Narrow-band, multi-cycle terahertz (THz) pulses have been generated in the pre-engineered domain structure of periodically-poled lithium niobate (PPLN) crystals. The mechanism for THz generation is quasi-phase-matching (QPM) optical rectification. Recently, THz generation of high conversion efficiency in a new material, QPM GaAs, were demonstrated using mid-IR femtosecond pulses. GaAs has several advantages for QPM THz wave generation, as compared to PPLN. First, it is highly transparent at THz frequencies (absorption coefficient below 1.5 THz < 1 cm^-1). Second, the mismatch between the optical group velocity and THz phase velocity is much smaller: the corresponding group (ng) and refractive (n) indices are ng=3.431 at 2&mgr;m and n=3.61 at 1 THz. In this work, we report on generation of THz wave packets in three different types of QPM GaAs, combined with their coherent detection using two-color THz time-domain spectroscopy. The QPM GaAs structures are optically-contacted GaAs, diffusion-bonded GaAs, and all-epitaxially-grown orientation patterned GaAs. The QPM optical rectification in GaAs is a nonresonant mechanism, as opposed to widely used photoconductive antenna technique in GaAs, where THz radiation is produced via ultrafast charge transport caused by photoexcitation with femtosecond laser pulses of the near-IR range. In order to avoid linear and two-photon absorption in GaAs, we use 2&mgr;m femtosecond pulses to generate THz pulses. We measure the THz waveforms via electro-optic sampling in ZnTe using 0.8&mgr;m probe pulses. The corresponding power spectra are also measured by a THz Michelson interferometer. Frequency tunability in the range 0.8-3 THz is achieved with several structure periods.

Date Published: 14 February 2007
PDF: 8 pages
Proc. SPIE 6455, Nonlinear Frequency Generation and Conversion: Materials, Devices, and Applications VI, 64550G (14 February 2007);

Published in SPIE Proceedings Vol. 6455:
Nonlinear Frequency Generation and Conversion: Materials, Devices, and Applications VI
Peter E. Powers, Editor(s)