Theoretical Modeling of Collective Mode Effects in Photocurrent Nanoscopy

Utilizing scanned probes to focus light beyond the diffraction limit has been shown to be effective for studying collective modes of two-dimensional plasmonic and polaritonic systems, such as graphene and van der Waals heterostructures. 

Scanned probes techniques have also been employed for measuring photocurrent response with nanoscale resolution. 

We develop a model that describes the latter class of measurements.

The model includes three typical photocurrent generation mechanisms: photothermal, photovoltaic, and bolometric.  

The relative strength of these mechanisms depends on frequency, temperature, applied bias, and sample geometry, so that any of them can dominate under certain conditions. 

Our model is able to account for plasmonic and polaritonic interference patterns observed near sample edges and inhomogeneities. 

We also compare the frequency dependence of the photocurrent with the signal obtained from another scanning technique, scattering-type near-field optical microscopy (s-SNOM).

We find a simple relation, analogous to the optical theorem, that connects the two for the case of a weakly absorbing substrate.