Exploring how light interacts with physical systems is an elegant and powerful way to unravel processes occurring at different scales, from bulk materials to single atoms. In this thesis, we employ several microscopy and spectroscopy techniques to investigate the local optoelectronic properties of metal halide perovskites, from thin films to large crystals, as well as to elucidate the physics of single-photon emitters in diamond.

First, the influence of the grain size on the low-temperature phase transition in methylammonium lead iodide perovskite polycrystalline thin films is assessed by means of temperature-dependent macro- and micro-photoluminescence measurements coupled with complementary X-ray diffraction and absorption measurements. The results suggest that local strain plays a role in inhibiting thel low-temperature tetragonal-to-orthorhombic phase transition, and in the extreme case of very small grains, can almost entirely suppress it.

We then unveil buried charge-carrier recombination pathways in both thin film and micro-crystal methylammonium lead halide perovskite structures through 3D photoluminescence tomography acquired using two-photon confocal microscopy. These measurements reveal that light-induced passivation approaches are primarily surface-sensitive and that even nominal single crystals still contain heterogeneous defects that impact charge-carrier recombination.

We build on the two-photon mapping by developing a technique to monitor the carrier diffusion at different depths in a semiconductor by monitoring the photoluminescence as a function of distance from the two-photon-excitation spot. The technique was applied to methylammonium lead bromide crystals, revealing a spatial heterogeneity in diffusion that is not captured in macroscopic diffusion measurements.

We outline a model to explain the observations by distinguishing the influences of carrier diffusion and photon reabsorption at different depths in the sample. Finally, a series of optical studies on the Silicon-Vacancy (SiV) colour-centre in diamond are reported. Coherent Population Trapping (CPT) experiments performed using electrically actuated diamond micro-cantilevers show that the ground state splitting, and therefore the strength of the electron-photon coupling limiting the coherence time in this system, depends upon mechanical strain. A route to the all-optical control of such single electron spins in diamond is then outlined.

The thesis overall introduces a number of powerful techniques to shed light on the intimate relationships between carrier recombination, defects, strain and other physical properties of novel light absorbing and light emitting materials.