ULTRAFAST MEASUREMENT IN GAAS THIN-FILMS USING NSOM

We combine near-field microscopy with ultrafast time-resolved spectros copy, By combining these two techniques, local carrier cooling and rel axation processes can be measured with submicron spatial resolution, s hedding light on their relationship to the microstructure in these mat erials. The first step in making these kinds of measurements is to cha racterize the measurement technique. NSOM measurements of differential transmission in GaAs thin films made using transmission mode NSOM com bined with the equal pulse correlation (EPC) technique (first demonstr ated by Tang and Erskine [C.L. Tang, D.J. Erskine, Phys. Rev. Lett. 51 (9) (1993)844]) are presented. These measurements show some features unique to the near-field: a high sensitivity to impurity concentration s(as low as 10(13) cm(-3)), a decrease in the depth of field as compar ed to a linear NSOM measurement, and a contribution due to carrier tra nsport. These measurements show the potential for making spatially res olved femtosecond time-resolved measurements at surfaces of semiconduc tor bulk materials and devices. (C) 1998 Published by Elsevier Science B.V. All rights reserved.