Macroscopic control of high-order harmonics quantum-path components for the generation of attosecond pulses

We present measurements of the different quantum path contributions to the high-order harmonic emission. Through spatial and spectral filtering, we evidence the strong correlation between the spatial and spectral distributions, which allows us to quantify the contribution of each quantum path. A systematic analysis as a function of the generating parameters has been done to identify the conditions for efficient generation and selection of a single quantum path. We show that combining phase matching and spatial filtering allows maximizing and selecting the short quantum path contribution, condition for the generation of “clean” and intense attosecond pulses.

Figure 1

Experimental setup for the spectral and spatial analysis of quantum path contributions.

Figure 2

(Color online) Average visibility of the fringes in the far-field interference pattern for the 17th harmonic, generated in argon, as a function of delay (1st-order autocorrelation trace): short (full circle) and long (open triangle) trajectories contributions. In dotted line are shown the Gaussian fits of the measured traces. The FWHM gives the coherence time. Inset: Interference pattern recorded at 0 delay.

Figure 3

(a) Spatial profile of the 17th harmonic generated in argon with a laser energy of $0.85\mathrm{mJ}$ when the laser is focused $4\mathrm{mm}$ after the gas jet. The laser diaphragm is $12.5\mathrm{mm}$. Fits of ${\tau }_1$ (Gaussian, dotted line) and ${\tau }_2$ contributions (superGaussian, dashed line). The arrow shows the size of the $1\mathrm{mm}$ closed HHG diaphragm. (b) Corresponding spectral profiles for an open and a closed HHG diaphragm.

Figure 4

(a) Integrated spectral intensity of H17 generated in argon as a function of the HHG diaphragm diameter, compared to the simulated signal, sum of the ${\tau }_1$ and ${\tau }_2$ contributions. The $12.5\mathrm{mm}$ apertured, $0.85\text{−}\mathrm{mJ}$ laser pulse is focused $4\mathrm{mm}$ after the gas jet. (b) HHG spectral width for two laser energies; the experimental widths are compared to the effective Gaussian width of the simulated profiles.

Figure 5

Spatial profiles of the 17th harmonic generated in argon when the slit in the spectral plane is opened or closed to $\Delta \lambda =0.2\mathrm{nm}$. The $1\text{−}\mathrm{mJ}$ laser pulse is focused $4\mathrm{mm}$ after the gas jet. The inset shows the corresponding spectral profile.

Figure 6

Measured spectral (a) and spatial (b) profiles of the 17th harmonic as a function of the gas jet position relative to the focus. The laser energy is $1.25\mathrm{mJ}$ and the laser diaphragm is $12.5\mathrm{mm}$.

Figure 7

(a) Contribution of the short trajectory for H17 as a function of the relative jet-to-focus position $z$ at $E=1.25\mathrm{mJ}$ and laser diaphragm $d=12.5\mathrm{mm}$ (open circle). The same dependance together with the total harmonic signal are shown for a laser energy of $0.85\mathrm{mJ}$ and $d=14\mathrm{mm}$ (respectively, open and full triangles). (b) Contribution of the short trajectory as a function of the laser energy at $z=+2\mathrm{mm}$ and $d=12.5\mathrm{mm}$. The total harmonic signal is plotted in full circle. The error bars (not shown) do not exceed 10% of the signal.