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Portable terahertz (THz) frequency comb sources are highly desired for applications in rotational molecular spectroscopy and sensing. To date, direct THz quantum cascade laser (QCL) frequency comb generation is not achievable at room temperature. However, THz comb generation based on intracavity difference frequency generation (DFG) in mid-infrared (mid-IR) QCLs is a promising alternative. Here, we present a numerical study of THz DFG-QCL comb formation in mid-IR QCLs based on a self-consistent multi-domain simulation approach. The dynamical simulations are performed using our open-source software tool mbsolve, which provides a flexible and efficient codebase for solving the generalized full-wave Maxwell–Bloch equations. Here, DFG in the active region of a dual-wavelength mid-IR QCL is considered for the generation of THz radiation. The mixing process and, thus, THz generation require a high second-order intersubband nonlinear susceptibility in the QCL active region and can be obtained by targeted quantum engineering. The associated nonlinear effects are included in the Hamiltonian of our Maxwell–Bloch simulation approach. All necessary input parameters for the description of the quantum system are determined self-consistently using our in-house ensemble Monte Carlo software tool for stationary carrier transport simulations. Notably, such simulations require a full-wave Maxwell–Bloch solver that does not employ the common rotating wave approximation, as a broadband optical field extending from the THz to the mid-IR region is investigated. Our modeling approach and the obtained simulation results for two THz DFG-QCL comb setups are validated against experimental data, showing reasonable agreement. Furthermore, we obtain a locked frequency modulated comb state for mid-IR and THz regimes. [ABSTRACT FROM AUTHOR]

Quantum cascade lasers (QCLs) are becoming a key tool for plenty of applications, from the mid-infrared (mid-IR) to the THz range. Progress in related areas, such as the development of ultra-low-loss crystalline microresonators, optical frequency standards, and optical fiber networks for time and frequency dissemination, is paving the way for unprecedented applications in many fields. For most demanding applications, a thorough control of QCLs emission must be achieved. In the last few years, QCLs’ unique spectral features have been unveiled, while multifrequency QCLs have been demonstrated. Ultra-narrow frequency linewidths are necessary for metrological applications, ranging from cold molecules interaction and ultra-high sensitivity spectroscopy to infrared/THz metrology. A review of the present status of research in this field is presented, with a view of perspectives and future applications.

Broadband metrological‐grade frequency comb (FC) synthesizers with a rich number of phase locked modes are the ideal sources for quantum sensing and quantum metrology. At terahertz (THz) frequencies, electrically pumped quantum cascade lasers (QCLs) have shown quantum‐limited frequency noise operation, phase/frequency absolute referencing and self‐starting FC operation, albeit over a rather restricted dynamic range, governed by the nature of the quantum gain media that entangles group velocity dispersion at the different bias points. Here, a technological approach is conceived to achieve FC operation over the entire available gain bandwidth at THz frequencies. The intracavity light intensity of a multistack QCL, inherently showing a giant Kerr nonlinearity, is altered by increasing the mirror losses of its Fabry‐Perot cavity through coating the back facet with an epitaxially‐grown multilayer graphene film. This enables a frequency modulated THz FC showing a proliferation of emitted modes over the entire gain bandwidth and across more than 60% of its operational range, with ≈0.18 mW per mode optical power. The QCL FC is then experimentally characterized to assess its phase coherence, reconstructing its intensity emission profile, instantaneous frequency, and electric field, thus proving its metrological nature. [ABSTRACT FROM AUTHOR]

Quantum cascade lasers (QCLs) represent the most fascinating achievement of quantum engineering, showing how artificial materials can be generated through quantum design, with tailor‐made properties. Their inherent quantum nature deeply affects their core physical parameters. QCLs indeed display intrinsic linewidths approaching the quantum limit, and show spontaneous phase‐locking of their emitted modes via intracavity four‐wave‐mixing, meaning that they can naturally operate as miniaturized metrological frequency rulers, also in frontier frequency domains, as the far‐infrared, yet unexplored in quantum science. Here, the authors discuss the fundamental quantum properties of QCLs operating at terahertz frequencies and their key technological performances, highlighting future perspectives of this frontier research field in disruptive areas of quantum technologies such as quantum sensing, quantum metrology, quantum imaging, and photonic‐based quantum computation. [ABSTRACT FROM AUTHOR]

Ultracold fermionic atoms are proposed to be used in 1D optical lattices to quantum simulate the electronic transport in quantum cascade laser (QCL) structures. The competition between the coherent tunneling among (and within) the wells and the dissipative decay at the basis of lasing is discussed. In order to validate the proposed simulation scheme, such competition is quantitatively addressed in a simplified 1D model. The existence of optimal relationships between the model parameters is shown, maximizing the particle current, the population inversion (or their product), and the stimulated emission rate. This substantiates the concept of emulating the QCL operation mechanisms in cold‐atom optical lattice simulators, laying the groundwork for addressing open questions, such as the impact of electron–electron scattering and the origin of transport‐induced noise, in the design of new‐generation QCLs. [ABSTRACT FROM AUTHOR]

Full phase control of terahertz (THz)‐emitting quantum cascade laser (QCL) combs has recently been demonstrated, opening new perspectives for even the most demanding applications. In this framework, simplifying the set‐ups for control of these devices will help to accelerate their spreading in many fields. This study reports a new way to control the emission frequencies of a THz QCL comb by small optical frequency tuning (SOFT), using a very simple experimental setup, exploiting the incoherent emission of an ordinary white light‐emitting diode. The slightly perturbative regime accessible in these conditions allows tweaking the complex refractive index of the semiconductor without destabilizing the broadband laser gain. The SOFT actuator is characterized and compared to another actuator, the QCL driving current. The suitability of this additional degree of freedom for frequency and phase stabilization of a THz QCL comb is shown and perspectives are discussed. [ABSTRACT FROM AUTHOR]

Terahertz quantum cascade laser sources based on intra-cavity difference frequency generation from mid-IR devices are an important asset for applications in rotational molecular spectroscopy and sensing, being the only electrically pumped device able to operate in the 0.6–6 THz range without the need of bulky and expensive liquid helium cooling. Here we present comb operation obtained by intra-cavity mixing of a distributed feedback laser at λ = 6.5 μm and a Fabry–Pérot device at around λ = 6.9 μm. The resulting ultra-broadband THz emission extends from 1.8 to 3.3 THz, with a total output power of 8 μW at 78 K. The THz emission has been characterized by multi-heterodyne detection with a primary frequency standard referenced THz comb, obtained by optical rectification of near infrared pulses. The down-converted beatnotes, simultaneously acquired, confirm an equally spaced THz emission down to 1 MHz accuracy. In the future, this setup can be used for Fourier transform based evaluation of the phase relation among the emitted THz modes, paving the way to room-temperature, compact, and field-deployable metrological grade THz frequency combs. [ABSTRACT FROM AUTHOR]

Broadband, quantum-engineered, quantum cascade lasers (QCLs) are the most powerful chip-scale sources of optical frequency combs (FCs) across the mid-infrared and the terahertz (THz) frequency range. The inherently short intersubband upper state lifetime spontaneously allows mode proliferation, with large quantum efficiencies, as a result of the intracavity four-wave mixing. QCLs can be easily integrated with external elements or engineered for intracavity embedding of nonlinear optical components and can inherently operate as quantum detectors, providing an intriguing technological platform for on-chip quantum investigations at the nanoscale. The research field of THz FCs is extremely vibrant and promises major impacts in several application domains crossing dual-comb spectroscopy, hyperspectral imaging, time-domain nanoimaging, quantum science and technology, metrology and nonlinear optics in a miniaturized and compact architecture. Here, we discuss the fundamental physical properties and the technological performances of THz QCL FCs, highlighting the future perspectives of this frontier research field. [ABSTRACT FROM AUTHOR]

Four-wave-mixing-based quantum cascade laser frequency combs (QCL-FC) are a powerful photonic tool, driving a recent revolution in major molecular fingerprint regions, i.e. mid- and far-infrared domains. Their compact and frequency-agile design, together with their high optical power and spectral purity, promise to deliver an all-in-one source for the most challenging spectroscopic applications. Here, we demonstrate a metrological-grade hybrid dual comb spectrometer, combining the advantages of a THz QCL-FC with the accuracy and absolute frequency referencing provided by a free-standing, optically-rectified THz frequency comb. A proof-of-principle application to methanol molecular transitions is presented. The multi-heterodyne molecular spectra retrieved provide state-of-the-art results in line-center determination, achieving the same precision as currently available molecular databases. The devised setup provides a solid platform for a new generation of THz spectrometers, paving the way to more refined and sophisticated systems exploiting full phase control of QCL-FCs, or Doppler-free spectroscopic schemes. THz dual comb spectroscopy may be performed using down-converted or quantum cascade laser systems, forcing a choice between absolute frequency referencing and high sensitivity. Here, these strengths are combined in a hybrid, dual frequency comb spectrometer, capable of high-accuracy measurements on molecular transitions. [ABSTRACT FROM AUTHOR]

Quantum cascade lasers (QCLs) are becoming a key tool for plenty of applications, from the mid-infrared (mid-IR) to the THz range. Progress in related areas, such as the development of ultra-low-loss crystalline microresonators, optical frequency standards, and optical fiber networks for time and frequency dissemination, is paving the way for unprecedented applications in many fields. For most demanding applications, a thorough control of QCLs emission must be achieved. In the last few years, QCLs' unique spectral features have been unveiled, while multifrequency QCLs have been demonstrated. Ultra-narrow frequency linewidths are necessary for metrological applications, ranging from cold molecules interaction and ultra-high sensitivity spectroscopy to infrared/THz metrology. A review of the present status of research in this field is presented, with a view of perspectives and future applications. [ABSTRACT FROM AUTHOR]

The need for highly performing and stable methods for mid-IR molecular sensing and metrology pushes towards the development of more and more compact and robust systems. Among the innovative solutions aimed at answering the need for stable mid-IR references are crystalline microresonators, which have recently shown excellent capabilities for frequency stabilization and linewidth narrowing of quantum cascade lasers with compact setups. In this work, we report on the first system for mid-IR high-resolution spectroscopy based on a quantum cascade laser locked to a CaF2 microresonator. Electronic locking narrows the laser linewidth by one order of magnitude and guarantees good stability over long timescales, allowing, at the same time, an easy way for finely tuning the laser frequency over the molecular absorption line. Improvements in terms of resolution and frequency stability of the source are demonstrated by direct sub-Doppler recording of a molecular line. [ABSTRACT FROM AUTHOR]

The infrared spectrum of the (0111–0110) ro-vibrational band of12C16O2in the range 2306–2312 cm−1is investigated with saturated-absorption sub-Doppler spectroscopy. The absolute frequencies of six transitions belonging to the P-branch of this band are measured by recording their Lamb-dip features in a pump-probe detection scheme employing a room-temperature quantum cascade laser. The laser is phase-locked to a subkilohertz-linewidth difference-frequency-generated radiation source, which is referenced to an optical frequency comb synthesiser. The achieved relative uncertainties range from 1×10−11to 5×10−11, improving by three to four orders of magnitude the previous tabulated values for such frequencies. Moreover, thanks to this precision level, self-pressure-shift coefficients due to collisional processes of CO2molecules are reported for the first time. [ABSTRACT FROM AUTHOR]

An external ring-cavity quantum cascade laser (QCL) is demonstrated. Gain competition between the clockwise and anticlockwise ring-cavity modes results in a transition from bidirectional to directional emission as current is increased. In the directional regime, spatial hole burning (SHB) is suppressed, and the spectrum evolves to a single longitudinal mode, in contrast with the multimode spectrum of a comparable Fabry-Pérot QCL. The absence of SHB and the long path-length of the external cavity make this laser an excellent candidate for active mode-locking and high-sensitivity spectroscopic applications in the mid-infrared. A proof-of-principle intracavity absorption spectroscopic detection of water vapor is demonstrated. [ABSTRACT FROM AUTHOR]

We report on a set of high-sensitivity terahertz spectroscopy experiments making use of QCLs to detect rotational molecular transitions in the far-infrared. We demonstrate that using a compact and transportable cryogen-free setup, based on a quantum cascade laser in a closed-cycle Stirling cryostat, and pyroelectric detectors, a considerable improvement in sensitivity can be obtained by implementing a wavelength modulation spectroscopy technique. Indeed, we show that the sensitivity of methanol vapour detection can be improved by a factor ≈ 4 with respect to standard direct absorption approaches, offering perspectives for high sensitivity detection of a number of chemical compounds across the far-infrared spectral range. [ABSTRACT FROM AUTHOR]

Quantum cascade lasers can be considered the primary achievement of electronic band structure engineering, showing how artificial materials can be created through quantum design to have tailor-made properties that are otherwise non-existent in nature. Indeed, quantum cascade lasers can be used as powerful testing grounds of the fundamental physical parameters determined by their quantum nature, including the intrinsic linewidth of laser emission, which in such lasers is significantly affected by the optical and thermal photon number generated in the laser cavity. Here, we report experimental evidence of linewidth values approaching the quantum limit in far-infrared quantum cascade lasers. Despite the broadening induced by thermal photons, the measured linewidth results narrower than that found in any other semiconductor laser to date. By performing noise measurements with unprecedented sensitivity levels, we highlight the key role of gain medium engineering and demonstrate that properly designed semiconductor-heterostructure lasers can unveil the mechanisms underlying the laser-intrinsic phase noise, revealing the link between device properties and the quantum-limited linewidth. [ABSTRACT FROM AUTHOR]

In article number 2000417, Luigi Consolino and co-workers demonstrate a novel degree of freedom for controlling the emission of quantum cascade lasers (QCLs). Quantum Cascade Lasers: Controlling and Phase-Locking a THz Quantum Cascade Laser Frequency Comb by Small Optical Frequency Tuning (Laser Photonics Rev. 15(6)/2021) The resulting additional tuning knob is capable of phase stabilizing the modes spacing of a QCL comb. GLO:B3K5/01jun21:lpor202170033-gra-0001.jpg PHOTO (COLOR): . gl. [Extracted from the article]

We report on the development, testing, and performance analysis of a bow-tie resonant cavity for terahertz (THz) radiation, injected with a continuous-wave 2.55 THz quantum cascade laser. The bow-tie cavity employs a wire-grid polarizer as input/output coupler and a pair of copper spherical mirrors coated with an unprotected 500 nm thick gold layer. The improvements with respect to previous setups have led to a measured finesse value F = 123, and a quality factor Q = 5.1·105. The resonator performances and the relevant parameters are theoretically predicted and discussed, and a comparison among simulated and experimental spectra is given. [ABSTRACT FROM AUTHOR]

Quantum cascade lasers (QCLs) are becoming a key tool in several advanced experiments in the field of precision molecular spectroscopy and absolute frequency metrology. In view of this, a thorough control of their emission properties must be achieved, including a narrow linewidth as well as a high frequency-stability combined with referencing to a primary standard. Here, we give a review on recent developments and next perspectives in this scope, with particular regard to the use of QCLs in fundamental physics spectroscopic searches. [ABSTRACT FROM AUTHOR]

We report on rovibrational fine structure spectroscopy of room-temperature trifluoromethane with a saturated absorption method, using a quantum cascade laser at around 8.6 µm phase-locked to a self-referenced mid-infrared frequency comb synthesizer. The absolute line-center frequencies of 36 lines have been measured by means of the wavelength modulation method with a best fractional uncertainty of ∼ 3 × 10 − 9 in a single acquisition scan from 34.7426 to 34.7443 THz. In addition, an extensive investigation of the measured Lamb-dip profiles using both direct pump-probe spectroscopy method and wavelength modulation technique has been performed for two selected lines, r R 40 (64) and r R 36 (38), belonging to the υ 5 vibrational band, as a function of either the gas pressure or the pump beam power. This approach allowed the measurement of the main spectroscopic parameters, such as saturation intensity, transition electric dipole moment, as well as self-pressure broadening coefficient. [ABSTRACT FROM AUTHOR]

The article presents a study which measured the free-running emission linewidth (LW), the tuning characteristics, and the absolute center frequency of individual emission lines of tetrahertz quantum cascade laser (QCL) sources based in intracavity difference-frequency generation (DFG). Topics discussed include the unveiled emission LW which indicates that DFG-QCLs are well suited to operate as local oscillators and to be used for a variety of applications that require narrow-LW THz sources.