All-Optical Experimental Control of High-Harmonic Photon Energy

We generate high-order harmonics in a gaseous medium with tunable photon energy using time-domain interferometry of double pulses in a noncollinear generation geometry. The method is based on the fact that the generated harmonics inherit certain spectral properties of the driving laser. The two temporally delayed laser pulses, identical in all parameters, are produced by a custom-made split-and-delay unit utilizing wave-front splitting without a significant energy loss. The arrangement is easy to implement in any attosecond pulse generation beamline, and is suitable for the production of an extreme ultraviolet source with simply and quickly variable central photon energy, useful for a broad range of applications.

Figure 1

Schematic experimental setup of the HR GHHG GAS attosecond beamline at ELI-ALPS. The main optical components of the setup are: holey-splitting mirror (HSM), split-and-delay unit (SDU), focusing mirror (FM), visible-infrared (VIS IR) spectrometer, gas cell, holey-dump mirror (HDM), aluminum filter, and XUV flat-field spectrometer. Red represents the IR beam and purple the generated XUV beam.

Figure 2

Simulated (a) and experimentally measured (b) spectra of the temporal double-pulse structure as a function of the time delay between the constituting pulses. The black (a) and gray (b) curves show the delay dependence of the central angular frequency.

Figure 3

Experimentally generated high-order harmonic spectra as a function of the delay between the driving pulses (colored map). The black dots represent the exact spectral positions of the harmonic peaks. The black curves show where each harmonic should appear according to the central angular frequency of the generation field, i.e., according to the black curve of Fig. 2.

Figure 4

(a) Simulated and (b) experimentally measured harmonics from the 37th to 41st harmonic orders around $-17$-fs delay. The black curves demonstrate where the harmonic peaks should appear according to the central angular frequency of the driving laser. The black dots in (b) show the measured positions of the harmonic peaks. (c) Demonstration of the achievable tuning ranges of different harmonics.

Figure 5

(a) Spectral bandwidth variation (black curve with open circles) and the flux (blue curve with filled circles) of the measured driving laser spectra as a function of the delay. (b) The 37th harmonic on a wide delay range showing the positions where the driving pulses interfere constructively. (c) The spectral bandwidth variation (black curve with squares) and flux (blue curve with triangles) of the 37th harmonic.