Confining optical modes well below the size determined by the light wavelength at the same frequency is beneficial for the design of compact and efficient optical devices because light concentration impacts the electromagnetic energy density, the ability of light to interact with analytes in sensors, and the nonlinear response of materials in all-optical light modulators. However, such confinement comes at the prize that coupling of propagating light to those modes is made more inefficient because of the mismatch with the light wavelength. Light coupling in and out of confined modes is in fact a major pending problem that limits the practical applicability of the optical confinement strategy in nanophotonics.

Small scatterers are commonly employed to assist light coupling because they allow targeting designated spatial regions in space, matching the area occupied by the confined modes. However, when those scatterers are placed close to the materials supporting the optical modes, the coupling is reduced by losses introduced through the coupling itself, acting as a loss channel. In this work, we have proposed the use of small scatterers placed at a suitable distance from a surface supporting optical modes as a strategy to realize complete optical coupling into such optical modes. By employing lossless, resonant scatterers such as silica particles supporting dipolar Mie resonances, and illuminating the system with focused light, we demonstrate through rigorous theory the possibility of achieving complete optical coupling, provided the angular profile of the incident light is appropriately shaped, and the surface-scatterer distance is fixed to satisfy the so-called critical coupling conditions.

The solution here proposed to solve the in and out optical coupling problem has general applicability in nanophotonics and provides a viable route toward the design of compact optical devices in which suitably engineered optical scatterers are used to funnel light into the surface modes of a planar surface, where they can be used for optical sensing or signal processing, and eventually coupled out into propagating light following the same scheme.

Hybrid lead halide perovskites are causing a revolution in photovoltaics, reaching light conversion efficiencies in excess of 25% after less than a decade of intense research. In nanocrystal form, these materials exhibit light-emission quantum yields close to 100%, which make them excellent candidates for light emitting devices too. However, a feature making hybrid perovskites so attractive for commercialization is the fact that they are produced by low-temperature, hence low-cost, scalable solution-based methods. In part as a consequence, hybrid perovskites are mechanically soft ionic solids with low energetic barriers for point-defect formation and yet they exhibit excellent optoelectronic properties. This can be explained if most native point defects (vacancies, interstitials and antisite defects) are shallow. Unlike deep traps, which are highly localized centers exhibiting high charge-carrier trapping rates, shallow defects are benign as far as the charge-carrier recombination is concerned. Shallow defects can be studied by means of low-temperature photoluminescence (PL). In PL experiments, correlated electron-hole pairs, called excitons, are excited with a laser. Excitons move freely through the crystal, like a binary-stars system, but they can become bound to different shallow defects. Free or bound excitons recombine radiatively emitting photons with precise energies, constituting an optical fingerprint (see Fig. 1). 

In a recent work [1], we performed the first systematic study of the evolution of shallow-defect signatures observed in low-temperature PL spectra of mixed organic-cation lead iodide perovskite single crystals. Based on state-of-the-art ab initio calculations, we were able to provide a first assignment for all PL features to different shallow-defects (vacancies & interstitials) of hybrid perovskites. 

In this way, our results provide a deeper insight into fundamental aspects of the photo-physics of native shallow defects in metal halide perovskites. This is instrumental for the optimization and further development of photovoltaic as well as light-emitting devices, based on this class of extraordinary semiconductor materials.