Time domain terahertz frequency comb spectroscopy
High-resolution spectroscopy in the terahertz (THz) region of the electromagnetic spectrum (0.1-10 THz) is crucial for molecular astrophysics and the study of hydrogen-bonded clusters. Unfortunately, progress in these fields has been hindered by the available THz technology. High-resolution techniques with electronic sources suffer from limited bandwidths, while broadband time-domain THz techniques with ultrafast lasers suffer from a limited resolution. We have recently overcome the resolution limitations of time domain THz spectroscopy using two offset-locked Ti:Saphhire lasers and asynchronous optical sampling THz time-domain spectroscopy. In this technique, the femtosecond optical pulse train from a pump laser is used to generate a broadband THz frequency comb, while a second optical pulse train from a probe laser detects the comb in the time domain through electro-optic sampling. The offset lock between the lasers ensures a repetitive phase-walkoff in delay time, allowing for a resolution that is only limited by the stability of the lasers. With this technique, we have achieved a decade-spanning THz bandwidth (0.2-2.5 THz) and a resolution of 10-10. In my talk, I will discuss the design of the spectrometer, some preliminary Doppler-limited measurements of gas phase spectra, and the future implications for molecular spectroscopy.