Solid-state single organic molecules, such as dibenzoterrylene (DBT) in organic matrices, are prominent deterministic single-photon sources, usually modeled as effective two-level systems (TLS). We show that single DBT molecules in anthracene nanocrystals, under strong continuous-wave driving, depart from this picture: instead of the expected saturation, fluorescence is strongly suppressed at high resonant pump power, while the linewidth broadens beyond the TLS prediction -- an \emph{anomalous saturation} regime. Comparing coherent and incoherent excitation and independently calibrating temperature via phonon-induced dephasing rules out laser-induced heating and intersystem crossing. Instead, a model of intensity-dependent excited-state absorption (ESA) toward a short-lived dark state quantitatively reproduces both the fluorescence suppression and the linewidth broadening. We further show that matrix quality mitigates this quenching, and that pulsed excitation schemes are strategic toward full population inversion, with direct implications for quantum nanophotonics and molecular optomechanics.
Solid-state single organic molecules, such as dibenzoterrylene (DBT) in organic matrices, are prominent deterministic single-photon sources, usually modeled as effective two-level systems (TLS). We show that single DBT molecules in anthracene nanocrystals, under strong continuous-wave driving, depart from this picture: instead of the expected saturation, fluorescence is strongly suppressed at high resonant pump power, while the linewidth broadens beyond the TLS prediction -- an \emph{anomalous saturation} regime. Comparing coherent and incoherent excitation and independently calibrating temperature via phonon-induced dephasing rules out laser-induced heating and intersystem crossing. Instead, a model of intensity-dependent excited-state absorption (ESA) toward a short-lived dark state quantitatively reproduces both the fluorescence suppression and the linewidth broadening. We further show that matrix quality mitigates this quenching, and that pulsed excitation schemes are strategic toward full population inversion, with direct implications for quantum nanophotonics and molecular optomechanics.
Single organic molecules hold great promise for generating, manipulating, and storing single photons. Such processes form the basis for new quantum-enhanced technologies such as sensing, communications, and computation. In this talk I will focus on a particularly promising molecule – dibenzoterrylene (DBT). When DBT is introduced into a solid crystal of anthracene it is photostable and emits light between 780 and 795 nm. When cooled to cryogenic temperature, DBT is isolated from phonon-induced dephasing meaning that the photons have a lifetime-limited. I will present recent results in growing DBT-doped anthracene crystals, introducing these to nanophotonic interfaces for enhancing the collection of photons, tuning the DBT emission wavelength to coincide with rubidium atomic absorption, and finally pump-probe spectroscopy experiments to find long-lived triplet states in DBT which will be useful for building a single-molecule quantum memory.
Quantum light sources are crucial for the future of quantum photonic technologies and, among them, single photons on-demand are key resources in quantum communications and information processing. Ideal quantum emitters providing indistinguishable photons in a clocked manner, negligible decoherence and spectral diffusion, and with potential for scalability are today still a major challenge. We report on photostable and indistinguishable single photon emission from dibenzoterrylene molecules isolated in anthracene nanocrystals (DBT:Ac NCs) at 3K. The visibility of two-photon interference is preserved even when they are separated more than thirty times the excited-state lifetime, or ten fluorescence cycles. One of the advantages of organic molecules is the low-cost mass production of nominally identical emitters, that also allow for on-chip integration. These aspects combined with high spectral stability and coherence make them promising for applications and future quantum technologies.
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. \n \nQuantum 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. \n \nIn 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. \n \nIn 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. \n \nIn 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.
Dibenzoterrylene (DBT) molecules embedded in an anthracene host crystal are promising sources of single photons. They have high photostability and can be readily included in photonic structures [1] . Furthermore, when cooled to cryogenic temperatures they are spectrally stable and display fully coherent, lifetime-limited emission in the zero-phonon line (ZPL) [2] , and have a large branching ratio for this decay compared to incoherent decay involving molecular vibrations [3] . Another key characteristic of DBT-based photon sources is the proximity of their emission wavelength compared to rubidium atoms [4] , which can be exploited for quantum technology applications. However, so far fine-tuning of the emission of DBT has mainly been achieved using Stark tuning with a static electric field.
Single molecules in solid-state matrices have been proposed as sources of single-photon Fock states back 20 years ago. Their success in quantum optics and in many other research fields stems from the simple recipes used in the preparation of samples, with hundreds of nominally identical and isolated molecules. Main challenges as of today for their application in photonic quantum technologies are the optimization of light extraction and the on-demand emission of indistinguishable photons. We here present Hong-Ou-Mandel experiments with photons emitted by a single molecule of dibenzoterrylene in an anthracene nanocrystal at 3 K, under continuous wave and also pulsed excitation. A detailed theoretical model is applied, which relies on independent measurements for most experimental parameters, hence allowing for an analysis of the different contributions to the two-photon interference visibility, from residual dephasing to spectral filtering.
Single molecules in solid state matrices have been proposed as sources of single photon Fock states back 20 years ago. Their success in quantum optics and in many other research fields stems from the simple recipes used in the preparation of samples, with hundreds of nominally identical and isolated molecules. Main challenges as of today for their application in photonic quantum technologies are the optimization of light extraction and the on-demand emission of indistinguishable photons. We here present Hong–Ou–Mandel (HOM) experiments with photons emitted by a single molecule of dibenzoterrylene in an anthracene nanocrystal at 3 K, under continuous wave and also pulsed excitation. A detailed theoretical model is applied, which relies on independent measurements for most experimental parameters, hence allowing for an analysis of the different contributions to the two-photon interference visibility, from residual dephasing to spectral filtering. A HOM interference visibility of more than 75% is reported, which, according to the model, is limited by the residual dephasing present at the operating temperature.