Eric Muller

Spectroscopic Nano-Imaging and Control of Molecular Vibrations

Functional molecular materials have emergent physical properties determined by multi-length scale structural order and dynamical interactions that are difficult to measure with conventional methods. To gain the desired nanometer spatial resolution with simultaneous spectroscopic specificity we combine scanning probe microscopy with vibrational spectroscopies using both tip-enhanced Raman (TERS) and IR scattering scanning near-field optical microscopy (IR s-SNOM). I demonstrate how the enhanced near-field light-matter interaction can be used to image domain-level structural information, record dynamical motions of single molecules, and ultimately control structural dynamics.

Tip-enhanced vibrational spectroscopy has emerged as a powerful tool for nanoscale imaging and chemical identification. We extend vibrational nanoimaging and spectroscopy to understand fundamental physical properties of molecular materials. We measure nanoscale heterogeneity in molecular order, structure, domain formation, intermolecular coupling, and the local chemical environment. From the symmetry-selective probing of vibrational normal modes we measure nanoscale maps of crystalline orientation in molecular semiconductors. Through vibrational solvatochromism of specific marker resonances, we image nanoscale variation of the the local chemical environment at the sub-domain level and across domain interfaces of diblock copolymers. Disorder and multi-timescale fluctuations continue to play a role in material properties down to the single molecule limit. We measure intra- and inter-molecular coupling through temperature-dependent single-molecule spectroscopy of rhodamine 6G and record dynamical motion and spectral diffusion on the timescale of seconds.

Finally, we can modify and control the molecular vibrations through the optical-antenna behavior of nanoscale metallic tips in IR s-SNOM. Near-field coupling between optical antenna modes and molecular vibrations leads to new hybrid light-matter states, altering the ground state of molecules. I discuss this in terms of recent experimental measurements showing strong coupling, Fano lineshapes, electromagnetically induced scattering, and tip-enhanced radiative emission of molecular vibrations. These hybrid light-matter states have potential applications in quantum information processing, nonlinear spectroscopies, and optical control of photochemical reactions.