We demonstrate an efficient room temperature source of narrow-bandwidth terahertz (THz) radiation using femtosecond pump pulses and periodic GaAs structure as a nonlinear material. In the past, several THz generation schemes exploited optical rectification in nonlinear crystals using femtosecond laser technology. Most of them generated single-cycle THz-pulses with broad bandwidth, using nonlinear crystals shorter than the phase-matching coherence length. Recently a novel technique to generate multi-cycle THz-pulses in the pre-engineered domain structure of periodically-poled lithium niobate (PPLN) crystals has been demonstrated. Quasi-phase matching (QPM) structures such as PPLN consist of a periodic system of domains of inverted crystal orientation. The sign of second order nonlinear polarization generated by femtosecond pulses is inverted at domain boundaries. If domain length is comparable with coherence length, QPM between THz-wave and nonlinear polarization extends the
interaction length between THz and optical pulses. In the present
work, using periodic GaAs structures we have achieved exceptionally high photon as well as energy conversion efficiency: 3% and 0.07% respectively. We have examined two different types of periodic QPM GaAs samples: diffusion-bonded GaAs wafers and all-epitaxially-grown orientation-patterned GaAs crystals with 3-10 mm thicknesses. The incident optical pulse energy was in the micro-Joule range and pulse duration was ~100 fsec. We measured spectral properties of THz radiation using Michelson interferometer and a bolometer. Narrow-bandwidth (~100GHz) THz output, tunable between 1 and 3 THz, was achieved. THz frequency was tuned either by tuning the light source wavelength between 2 and 4.4 microns, or by selecting GaAs samples with different QPM periods. Our theoretical analysis, based on known GaAs dispersion properties, shows good agreement between the measured and predicted THz frequencies.© (2006) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.