Optoelectronic Interferometry and Spectroscopy beyond 6THz

Abstract

We report a new interferometric technique1, to measure freely propagating THz radiation. Via this new technique which uses unamplified, 100MHz, 60fs laser pulse excitation of a two source interferometer, we demonstrate useful THz power beyond 6THz. Additionally, we were able to demonstrate the applicability of the technique to spectroscopy by measuring the absorption of GaAs beyond 5.0 THz. We used the newly developed THz radiation source2, based trap enhanced electric fields, to produce THz radiation with a total average power of 30 nW for 4 mW of incident laser power. The optoelectronic interferometer consists of two identical THz radiation sources, a high resistivity Silicon beam splitter and a cooled bolometer. The interferometer is mechanically fixed; the interferogram is obtained by scanning the relative time-delay between the two laser excitation beams. The signal to noise ratio of a single 2Hz scan was often greater than 50:1, thereby permitting iterative adjustments in real time. The interference is characterized by excellent overall symmetry and near total extinction and a temporal width of 200fs. The spectroscopic demonstration establishes the usefulness of the interferometric technique with the advantage of increased bandwidth as compared to direct measurements with fast photoconductors.