Light-Controlled in Situ Bidirectional Tuning and Monitoring of Gold Nanorod Plasmon via Oxidative Etching with FeCl3

We report on light-controlled in situ bidirectional tuning of longitudinal surface plasmon resonance (LSPR) of single gold nanorods via oxidative etching with ferric chloride. By removing the surfactant layer from the surface of a gold nanorod, we demonstrate that the etching happens only in the presence of an excitation laser, and the etching rate and directionality can be controlled by the intensity of excitation light. At a low excitation power, a blue shift of a nanorod's LSPR of up to 50 nm was observed, which indicates preferential etching from its tips. Whereas at a high power, we see a red shift of the nanorod's LSPR of up to 140 nm indicating etching from sides. These results present a new approach for in situ finer adjustments of a selected nanorod's plasmon resonance.