Few-photon coherent nonlinear optics with a single molecule

This thesis examines the interaction of light with single dibenzanthanthrene (DBATT) dye molecules. DBATT serves as a model for a quantum-mechanical two-level system. By means of strong focusing of the incident light and cooling of the dye molecules to temperatures below 2 K, a particularly efficient light-matter interaction can be realized. This enables observation of the nonlinearity inherent to a two-level system, e.g., in the form of saturation of the fluorescence signal with light beams containing only a few photons per lifetime of the excited molecular state. Various nonlinear phenomena arise when two light beams of different frequency are sent to a single molecule. These processes can be exploited to coherently manipulate the transmission of a beam focused onto a single molecule via a second light beam. The occurrent effects, i.e., the AC-Stark shift, stimulated Rayleigh scattering, and three-photon amplification, are detected in the transmitted signal. In addition, four-wave mixing and the dependence of the excited state population on the phase difference of the two incident beams are demonstrated by the use of measurements with subnanosecond time resolution. These results show the possible application of organic dye molecules in the field of quantum information processing where nonlinearities on the single photon and single emitter level are highly sought-after. In this work, the experimental and theoretical principles of single molecule spectroscopy are discussed. Particular attention is placed on the investigation of the coherent light-matter interaction using transmission measurements. A significant change in this transmitted signal, via the scattering of a single molecule, requires a strong light-matter coupling. To quantify the efficiency of this interaction, the maximum possible coupling of a focused beam and a single emitter is discussed. The results presented herein show that the achieved coupling is typically 5% of the theoretical maximum. Thus, the interaction of a molecule with two light fields with different frequencies is investigated. The nonlinear effects that arise are described qualitatively within the dressed-atom model and quantitatively with a Fourier ansatz. The experimental techniques are explained in detail and the measurement results are presented and discussed.