  Semiconductor nanowires have been demonstrated as an efficient platform for
fluorescence-based biosensors. Here, we study theoretically how GaP nanowires
(i) enhance the excitation intensity at the position of fluorophores attached
to the nanowire sidewall, (ii) enhance the probability to collect photons
emitted from the fluorophores by directing them preferentially into the
numerical aperture of collection objective, and (iii) through the Purcell
effect increase the quantum yield of fluorophores. We find that the optimum
diameter depends strongly on the fluorophore emission wavelength. In addition
to an overall signal-detection scheme, we model imaging-based detection of the
fluorescence.
