Impact of interference between two infrared pulses driving high harmonic generation

TL;DR

This study investigates phase-stable XUV interference generated by driving high-harmonic generation in Ar with two temporally overlapping collinear IR pulses. By combining phase-modulation interferometry with both perturbative and Lewenstein-based non-perturbative theories, the authors map the complex nonlinear response and decompose the H15 signal into q-order contributions up to q = 21. Experimental results reveal slower-amplitude decay and extended higher-order contributions than perturbation theory predicts, in good agreement with the non-perturbative model, underscoring the importance of strong-field dynamics in two-pulse HHG. The findings have implications for XUV interferometry and coherent spectroscopy, highlighting both the utility and limitations of current models and the need for high-dynamic-range detection to fully characterize the nonlinear response during pulse overlap.

Abstract

Extreme ultraviolet (XUV) interferometry is technically challenging to implement. One approach to generating interference between two XUV pulses relies on driving high-harmonic generation in a gas jet with two collinearly overlapping infrared laser pulses. We investigate this scheme through a combined experimental and theoretical study, with particular emphasis on the regime of temporal overlap between the driving pulses. A special phase-modulation interferometry technique is implemented to increase the sensitivity for the comprehensive mapping of the strong-field induced high-order nonlinear response. We find that the dynamics arising from the interference of the two electric fields can be adequately described by the non-perturbative model developed by Lewenstein and co-workers.

Figures (7)

• Figure 1: Experimental setup. The stretched IR pulses enter a phase-modulation interferometer producing two pulses with modulated carrier-envelope phases $\phi_1$ and $\phi_2$ and time delay $\tau$. The pulses are compressed with chirped mirrors and generate high harmonics in an Ar gas jet. The produced XUV radiation is analyzed with a home-built spectrometer. Inset: Electric fields $E_1$ and $E_2$ generate high-order harmonics resulting in pulses at the $n\mathrm{^{th}}$ harmonic along with additional mixing contributions. CMP: chirped mirror pairs, Al: aluminium filter, CM: cylindrical mirror, MCP: microchannel plate, HG: harmonic generation.
• Figure 2: (a) Intensity autocorrelation traces of stretched input pulses (blue) and pulses compressed with the chirped mirror pairs (orange), (b) harmonic spectra recorded without (blue) and with (orange) the phase-modulated interferometer inserted in the beamline.
• Figure 3: H15 generation calculated with perturbation theory for two IR driving pulses. (a) time domain interferogram of the produced H15 radiation and (b) it's Fourier transform.
• Figure 4: Calculation steps in the non-perturbative model. (a) Flow chart. (b) Calculated $x(\omega ; \tau)$, dashed lines indicate the boundaries of the band pass filter to select H15. (c) $I_{15}(\tau)$, (d) $I_{15}(\omega)$ signals.
• Figure 5: Amplitude comparison: the peak amplitudes of the $q$-order signal components are shown for the experimental data (green), the perturbative (blue) and the non-perturbative model (orange). For $q \leq 15$, experimental error bars reflect the fluctuations between three consecutive experiments and for $q>15$ the error of the fitting routine for a single data set.