  We report on the analysis of electroabsorption in thin
GaAs/Al$_{0.3}$Ga$_{0.7}$As nanophotonic waveguides with an embedded
$p$-$i$-$n$ junction. By measuring the transmission through waveguides of
different lengths, we derive the propagation loss as a function of electric
field, wavelength, and temperature. The results are in good agreement with the
Franz-Keldysh model of electroabsorption extending over 200 meV below the GaAs
bandgap, i.e. in the 910--970 nm wavelength range. We find a pronounced
residual absorption in forward bias, which we attribute to Fermi-level pinning
at the waveguide surface, producing over 20 dB/mm loss at room temperature.
These results are essential for understanding the origin of loss in
nanophotonic devices operating in the emission range of self-assembled InAs
semiconductor quantum dots, towards the realization of scalable quantum
photonic integrated circuits.
