Single organic molecules and light transport in thin films

Two important processes are at the base of light-matter interaction: absorption and scattering. The first part of this work focuses on the interaction of light with single absorbers/emitters embedded in thin films. In the second part, diffusion of light through thin films of scattering materials is numerically investigated. Quantum emitters based on organic fluorescent molecules in thin films are investigated in the first part of this thesis. The focus of this work is on the experimental characterization of a specific system consisting of single DBT molecules embedded in a thin crystalline matrix of anthracene. The system under investigation exhibits some unique optical properties that enable its use in many applications, especially as a single-photon source and as a sensitive nanoprobe. In particular, single DBT molecules are very bright and stable within the anthracene matrix. At cryogenic temperatures, dephasing of the molecular dipole due to interactions with the phonons of the matrix vanishes, and as a result the purely electronic transition or 00-ZPL becomes extremely narrow, approaching the limit set by its natural linewidth. Under pulsed excitation, the system can be operated as a source of indistinguishable, lifetime-limited single photons. Furthermore, the spectral shifts of the narrow ZPL can be exploited as a sensitive probing tool for local effects and fields. In this work we perform a complete optical characterization of the DBT in anthracene system. Using a home-built scanning epifluorescence microscope, we study its optical properties at room temperature: fluorescence saturation intensity, dipole orientation and emission pattern, fluorescence and triplet lifetime are investigated. At temperatures down to 3K, we observe a lifetime-limited absorption line. Also, we demonstrate photon antibunching from this system. We then show that single DBT molecules can be effectively used for sensing applications. Indeed, at the nanometre scale, i.e. on a scale of the order of their physical size, the optical properties of a single molecule are affected by the surrounding environment. In particular, we here demonstrate energy transfer between single DBT molecules and a graphene sheet, a process that can be exploited to measure the distance d between a single molecule and the graphene layer. Based on the universality of the energy transfer process and its sole dependence on d, we provide a proof of principle for a nanoscopic ruler. In the second part of this thesis we look at the interaction of light with matter from a different perspective. By means of numerical simulations, we address the problem of light transport in turbid media, with a particular focus on optically thin systems. The problem is usually modelled by the Radiative Transfer Equation and its simple Diffusive Approximation which holds for the case of a single, thick slab of turbid material but fails dramatically for thin systems. Alternatively, the problem of light transport can be modelled as a random walk process and therefore it can be numerically investigated by means of Monte Carlo algorithms. In this work we develop a Monte Carlo software library for light transport in multilayered scattering samples, introducing several advancements over existing Monte Carlo solutions. We use the software to build a lookup table which allows us to solve the so-called inverse problem of light transport in a thin slab, i.e. the determination of the microscopic properties at the base of light propagation (such as the scattering mean free path ls and the scattering anisotropy g) starting from macroscopic ensemble observables. We then study diffusion of light in thin slabs, with a particular attention on transverse transport. Indeed, even if a diffusive behaviour is usually associated with thick, opaque media, as far as in-plane propagation is concerned, transport is unbounded and will eventually become diffusive provided that sufficiently long times are considered. By means of Monte Carlo simulations, we characterise this almost two-dimensional asymptotic diffusive regime that sets in even for optically thin slabs (OT=1). We show that geometric and boundary conditions, such as the refractive index contrast, play an active role in redefining the very asymptotic value of the diffusion coefficient by directly modifying the statistical distributions underlying light transport in a scattering medium.