High-Resolution Spectroscopy of Vibronic Transitions in Single Molecules

Description

Dye molecules in organic crystals are excellent single-photon sources and can act as non-linear elements in optical circuits. Since these molecules lack a long-lived spin state, they have previously not been considered for applications that require a quantum memory. In principle, the vibrational modes of single dye molecules in organic crystals could serve that purpose and act as qubits. Due to their coupling to the crystal lattice, the lifetimes of molecular vibrations are often limited to around 10 ps. In the field of cryogenic single molecule spectroscopy, however, the decay constants of vibrational modes have never been studied systematically. This leaves open the possibility that some modes exhibit considerably longer lifetimes.

In this work, we study the linewidths of vibrational modes of single dibenzoterrylene (DBT) molecules in paradichlorobenzene (pDCB) and anthracene (AC) crystals at cryogenic temperatures using their vibronic spectra. To identify long-lived modes, these spectra are measured at a high spectral resolution via fluorescence excitation spectroscopy and stimulated emission depletion (STED) spectroscopy with narrowband tunable lasers. We show that the linewidths of some vibrational modes of DBT in pDCB reach values around 2 GHz. This corresponds to a lifetime of 80 ps and is, thus, significantly longer than the typical lifetimes of vibrational modes in the solid state.

We also observe indications of the coherent excitation of a vibronic mode in the electronic ground state of DBT in pDCB. The associated splitting of the absorption profile of a vibronic transition is achieved by tuning an intense control laser on resonance with a transition between two vibronic states with a high Franck-Condon overlap. According to our model calculation, 80 % of the population that is transferred to the vibronic state of the electronic ground state is coherent in this process. If the vibrational lifetimes of certain modes can be extended by decoupling them from crystal phonons, similar schemes may be exploited in future for the coherent transfer of a flying qubit state to a vibrational state of a single molecule.