Sub-additivity in Electron Emission from GaAs

When two spatially-overlapped laser pulses (775 nm center wavelength, 75 fs duration) are incident on an untreated \textless 100\textgreater GaAs crystal surface, the electron emission rate depends on the temporal separation between the two pulses [1]. We have shown that for delays between 0.2 and 1000ps, the emission rate is ``sub-additive'', i.e., is lower than when the beams have separation \textgreater \textgreater 1 ns. We believe the cause of this sub-additivity is an increase in reflectance and transmittance due to electrons occupying the excited state of the GaAs. We are now able to manipulate the magnitude of the sub-additivity by changing the number of electrons that are in the excited state. Sub-additivity is not observed with tungsten tip surfaces which have no excited state. [1] E. Brunkow \textit{et al.}, Bull. Am. Phys. Soc. \textbf{60} (2015)