Detection of THz waves by GaAs photoconductive antennas excited at 1560 nm with tight focusing

We investigated the use of low-temperature-grown (LTG) GaAs photoconductive antenna (PCA) excited at 1560 nm for the detection of terahertz (THz) pulses. The decrease in the excitation spot size with a PCA of properly reduced gap length was found to be efficient to enhance the signal intensity. With a spot size of 2.7 μm exciting a 1.5 μm gap PCA, the amplitude of the detected THz wave is an order of magnitude larger than that obtained in conventional 5 μm gap PCAs. The enhancement of the signal intensity by the reduction of the spot size stems mostly from the super-linear response of the LTG-GaAs excited at 1560 nm. The results suggest that the present scheme is very useful in fiber laser based time domain THz systems.