Fig. 1. Schematic diagram of the experimental setup. TS: telescope system.

Fig. 2. Typical spectra of (a) 3rd harmonic and (b) 7th harmonic; (c) measured 7th harmonic signal as a function of pump–probe time delay; (d) measured beam profiles of 3rd harmonic (red squares) and 7th harmonic (black circles).

Fig. 3. (a) Measured 7th (black circles) and 3rd (red squares) harmonic signals from excited argon atoms as a function of the probe pulse energy; (b) measured 7th and 3rd harmonic signals from unexcited argon atoms as a function of the probe laser energy. The green dashed and blue dotted curves are the corresponding fitting curves with function $I^{q}$, where $q$ is the harmonic order. Inserts: measured harmonic signals as a function of the probe laser energies with logarithmic scales.

Fig. 4. Measured dependences of 7th (black circles) and 3rd (red squares) harmonic signals from excited argon atoms on the elliptical polarization of the probe pulse. The green solid and blue dotted curves represent the theoretical predictions.

Fig. 5. The physical picture of the 3rd and the 7th harmonic generation from the excited argon atoms.