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Schematic illustration of experimental setup. An optical beam from an amplifier Ti:sapphire laser is split into three beams as probe, control, and signal beams. Both control and signal beams create plasma, where terahertz waves are generated through ponderomotive force. The signal beam is modulated by an optical chopper. Delay-1 varies the time delay between the control and signal pulses. Delay-2 is used for the probe pulse to sample the terahertz wave form.
Terahertz wave form from the plasma generated by the signal pulse and enhanced by the control pulse: (a) the control beam is blocked and the signal beam has pulse energy. (b) The control pulse is after the signal pulse. [(c) and (d)] Control beam is 22 and before the signal beam, respectively. For the curves (b), (c), and (d), both the signal beam and the control beam have pulse energy.
Enhancement as a function of the relative timing between the control pulse and the signal pulse (delay-1). The dash-dot line is the zero offset and the dash line is the exponential decay fit. A decay time is extrapolated.
Dependence of the enhancement on the control beam pulse energy. The terahertz pulse energy is obtained from integrating over the square of the whole wave form. The solid curve is the power fit with slopes of 1.2 (when ) and 1.4 (when ). An estimated threshold of corresponding to laser intensity of is observed.
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