  Superconducting nanowire single photon detectors (SNSPDs) are typically used
as single-mode-fiber-coupled single-pixel detectors, but large area detectors
are increasingly critical for applications ranging from microscopy to
free-space quantum communications. Here, we explore changes in the rising edge
of the readout pulse for large-area SNSPDs as a function of the bias current,
optical spot size on the detector, and number of photons per pulse. We observe
a bimodal distribution of rise times and show that the probability of a slow
rise time increases in the limit of large spot sizes and small photon number.
In the limit of low bias currents, the dark-count readout pulse is most similar
to the combined large spot size and small-photon-number bright-count readout
pulse. These results are consistent with a simple model of traveling microwave
modes excited by single photons incident at varying positions along the length
of the nanowire.
