Visible Pump-THz Probe Spectroscopy of the Undoped Cuprate Sr$_2$CuCl$_2$O$_2$

We present experimental results on the mobility of photoexcited carriers in Sr$_2$CuCl$_2$O$_2$, an undoped cuprate. We use ultrafast laser pulses to excite photocarriers in the antiferromagnetic insulating lattice. We then probe the low-frequency dynamical conductivity of this nonequilibrium state with time-domain terahertz spectroscopy. Our current measurements place an upper bound on the mobility that is consistent with Hall mobility measurements.\footnote{Y. Ando {\em et al.} PRL {\bf 87} 017001 (2001)}