## 3 Detection, Imaging and Spectroscopy  
The manipulation of light and light-matter interactions have been largely investigated for applications in the fields of detection, imaging and spectroscopy. In the paper published by [PERSON] and co-workers [3], the Magneto-Optic Kerr Effect in the guided mode of an unbalanced Mach Zehnder Interferometer (MZI) was designed and simulated to optically detect the magnetization direction of ultra-thin (\(\sim\)12 nm) metal cladding. In fact, the device was an unbalanced MZI based on InP membrane on silicon. The MZI arms were made up of a polarization converter from one side and ferromagnetic thin-film cladding and a delay line from the other side. The device read a nanoscale memory bit (400 nm \(\times\) 50 nm \(\times\) 12 nm) with a signal-to-noise ratio \(\sim\)10 dB and tolerated performance reductions that arose during the fabrication. While this hybrid device based on ultra-thin metal membrane on silicon was demonstrated to be an all-optical magnetic memory reading tool, hybrid devices with an ultra-thin conductive layer jointed with semiconductors were presented by [PERSON] and co-authors [4], such as all-optical Schottky photodetectors that operated at room temperature. In particular, in [4], two Schottky photodetectors based on graphene/n-silicon (Si) and graphene/n-germanium (Ge) Schottky barriers were theoretically investigated and operated at 1550 nm and at 2000 nm, respectively. The responsivity/noise equivalent power (NEP) ratio was analysed, and a strong addiction on the Schottky barrier height of the junction was demonstrated. The authors derived a closed analytical formula for use in maximizing the responsivity/NEP ratio and theoretically discussed how the Schottky barrier height is related to the reverse bias applied to the junction. Moreover, they found that at 1550 nm, the optimized graphene/n-silicon (Si) Schottky PDs with a reverse bias of 0.66 V showed a responsivity and NEP of 133 mA/W and 500 fW/Hz, respectively. Finally, at 2000 nm, the optimized graphene/n-germanium (Ge) Schottky PDs showed a responsivity and NEP of 233 mA/W and 31 pW/Hz, respectively. The detection and imaging of optical signals were investigated and reviewed in this Special Issue. In particular, two innovative imaging techniques were addressed: Polarization-sensitive Digital Holographic Imaging (PDHI) [5] and Stimulated Raman Scattering (SRS) microscopy [6]. [PERSON] and [PERSON] [5] reported a review on the state of the art of PDHI techniques, focusing on the theoretical principles and important applications. The paper not only provided an exhaustive review of applications in several fields, from biology to microelectronics and micro-phototototots, but also emphasized the merits of this new technique based on the interference between different polarized optical beams. PSDHI, in fact, simultaneously allows the three-dimensional reconstruction and the quantitative evaluation of the polarization properties of a sample with a resolution on the micrometric scale, a good acquisition speed and the absence of labels/markers. [PERSON] and co-workers [6] measured the spectral resolution of SRS, i.e., the ability to distinguish closely lying resonances, and their paper focused on the spectral splitting of protein and lipid bands in the C-H region, which is of great interest in the field of biochemistry. In particular, the paper addressed the interplay among pump and Stokes bandwidth and the degree of chirp-matching. Moreover, the spectral resolution of femtosecond SRS microscopy was experimentally investigated. Spectroscopy systems were further reviewed by [PERSON] and [PERSON] [7] in the field of instrumentation working at near-infrared NIR frequencies. The authors discussed NIR spectroscopy systems from the instrumentation point of view with regard to state-of-the-art approaches and the associated challenges.