Femtosecond demodulation in low-temperature-grown gallium arsenide: A high-spectral-purity submillimeter and terahertz source for spectroscopy

The development of a submm source capable of producing high spectral purity THz radiation for high resolution Doppler limited spectroscopy is technologically difficult. However, the potential usefulness of such a source for spectroscopy has driven our development of a technique whereby pulses from an 800 MHz, 200 femtosecond mode-locked Ti:sapphire laser are demodulated with a Low Temperature Grown GaAs Photo-Conductive Switch (LTG GaAs PCS) and the resulting submm radiation used for molecular spectroscopy. Current technology incorporates a sputtered gold antenna on the surface of the LTG GaAs substrate and the pulse train from the mode-locked laser is focused onto the 5 mum gap at the center of the biased antenna structure. The antenna radiates all the spectral components produced by the demodulation process into free space. A single component from this "comb" of frequencies may then be employed for molecular spectroscopy. Scanning through the molecular transition is achieved by changing the length of the laser cavity, and, therefore, the spacing of the spectral components.;The spectral purity of the source is dependent upon fluctuations in the timing of the optical pulses. In this thesis, the noise on passively mode-locked Ti:sapphire lasers is developed completely from the frequency domain using simple Frequency Modulation (FM) theory. It will be shown that the primary source of "noise" on these systems is a result of AM noise on the Argon Ion pump laser. It will also be shown that the inclusion of low frequency amplitude fluctuations and white noise from the Argon Ion pump laser as sources of FM results in power spectrum "skirts" which match those commonly found on higher order spectral components.;While understanding the spectral purity and laser characteristics is a major portion of this research, the primary goal of this thesis work was to produce a spectrometer capable of measuring molecular rotational transitions at frequencies above 1 THz. Towards this goal, initial studies involving rotational transitions in the molecules carbonyl sulfide and carbon monoxide were used to characterize and calibrate the system at frequencies around 1 THz. Novel studies of hydrogen peroxide at frequencies around 1.3 THz were also successfully performed.