The generation, shaping and measurement of coherent terahertz radiation.

摘要

Coherent terahertz (THz) radiation has been studied extensively recently. It has seen great success in spectroscopy, and more applications are being developed. In this research, we studied various aspects of THz radiation technology which cover the generation, shaping and detection of coherent THz radiation. Firstly, we compared the THz waveforms measured by electro-optic (E-O) and photoconductive (PC) sampling techniques. For the first time, by quantitative comparison of the waveforms measured by E-O and PC sampling, we found out that the relation between those can be expressed by a simple formalism. And, by using E-O sampled waveforms as references, we could determine the response of the dipole antenna system used in the PC-sampling. This is to our knowledge the first verification of the antenna response function based on the direct measurement of electric field. Also, we demonstrated near-field effects in the THz waveforms and observed the evolution of the THz radiation from the near-field into the far-field regime. Secondly, for the first time, we demonstrated THz pulse shaping using femtosecond optical pulse-shaping. By engineering the temporal profile of the optical pulses exciting PC-emitters using a optical pulse-shaper, we could generate a number of THz waveforms including binary bit streams, and tunable narrow-band radiation. For many applications, high power THz radiation is desired. Usually PC-antennas generate strongest THz radiation below the 2 ∼ 3 THz region, but they suffer from saturation effects when excited by high fluence optical pulses. We were able to generate high power narrow-band THz radiation avoiding saturation effects by employing multiple pulse excitation on PC-antennas fabricated on fast carrier lifetime LT-GaAs. This was demonstrated using a dipole emitter and then extended to large aperture emitters with amplified excitation pulses. We could obtain more than a factor of 7 improvement in the peak spectral power density. Also our research clearly demonstrates the near-field nature of the saturation dynamics of large-aperture PC-emitters.