] these sources display spontaneous FC operation by self‐phase locking of the cavity modes through intracavity four‐wave mixing (FWM). ] Owing to the large third‐order nonlinearity of their gain media ( χ (3) ≈0.9 × 10 −15 m 2 V − 2 for mid‐infrared QCLs and χ (3)≈0.7 × 10 −15 m 2 V − 2 for THz QCLs), [ ] in the far‐field in combination with hot‐electron bolometers. ] in the mid‐IR and THz spectral ranges and widely exploited for dual‐comb spectroscopy [ Miniaturized quantum cascade lasers (QCLs) have emerged over the last few years as the most promising source of FCs [ ] However, the widespread use of THz FCs has been hampered by the size and complexity of the associated optical apparatus. Alternative techniques have, therefore, been developed, including use of nonlinear crystals, [ across the terahertz (THz) and millimeter ranges, since the physics behind the operation of sources and optical components is substantially different at longer wavelengths. ] However, such approaches cannot be easily translated to the low‐energy regions of the electromagnetic spectrum, i.e. ] They can be generated by several well‐established techniques, most notably through the stabilization of the repetition rate and carrier envelope phase of mode‐locked lasers. ] with an ever‐growing range of applications in a plethora of distinctive fields including molecular spectroscopy, [ This technique can be applied to the entire infrared range, opening up a new approach to hyper‐spectral near‐field imaging with wide‐scale applications in the study of plasmonics and quantum science, inter alia.įrequency combs (FCs) have revolutionized the fields of optical synthesis, [ This nanoscope provides 160 nm spatial resolution, coherent detection of multiple phase‐locked modes, and mapping of the THz optical response of nanoscale materials up to 3.5 THz, with noise‐equivalent‐power (NEP) ≈400 pW √Hz −1. Stable phase‐locked regimes are used to construct a FC‐based hyperspectral, THz s‐SNOM nanoscope. This enables to exploit the sensitivity of FC phase‐coherence to optical feedback and, for the first time, manipulate the amplitude, linewidth and frequency of the intermode THz FC beatnote using the feedback itself. Here, a self‐mixing intermode‐beatnote spectroscopy system is demonstrated, based on THz quantum cascade laser FCs, in which light is back‐scattered from the tip of a scanning near‐field optical‐microscope (SNOM) and the intracavity reinjection monitored. Simultaneous monitoring and manipulation of the intermode phase coherence, without any external seeding or active modulation, is a very demanding task for which there has hitherto been no technological solution. Chip‐scale, electrically‐pumped terahertz (THz) frequency‐combs (FCs) rely on nonlinear four‐wave‐mixing processes, and have a nontrivial phase relationship between the evenly spaced set of emitted modes.

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