Spectroscopy and Imaging of Optical Near-fields on Plasmonic Nanostructures with UV-vis-NIR PiFM

Background

Early on in PiFM’s young history, a research group in Rice University published a paper titled “Photoinduced Force Mapping of Plasmonic Nanostructures” where they mapped the z-field associated with a single gold nanorod near its plasmon resonance wavelength (see Figure 1a) [1]. The paper demonstrated that the results agreed with the Maxwell’s stress tensor simulations and concluded the following: “we believe that photoinduced force microscopy is a promising technique to characterize heterogeneities of precisely manufactured nanostructures, SERS substrates, and heterogeneous (photo)catalytic samples.” Unfortunately, not much more work has been performed in this field, mostly supplanted by the exciting development of infrared (IR) PiFM [2]. However, as the researchers at Rice noted, “The ability to image the optical near-fields of nanoscale structures, map their morphology, and concurrently obtain spectroscopic information, all with high spatiotemporal resolution, is a highly sought-after technique in nanophotonics” [1]. Therefore, in this application note, we will return to one of the original applications and highlight the unique capabilities of visible PiFM for studying plasmonic nanostructures with unprecedented spatial resolution.

A major advance since the Rice paper has been the availability of tunable visible – near infrared (NIR) light sources. These light sources can tune their wavelengths continuously from 430 nm to 1450 nm. The light source consists of a supercontinuum source and AOTF filters, with the linewidth ranges from ~2 nm (at shorter wavelengths) to ~10 nm (at longer wavelengths). In this application note, gold nanorod and nanodisk samples are analyzed by vis-NIR PiFM.