Coherent Control of High Harmonic Generation via Dual-Gas Multijet Arrays

High harmonic generation (HHG) is a central driver of the rapidly growing field of ultrafast science. We present a novel quasiphase-matching (QPM) concept with a dual-gas multijet target leading, for the first time, to remarkable phase control between multiple HHG sources ($>2$) within the Rayleigh range. The alternating jet structure with driving and matching zones shows perfect coherent buildup for up to six QPM periods. Although not in the focus of the proof-of-principle studies presented here, we achieved competitive conversion efficiencies already in this early stage of development.

Figure 1

(a) The dual-gas QPM principle. (b) Principle of the prototype target. It consists of two blocks with electroeroded Laval nozzles. The jets have a crossing angle of 75 deg. (c) Stacked foil target with 6 argon nozzles of $d1=100\mu \mathrm{m}$ and $s=1\mathrm{mm}$ used as a merged jet with two thin hydrogen jets for enclosing the argon density. The jets are separated by $20\mu \mathrm{m}$ thin foils so that $d2=700\mu \mathrm{m}$. (d) QPM configuration with hydrogen jets separating the six argon jets. The hydrogen jets for density confinement remain.

Figure 2

(a) Selective enhancement due to QPM in the two nozzle array for two distinct values of hydrogen pressure (black solid, 0.9 bar of hydrogen; shaded gray, 2.35 bar). (b) Zoomed area of the harmonics 29–41. (c) Order-dependent enhancement due to the phase added by hydrogen. The argon backing pressure is 2 bar for all curves.

Figure 3

Relative enhancement of the 41st harmonic (19.5 nm) between two sources (shaded dark gray) and four sources (shaded light gray). (a) Spectrally integrated tuning curve with variable hydrogen backing pressure and fixed argon pressure at 2 bar. (b) Peak-to-peak comparison in the angularly integrated spectrum at 1 bar hydrogen and 2 bar argon pressure.

Figure 4

(a) Comparison of the angularly and spectrally integrated harmonic yield for the 27th harmonic using a $700\mu \mathrm{m}$ merged argon jet (circles) and a dual-gas six-sources jet (triangles) which has five hydrogen jets placed between the six argon jets. For comparison the theoretical curves are included for $N_{\mathrm{QPM}}=1$ (dark shaded) and $N_{\mathrm{QPM}}=6$ (light shaded). (b) Close-up of the merged-jet oscillations with increasing argon backing pressure.