feed and a synthesized elliptical lens. The radiation pattern and the impedance of the planar feed are computed using the method of moments; the action of the lens is evaluated using the physical-optics approach based on the Huygens principle. In the frequency range from 0.6 to 5 THz measured antenna efficiency and radiation pattern correspond fairly well with the results of simulations approving this simulation technique as a reliable tool for the antenna design.

Integrated planar log-spiral antenna at terahertz waves

Erratum: “Submegahertz frequency stabilization of a terahertz quantum cascade laser to a molecular absorption line” [Appl. Phys. Lett. 96, 071112 (2010)]

H. Richter

Papers

High-resolution gas phase spectroscopy with a distributed feedback terahertz quantum cascade laser

Terahertz Performance of Integrated Lens Antennas With a Hot-Electron Bolometer

Molecular Spectroscopy with TeraHertz Quantum Cascade Lasers

Integrated planar log-spiral antenna at terahertz waves

Erratum: “Submegahertz frequency stabilization of a terahertz quantum cascade laser to a molecular absorption line” [Appl. Phys. Lett. 96, 071112 (2010)]