  We present a basic framework for modeling collective mode effects in
photocurrent measurements performed on two-dimensional materials using
nano-optical scanned probes. We consider photothermal, photovoltaic, and
bolometric contributions to the photocurrent. We show that any one of these can
dominate depending on frequency, temperature, applied bias, and sample
geometry. Our model is able to account for periodic spatial oscillations
(fringes) of the photocurrent observed near sample edges or inhomogeneities.
For the case of a non-absorbing substrate, we find a direct relation between
the spectra measured by the photocurrent nanoscopy and its parental scanning
technique, near-field optical microscopy.
