Miniaturized frequency comb sources across hard-to-access spectral regions, i.e. mid- and far-infrared, have long been sought. Four-wave-mixing based Quantum Cascade Laser combs (QCL-combs) are ideal candidates, in this respect, due to the unique possibility to tailor their spectral emission by proper nanoscale design of the quantum wells. We demonstrate full-phase-stabilization of a QCL-comb against the primary frequency standard, proving independent and simultaneous control of the two comb degrees of freedom (modes spacing and frequency offset) at a metrological level. Each emitted mode exhibits a sub-Hz relative frequency stability, while a correlation analysis on the modal phases confirms the high degree of coherence in the device emission, over different power-cycles and over different days. The achievement of fully controlled, phase-stabilized QCL-comb emitters proves that this technology is mature for metrological-grade uses, as well as for an increasing number of scientific and technological applications.

We measure the absolute frequency of seven out of the nine allowed transitions between the $2{}^{3}S$ and $2{}^{3}P$ hyperfine manifolds in a metastable ${}^3\mathrm{He}$ beam by using an optical frequency comb synthesizer-assisted spectrometer. The relative uncertainty of our measurements ranges from $1\times {10}^{-11}$ to $5\times {10}^{-12}$, which is, to our knowledge, the most precise result for any optical ${}^3\mathrm{He}$ transition to date. The resulting $2{}^{3}P–2{}^{3}S$ centroid frequency is 276 702 827 204.8(2.4) kHz. Comparing this value with the known result for the ${}^4\mathrm{He}$ centroid and performing ab initio QED calculations of the ${}^4\mathrm{He}\mathrm{\text{−}}{}^3\mathrm{He}$ isotope shift, we extract the difference of the squared nuclear charge radii $\delta r^2$ of ${}^3\mathrm{He}$ and ${}^4\mathrm{He}$. Our result for $\delta r^2=1.074(3)\mathrm{f}{\mathrm{m}}^2$ disagrees by about $4\sigma$ with the recent determination [R. van Rooij et al., Science 333, 196 (2011)].

Coherent imaging in the THz range promises to exploit the peculiar capabilities of these wavelengths to penetrate common materials like plastics, ceramics, paper or clothes with potential breakthroughs in non-destructive inspection and quality control, homeland security and biomedical applications. Up to now, however, THz coherent imaging has been limited by time-consuming raster scanning, point-like detection schemes and by the lack of adequate coherent sources. Here, we demonstrate real-time digital holography (DH) at THz frequencies exploiting the high spectral purity and the mW output power of a quantum cascade laser combined with the high sensitivity and resolution of a microbolometric array. We show that, in a one-shot exposure, phase and amplitude information of whole samples, either in reflection or in transmission, can be recorded. Furthermore, a 200 times reduced sensitivity to mechanical vibrations and a significantly enlarged field of view are observed, as compared to DH in the visible range. These properties of THz DH enable unprecedented holographic recording of real world dynamic scenes.