  We experimentally investigate the detection mechanism in a meandered
molybdenum silicide (MoSi) superconducting nanowire single-photon detector by
characterising the detection probability as a function of bias current in the
wavelength range of 750 to 2050 nm. Contrary to some previous observations on
niobium nitride (NbN) or tungsten silicide (WSi) detectors, we find that the
energy-current relation is nonlinear in this range. Furthermore, thanks to the
presence of a saturated detection efficiency over the whole range of
wavelengths, we precisely quantify the shape of the curves. This allows a
detailed study of their features, which are indicative of both Fano
fluctuations and position-dependent effects.
