Laser-induced plasma formation in solid dielectrics
When strong-field ionization (SFI) occurs in a dielectric solid, the plasma density increases in a nearly stepwise fashion with two steps per optical cycle of the driving field [Fig. 1 (a)]. The Fourier series of the plasma density evolution ρSFI(t) contains even harmonics of the incident laser field [Fig. 1 (b)], the so-called Brunel harmonics. Since only the rapid variations of ρSFI(t) contribute to the harmonic emission, time-resolved detection of Brunel harmonics allows to analyze sub-cycle ionization dynamics in the frequency domain. In a two-color pump-probe scheme Brunel harmonics occur at frequencies wn = wprobe + 2n x wpump where n is a natural number. An experimental spectrogram displaying the first three orders of Brunel harmonics obtained from a two-color pump-probe experiment (lpump = 2300 nm, tpump = 150 fs, lprobe = 790 nm, tprobe = 40 fs) in the bulk of fused silica is shown in Fig. 1(c). By performing an iterative phase-retrieval on the experimental spectrogram, it is possible to reconstruct the plasma formation dynamics due to SFI. A simultaneous measurement of the total plasma density generated in the material reveals the relative importance of the two competing ionization mechanisms (SFI and electron impact ionization), thus allowing unprecedented insights into ultrafast plasma formation dynamics in solid dielectrics.
Fig 1: (a) Temporal evolution of the plasma density in fused silica irradiated by an ultrashort laser pulse. (b) Fourier series expansion of (a) containing even harmonics of the driving laser field. (c) Time-resolved experimental spectrogram resulting from two-color irradiation of a 0.5 mm fused silica sample.