Nanoscale probing of Modified Surfaces with Surface Plasmons

A material can be uniquely identified by its optical properties. Based on this principle,the Kretschmann configuration is widely used in chemical analysis to identify materials by determining optical properties of the sample material upon surface plasmon resonance.This conventional approach measures average optical properties within the diffraction-limited illumination beam spot, therefore is incapable of identifying or mapping samples that are inhomogeneous on the nanometer scale. On the other hand, near-field optical microscopy is capable of nanoscale imaging but quantitative optical property maps cannot be inferred from the obtained images. This work demonstrates the principles of surface plasmon scanning tunneling microscopy (SPSTM), a new method to map the optical properties of a material at the nanometer scale, thus providing the foundation of a novel approach towards nanoscale chemical mapping. In this demonstration of principles, surface plasmons are excited in a gold thin film that supports micrometer-size flakes of a two-dimensional (2D) layered material in some areas. The surface plamons are coupled to a gold-coated, nanometer-sized optical fiber tip, and the resulting optical signal is guided by the fiber to a detector. The nanoscale imaging capability stems from the exponential decay of the evanescent field of surface plasmon modes, resulting in the exponential decrease in plasmon coupling as the tip-substrate separation increases, as well as the sharpness of the probe tip. By fabricating Au-coated, sharp optical fiber tips and constructing an SPSTM setup, I have demonstrated exponential decrease in optical signal intensity with increasing probe height, with faster decay in the presence the 2D material flakes than on bare Au, consistent with theoretical prediction, laying down the groundwork for a future SPSTM prototype.