Ultrafast measurement in GaAs thin films using NSOM

We combine near-field microscopy with ultrafast time-resolved spectroscopy. By combining these two techniques, local carrier cooling and relaxation processes can be measured with submicron spatial resolution, shedding light on their relationship to the microstructure in these materials. The first step in making these kinds of measurements is to characterize the measurement technique. NSOM measurements of differential transmission in GaAs thin films made using transmission mode NSOM combined with the equal pulse correlation (EPC) technique (first demonstrated 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 concentrations (as low as 1013 cm−3), a decrease in the depth of field as compared to a linear NSOM measurement, and a contribution due to carrier transport. These measurements show the potential for making spatially resolved femtosecond time-resolved measurements at surfaces of semiconductor bulk materials and devices.