Fig. 1. Experimental setup for the synchronous measurements of the THz and TH emission. DM, dichroic mirror; PD, photodiode detector.

Fig. 2. (a) Measured TH intensity versus the fundamental pulse energy at a pressure of 1000 mbar in argon, nitrogen, and neon. According to the measured beam waist radius, the pulse energy of 100 µJ corresponds to the peak intensity of about 5 × 10¹³ W/cm². The insert shows the square root of the TH intensity versus the gas pressure at a fundamental pulse energy of 400 µJ in argon, nitrogen, and neon. (b) TH intensity as a function of the pulse time delay and fundamental pulse energy at an SH pulse energy of 1 µJ. The insert shows the TH intensity versus the fundamental pulse energy at an SH pulse energy of 1 µJ and the SH pulse energy at a fundamental pulse energy of 1 µJ, respectively.

Fig. 3. Synchronous measurements for the THz and the THG. (a) The measured THz and TH intensities as functions of the time delay between the fundamental and the second harmonic pulses. (b) The Fourier analysis of the TH curve in (a). (c) The comparison of the phase-dependent intensity modulation between the THz and the TH at three different time delays: −50 fs, 0 fs, and 50 fs.

Fig. 4. TH spectra obtained at three different time delays: (a) −40 fs, (b) 0 fs, and (c) 40 fs.

Fig. 5. The blue curves show the optimized phase for the TH (circles) and the THz (squares) (left axis), and the red curves show the optimized TH (asterisks) and the THz (crosses) yield (right axis).