The Quantum Cascade Laser is becoming a key tool for plenty of applications, from the IR to the THz range. Progress in nearby areas, such as the development of ultra-low loss crystalline microresonators, optical frequency standards and optical fiber networks for time&frequency dissemination, are paving the way to unprecedented applications in many fields. For the most demanding applications, a thorough control of quantum cascade lasers (QCLs) emission must be achieved. In the last few years, QCLs unique spectral features have been unveiled, while multifrequency, comb-like 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. In our group, we are combining crystalline microresonators, with a combined high quality factor in the infrared and ultra-broadband spectral coverage, with QCLs and other nonlinear highly coherent and frequency referenced sources. Frequency referencing to optical fiber-distributed optical primary standards offers astonishing stability values of 10<sup>-16</sup> @1-sec timescales in laboratory environments but several hundred kilometres far away from the primary clocks. A review will be given of the present status of research in this field, with a view to perspectives and future applications.
A distributed-feedback quantum-cascade laser working in the 4.3÷4.4 mm range has been frequency stabilized to the Lamb-dip center of a CO<sub>2</sub> ro-vibrational transition by means of first-derivative locking to the saturated absorption signal, and its absolute frequency counted with a kHz-level precision and an overall uncertainty of 75 kHz. This has been made possible by an optical link between the QCL and a near-IR Optical Frequency Comb Synthesizer, thanks to a non-linear sum-frequency generation process with a fiber-amplified Nd:YAG laser. The implementation of a new spectroscopic technique, known as polarization spectroscopy, provides an improved signal for the locking loop, and will lead to a narrower laser emission and a drastic improvement in the frequency stability, that in principle is limited only by the stability of the optical frequency comb synthesizer (few parts in 10<sup>13</sup>). These results confirm quantum cascade lasers as reliable sources not only for high-sensitivity, but also for highprecision measurements, ranking them as optimal laser sources for space applications.
We discuss novel approaches to improve the tuning bandwidth and power output of terahertz (THz) sources based on difference-frequency generation (DFG) in mid-infrared quantum cascade lasers (QCLs). Using a double Littrow external-cavity system, we experimentally demonstrate that both doubly-resonant terms and optical rectification terms in the expression for the intersubband optical nonlinearity contribute to THz generation in DFG-QCLs and report THz DFG-QCLs with the optimized optical rectification terms. We also demonstrate a hybrid DFG-QCL device on silicon that enables significant improvement on THz out-coupling efficiency and results in more than 5 times higher THz output power compared to that of a reference device on its native semi-insulating InP substrate. Finally, we report for the first time the THz emission linewidth of a free-running continuous-wave THz DFG-QCL.
Infrared (IR) digital holography (DH) based on CO<sub>2</sub> lasers has proven to be a powerful coherent imaging technique due
to the reduced sensitivity to mechanical vibrations, to the increased field of view, to the high optical power and to
possible vision through scattering media, such as smoke. In this contribution we report IR DH based on the combination
of quantum cascade laser (QCL) sources and a high resolution microbolometric camera. QCLs combine highly desirable
features for coherent imaging, such as compactness, high optical power, and spectral purity. The present availability of
external cavity mounted QCLs having a broad tuning range, makes them suitable sources for multiple wavelength
holographic interferometry. In addition, QCL emission covers several windows throughout a large portion of the IR
spectrum, from the mid-IR to the terahertz region. This allows taking advantage of the different optical response of the
imaged objects at different frequencies, which is crucial for applications such as non-destructive testing and biomedical
imaging. Our holographic system is suitable for the acquisition of both transmission holograms of transparent objects
and speckle holograms of scattering objects, which can be processed in real time to retrieve both amplitude and phase.
The realization and control of radiation sources is the key for proper development of THz-based metrology. Quantum
Cascade Lasers (QCLs) are crucial, towards this purpose, due to their compactness and flexibility and, even more
important, to their narrow quantum-limited linewidth. We recently generated an air-propagating THz comb, referenced to
an optical frequency comb by nonlinear optical rectification of a mode-locked femtosecond Ti:Sa laser and used it for
phase-locking a 2.5 THz QCL. We have now demonstrated that this source can achieve a record low 10 parts per trillion
absolute frequency stability (in tens of seconds), enabling high precision molecular spectroscopy. As a proof-ofprinciple,
we measured the frequency of a rotational transition in a gas molecule (methanol) with an unprecedented
precision (4 parts in one billion). A simple, though sensitive, direct absorption spectroscopy set-up could be used thanks
to the mW-level power available from the QCL. The 10 kHz uncertainty level ranks this technique among the most
precise ever developed in the THz range, challenging present theoretical molecular models. Hence, we expect that this
new class of THz spectrometers opens new scenarios for metrological-grade molecular physics, including novel THzbased
astronomy, high-precision trace-gas sensing, cold molecules physics, also helping to improve present theoretical
Quantum cascade lasers (QCLs) are powerful testing grounds for the fundamental physical parameters determined by
their quantum nature. The associated wealth of unique physical properties makes the QCL a subject of extensive
research, especially across the far-infrared, where the electron-phonon relaxation dynamics, the gain mechanisms and
the related intrinsic QCL features need to be extensively investigated. Here, we report a complete overview of the
frequency-noise power spectral density of a THz QCL, giving an experimental and analytical evaluation of its intrinsic
linewidth and a full investigation of the physics beyond it, offering a new perspective of use of ultra-narrow THz QCL
sources as a metrological grade tool for a widespread range of photonic applications.
Recently, we have demonstrated that the "intrinsic" linewidth of Quantum Cascade Lasers (QCLs) can go beyond
the radiative lifetime of the upper level. This represents the first demonstration of a sub-radiative linewidth for
any laser. The intrinsic linewidth of a QCL can be as narrow as hundreds Hz, paving new ways for ultra-sensitive
and precise harnessing and detection of molecules. We are working towards full exploitation of such
intrinsic properties by designing appropriate phase-lock loops and enhancement-cavities for interaction with
molecules. Combination with optical-frequency-comb-synthesizers and appropriate spectroscopic techniques,
like saturated-cavity-ring-down-SCAR or polarization spectroscopy can provide unprecedented sensitivity and
frequency accuracy for molecular detection.
Despite the growing interest that quantum cascade lasers (QCLs) are gaining, they still present a few unclear aspects of their fundamental properties, such as spectral purity, that need to be deeply investigated when aiming to make these innovative laser sources suitable for high-resolution spectroscopy and metrology. This paper is a review of our efforts towards QCL-based high-resolution spectroscopy and of our experimental investigation of QCLs' frequency noise, aimed to discover the ultimate performances attainable by QCLs and to develop the experimental techniques required to achieve them. Our results, confirmed by several independent measurements, show that QCLs have a very small intrinsic linewidth buried under a large frequency-noise background. The development of appropriate frequency stabilization techniques will make QCLs well suited for high-resolution spectroscopy and metrology in the mid and far IR.
We recently reported the first Doppler-limited absolute frequency measurement of CO<sub>2</sub> transitions around 4.4 μm
wavelength, by linking a DFB Quantum Cascade Laser (QCL) to an Optical Frequency Comb Synthesizer
(OFCS). We further achieved sub-Doppler recording of these transitions, improving of about three orders of
magnitude the measurement precision. We are exploring techniques able to significantly reduce the QCL jitter,
in order to get metrological-grade QCLs for very demanding experiments in the frequency-domain. The latest
experimental results in our group will be reported.