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(Color online) Schematic diagram of the experimental setup. L1 and L2, optical lenses in the pump and gain beams with focal lengths of 150 and , respectively; BBO, -thick type-I beta barium borate crystal; F1, high-resistivity silicon filter; HWP, half-wave-plate; QWP, quarter-wave-plate; WP, Wollaston prism; BD, balanced detector.
Typical pump-probe decay obtained by scanning gain beam delay 2 while keeping seed beam delay 1 and probe beam delay 3 fixed at the peak of the seed (or amplified) terahertz waveform: (a) without BBO crystal in the gain beam; (b) with BBO crystal in the gain beam; (a) and (b) reflect the amplification change at different timings of the gain pulse. The highest amplification obtained is about 65% of the input peak terahertz signal with BBO crystal inserted in the gain beam.
(a) Terahertz waveforms (solid lines) obtained by scanning probe beam delay 3 with three different gain beam delay 2 set at (upper), (middle), and (lower), respectively, while fixing seed beam delay 1 at zero timing. The vertical arrows indicate the corresponding gain pulse positions. Three identical seed terahertz waveforms (dotted lines with circles) are listed in each case for comparison. (b) The amplification effect is experimentally verified by reducing the seed terahertz signal (middle curves) by a factor of , and by flipping the seed terahertz waveform (lower curves). Unamplified (seed) terahertz waveforms obtained by blocking the gain beam are listed for each case for comparison. In all the cases, the amplification percentage is almost kept the same. The base lines are shifted and the waveforms are enlarged for clarity.
Total net peak gain as a function of the estimated total gain beam intensity. The solid dots indicate the experimental result and the solid line is the fit using the four-wave-mixing model.
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