Optical frequency combs are one of the most remarkable inventions in recent decades. Originally conceived as the spectral counterpart of the train of short pulses emitted by mode-locked lasers, frequency combs have also been subsequently generated in continuously pumped microresonators, through third-order parametric processes. Quite recently, direct generation of optical frequency combs has been demonstrated in continuous-wave laser-pumped optical resonators with a second-order nonlinear medium inside. Here, we present a concise introduction to such quadratic combs and the physical mechanism that underlies their formation. We mainly review our recent experimental and theoretical work on formation and dynamics of quadratic frequency combs. We experimentally demonstrated comb generation in two configurations: a cavity for second harmonic generation, where combs are generated both around the pump frequency and its second harmonic and a degenerate optical parametric oscillator, where combs are generated around the pump frequency and its subharmonic. The experiments have been supported by a thorough theoretical analysis, aimed at modelling the dynamics of quadratic combs, both in frequency and time domains, providing useful insights into the physics of this new class of optical frequency comb synthesizers. Quadratic combs establish a new class of efficient frequency comb synthesizers, with unique features, which could enable straightforward access to new spectral regions and stimulate novel applications.

Optical frequency comb synthesizers with a wide spectral range are an essential tool for many research areas such as spectroscopy, precision metrology, optical communication, and sensing. Recent studies have demonstrated the direct generation of frequency combs, via second-order processes, that are centered on two different spectral regions separated by an octave. Here, we present the capability of optical quadratic frequency combs for broad-bandwidth spectral emission in unexplored regimes. We consider comb formation under phase-matched conditions in a continuous-wave pumped singly resonant second-harmonic cavity, with large intracavity power and control of the detuning over several cavity linewidths. The spectral analysis reveals quite distinctive sidebands that arise far away from the pump, singularly or in a mixed regime together with narrowband frequency combs. Notably, by increasing the input power, the optical frequency lines evolve into widely spaced frequency clusters, and at maximum power, they appear in a wavelength range spanning up to 100 nm. The obtained results demonstrate the capacity of second-order nonlinearities to produce direct comb within a wide range of wavelengths.

Entang-Like is an interactive game that intuitively illustrates quantum entanglement using bistable images. Designed to make quantum mechanics more accessible, it provides a hands-on analogy for this fascinating phenomenon, turning complex concepts into an engaging and funny experience.

We demonstrate optomechanical spatial projection of azimuthally structured optical beams. The system is based on an ultra-low loss circular membrane integrated into an interferometric setup, that exploits the spatial analogies between the mechanical modes and the structured optical fields. A weak anisotropy lifts the degeneracy of the membrane modes, producing a spectrally resolved mechanical doublet composed of two orthogonal eigenmodes. Petal-shaped optical intensity distributions, formed by coherent superpositions of orbital-angular-momentum eigenstates with opposite topological charges and generated via a q-plate, are used to probe the membrane in a readout-only regime, where the motion is thermally excited. By rotating the azimuthal orientation of the optical pattern, we observe a controlled redistribution of spectral weight between the two members of the mechanical doublet. The split doublet, therefore, acts as a two-channel mechanical spatial analyzer for azimuthal quadratures, providing a wavelength-independent platform for the projection and processing of structured optical fields. Since the analyzer relies on spatial mode matching rather than optical spectral properties, the platform can interface with a wide range of optical sources and channels, making it an ideal candidate for optical and quantum communication applications.