Development of ultrafast narrow -bandwidth terahertz sources with applications to exciton spectroscopy.

Abstract

This dissertation discusses work in the development and characterization of narrow-bandwidth terahertz waveforms generated by optical rectification of ultrafast laser pulses in periodically-poled lithium niobate (PPLN). The sign of the second order nonlinear susceptibility, chi<super>(2)</super> of lithium niobate follows the sign of the local ferroelectric domain. The domain structures of the crystals are designed so that chi<super>(2)</super> changes sign when the optical pulse leads the locally generated terahertz polarization by one optical pulse-length. In this way, the opposite sign of terahertz polarization is generated in neighboring domains; the fields from neighboring domains connect smoothly, and an oscillatory terahertz field is radiated. The terahertz electric field is temporally resolved via free-space electro-optic sampling in a ZnTe sensor crystal. A 7.2 mm long PPLN crystal containing 240 domains of 30 microns each produced a waveform consisting of 120 cycles of the electric field. This signal was peaked at 1.7THz with a bandwidth (FWHM) of approximately 18GHz. By varying the domain length and the temperature of the crystals, nearly continuous frequency coverage from 0.8THz to 2.5THz was demonstrated. The capability of aperiodic crystals to generate more complex waveforms was also demonstrated. The terahertz radiation produced from a broad-bandwidth source (ZnTe) is used to excite 1s-2p transitions in excitons in an optically excited. In _.04Ga_0.96As/GaAs multiple quantum well sample. The existence of an exciton population is verified by the presence of absorption lines in the spectrum of the terahertz field at the 1s-2p resonance. This work confirms an exciton population in the sample as early as 200ps following an above-bandgap optical excitation pulse.