Turning an Organic Molecule into an Efficient Optomechanical System

Molecules lie at the heart of chemical and biological processes, and are regarded as building blocks of organic matter. Their small size, rich and controllable quantum mechanical properties such as the electronic, vibrational and spin degrees of freedom also make them ideal candidates for building blocks of future quantum technologies. However, the quantum coherence of their vibrational states is lost within picoseconds due to coupling to the solidstate environment. While organic molecules act as effective two-level optical emitters, the lack of further coherent transitions limits their promise for certain quantum technologies. In this thesis, we theoretically investigate the nature of the coupling between the electronic and mechanical degrees of freedom of a molecule embedded in a solid. Based on these insights, we propose a new molecular platformto revive the optomechanical character and multi-level structure of an organic molecule in the solid state. We investigate the vibrational landscape of organic molecular systems and model the photophysics of a single molecule in solid state from first principle considerations. Using the polaron master equations and quantum Langevin formalism, we derive analytical formulas for the excitation and emission spectra at the weak laser excitation limit, and study the optomechanical interactions. Our results retrieve the well-known Franck-Condon and Debye-Waller physics. We discuss the relevance of various phonon modes to the the optical dephasing process. In addition, the importance of anharmonic interactions in the vibrational relaxation mechanism is demonstrated. Using finite element simulations, we study the effect of the substrate, crystal size and shape on the phonon lifetimes. We ultimately engineer the phononic environment of a molecule, and achieve molecular coherence times beyond the millisecond timescale. To predict the electron-phonon coupling strengths, we develop the quasi-normal mode theory for phononic environments. Our results demonstrate that the proposed molecular platformcould achieve high electron-phonon coupling strengths compared to the electronic and phonon decoherence rates. These enhanced optomechanical qualities of a molecule ensure optically accessible long-lived molecular states. We highlight the optomechanical character of the proposed molecular platformfurther by studying its spectroscopic signatures. The excitation spectrum shows well-defined phononic transitions and breakage of Kasha’s rule. Moreover, the resonance fluorescence spectrum reveals anti-Stokes Raman scattering features at zero phonon bath temperature, and evidences the high optomechanical cooperativity at the single-photon level. These high-cooperativity interactions in the proposed molecular platform further allows us to demonstrate a low threshold molecular phonon laser. Exploiting the improved optomechanical qualities in the molecular platform, we demonstrate a single-molecule quantum memory for a single-photon pulse. The memory dynamics is modelled with analytical formulas based on the quantum Langevin equations. We characterize the storage and retrieval efficiencies by using the input-output formalism. The analytical results reveal the universal memory constant uniquely determining the memory efficiencies. Moreover, we perform a parametric study and pulse optimization to explore the limits of the memory efficiencies. Our proposal provides ways of restoring the multi-level character of a molecule, and render organic molecules high-performance quantum optomechanical systems. The molecular quantum memory proposed in this thesis opens up prospects for realizing quantum networks via photonic and phononic interconnections.