PNNL

Abstract

The manipulation of light near the apex of a metallic nano-tip has enabled single molecule detection, identification, and imaging. The distinct advantages of the so-called tip-enhanced optical nano-spectroscopy/nano-imaging approaches are self-evident: ultra-high spatial resolution (nanometer or better) and the ultimate sensitivity (down to yoctomolar) are both attainable, all while retaining the ability to chemically fingerprint one molecule at a time (e.g., through Raman scattering). An equally interesting aspect of the same approach stems from using the properties of a single molecule to characterize the local environment in which it resides. This concept of single molecule spectroscopy on the left hand side of the Schrödinger equation is certainly not novel and has been discussed in pioneering single molecule studies that ultimately led to a Nobel prize in chemistry. That said, local environment mapping through ultrasensitive optical spectroscopy acquires a unique flavor when executed using tip-enhanced Raman scattering (TERS). This is the subject of this account. In a series of recent reports, our group utilized TERS to characterize different properties of nano-localized and enhanced optical fields. The platforms that were used to this end consist of chemically functionalized plasmonic nanostructures and nanoparticles imaged using visible light-irradiated gold or silver-coated probes of an atomic force microscope. Through a detailed analysis of the recorded spectral nano-images, we found that molecular Raman spectra may be used to track the magnitudes, resonances, spatio-temporal gradients, and even vector components of optical fields with nanometer spatial resolution under ambient conditions. On the other side of the equation, understanding how spatially varying optical fields modulate molecular nano-Raman spectra is of utmost importance to emerging areas of nanophotonics. For instance, tracking plasmon-enhanced chemical transformations via TERS necessitates a deeper fundamental understanding of the optical signatures of molecular re-orientation and multipolar Raman scattering, both which may be driven by local optical field gradients that are operative in TERS. We illustrate these concepts and introduce the readers to the generally less appreciated and equally exciting world of TERS on the left hand side of the Schrödinger equation.