Optical Detection of Tunneling Ionization

We have experimentally detected optical harmonics that are generated due to a tunneling-ionization-induced modulation of the electron density. The optical signature of electron tunneling can be isolated from concomitant optical responses by using a noncollinear pump-probe setup. Whereas previously demonstrated tools for attosecond metrology of gases, plasmas, and surfaces rely on direct detection of charged particles, detection of the background-free time-resolved optical signal, which uniquely originates from electron tunneling, offers an interesting alternative that is especially suited for systems in which free electrons cannot be directly measured.

Figure 1

A high-intensity few-cycle pump pulse with a frequency ${\omega }_0$ modulates the electron density through tunneling ionization. The pump field $E(t)$ (green line), its instantaneous intensity $[E(t){]}^2$ (red stripes) and the calculated temporal profiles of the electron density (blue line) and ionization-induced loss (black line) are shown in the left panel. A weak noncollinear probe pulse applied with a variable time delay with respect to the pump reads out the pump-induced modulation of the plasma current, giving rise to odd-harmonic emission exactly along the probe-beam direction. The right panel displays the input laser field (dashed line) and the probe field with harmonics generated through plasma current modulation in the presence of a pump pulse with different intensities (encoded by different colors).

Figure 2

(a) Experimental maps of the third, fifth, and seventh harmonic of the probe beam detected as a function of delay between the pump and probe pulses from a 2.5 mm long krypton target. (b) Temporally integrated tunneling current harmonic signal spectra: measurement (solid lines) vs simulation (dashed lines). (c) The spectrally integrated harmonic signals as a function of pump-probe delay all show the same temporal structure, which agrees well to the calculated convolution of the amplitude mask with the probe pulse (dashed line). (d) Simulated tunneling harmonic spectra at zero pump-probe delay calculated without (black curve) and with the pulse propagation included (color coded curves).

Figure 3

Experimentally measured (a) and simulated (b) spectrograms obtained from a 1 mm long argon target. The time integrated signal is collapsed on the spectral axis, and the spectrally integrated signal is collapsed on the delay axis (yellow lines). The experimentally measured signal below 220 nm is absorbed before the spectrometer. The grey lines show the normalized spectrum of the third harmonic in the absence of the strong pump pulse. Panel (c) shows the third-harmonic signal measured (■) and simulated (+) as a function of the probe energy in the presence of the strong pump pulse (black) and in the absence of the strong pump (red).

Figure 4

Amplitude (a) and phase (b) masks, correspondingly proportional to the time dependences of the ionization rate and the normalized electron density, in the tunneling current emission as a result of the interplay between the pump field and the pump-induced plasma, calculated on the beam axis at the input of the jet (blue curves) and at the position in the jet corresponding to the peak electron yield (red curves).