Quantum Path Interferences in High-Order Harmonic Generation

We have investigated the intensity dependence of high-order harmonic generation in argon when the two shortest quantum paths contribute to the harmonic emission. For the first time to our knowledge, experimental conditions were found to clearly observe interference between these two quantum paths that are in excellent agreement with theoretical predictions. This result is a first step towards the direct experimental characterization of the full single-atom dipole moment and demonstrates an unprecedented accuracy of quantum path control on an attosecond time scale.

Figure 1

Calculated argon 15th harmonic: single atom dipole strength including all quantum paths (black curve) and restricted to the two shortest quantum paths (red curve); macroscopic response without filtering (black dot curve) and with off-axis spatial filtering (blue curve, far-field filtering from 14 to 20 mrad).

Figure 2

Single argon atom harmonic spectra as a function of peak laser intensity for a 30-fs pulse duration.

Figure 3

Calculated macroscopic spectra of the argon 15th harmonic as a function of peak laser intensity without (left) and with (right) a far-field 6-mrad off-axis window. The harmonic signal is plotted with a linear color scale defined as blue at zero strength to red at maximum strength (${10}^{10}$ without window and $4.5\times {10}^7$ with window).

Figure 4

Measured harmonic spectral dependence on the peak laser intensity with far-field 6-mrad spatial filtering. (a) on-axis filtering; (b) off-axis filtering.