Terahertz generation via optical-heterodyne conversion: Development of a new far-infrared spectrometer and its applications toward a better understanding of nonrigid, astronomically important molecules

Generation of far-infrared radiation via terahertz optical heterodyne conversion (TOHC) has been implemented in high-resolution spectroscopic applications. Using this newly developed spectrometer technology, spectroscopic investigations of NH3 in the excited nu2 state and H2O in the ground and excited nu2 states were conducted.; To overcome the linewidth, problems associated with using diode lasers as heterodyne pump sources, optical feedback techniques were employed to develop a compact, narrow-linewidth, and tunable source of THz radiation for far-infrared spectroscopy. Distributed-Bragg-reflector (DBR) semiconductor lasers at 850 nm pump a low-temperature-grown GaAs (LTG-GaAs) photomixer. Resonant optical feedback stabilizes the center frequencies and narrows the linewidths of the DBR lasers. The heterodyne linewidth (full-width at half-maximum) of two optically locked DBR lasers was reduced from 40 MHz to 50 kHz on the 20-ms timescale and from 90 MHz to 2 MHz on the 10-s timescale. To demonstrate spectroscopic capabilities, fully resolved rotational spectra of acetonitrile near 312 GHz were acquired. The detection limit was 1 x 10--4 Hz--1/2 (noise/total power).; Subsequently, a three-laser, fully fiber-coupled TOHC spectrometer was constructed to implement absolute frequency calibration. In this setup, two DBR lasers (lasers #1 and #2) are locked to different longitudinal modes of the same Fabry-Perot cavity (using a Pound-Drever-Hall scheme). Another DBR laser, laser #3, is offset locked to laser #2. The outputs of lasers #1 and #3 pump the photomixer. A microwave sweeper controls the offset-locking frequency and enables continuous tuning. All lasers are subject to simple reflector feedback, and the resulting FIR linewidth is ≈1 MHz. Rotational spectra of CO have been acquired to calibrate FSR to an accuracy of better than 100 ppb.; Using the three-laser TOHC spectrometer, inversion and rotation-inversion spectra of nu2 ammonia were acquired with frequency accuracies (≈300 kHz) much higher than existing data. Fits of these new and existing transition frequencies determined two additional centrifugal-distort ion constants in the effective Hamiltonian. Additionally, submillimeter transitions of water in the ground and nu2 states were measured. These transitions are of great interest to the astrophysics and planetary sciences communities as they are expected to be particularly important for deciphering forth-coming remote-sensing data.