Ultrafast photoconductive response of LaAlO$_{3}$/SrTiO$_{3}$ nanoscale photodetectors*

Conducting AFM lithography can be used to create nanoscale field effect transistors at the LaAlO$_{3}$/SrTiO$_{3}$ interface.\footnote{C. Cen et al., Nature Material, 7, 2136(2008)}$^,$\footnote{C.Cen et al., Science, 323, 1026 (2009)} Such devices exhibit gatable photoconductive response, which spans from visible to near-infrared regime.\footnote{P.Irvin et al., Nature Photonics advanced online publication, 14 Nov.2010 (DOI 10.1038/nphoton.2010.238)} By implementing the pump-probe measurement with a home-built femtosecond laser, we observe an ultrafast nonlinear optical response of these nanoscale photodetectors. We explore the feasibility of these devices for molecular-scale THz spectroscopy applications.