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The measurement and manipulation of high-intensity femtosecond laser pulses in-situ enables pulse optimization and the quantification of strong-field processes within the same experiment and is also very important for testing and validating new models, but it has proven very difficult to achieve.
Here we present an all-optical and fully in-line technique for the temporal characterization of ultrashort laser pulses directly on target and at full laser power during high-intensity laser-matter interactions, using the concurrent third harmonic (TH) emission from the target. This TH is generated at the interaction location and thus it depends on and carries information of the driving laser field. This technique permits the full (amplitude and phase), in situ, in-parallel characterization of the pulses in a gas or solid target over a very wide intensity range that encompasses the 1013–1015 Wcm−2 regime of high harmonic generation (HHG) and other important strong-field phenomena. Despite HHG being a non-perturbative process, the TH signal shows a perturbative-like behavior over a very large intensity range extending well into the relativistic regime.
We demonstrated the technique by measuring intense 4-fs pulses in conditions optimized for HHG and show numerically its extension to relativistic intensities, from 1018 to at least 1021 Wcm−2, which are presently inaccessible to other diagnostics.
By combining a pulse compressor (chirped mirrors and a pair of wedges) already present in typical HHG setups with the TH signal, we implemented a dispersion-scan (d-scan) variant, whereby with a single measurement we can then relate the pulse at any compressor position with its respective HHG spectrum, providing a way to quantify and optimize the process. We refer to this new method as THIS:d-scan (Third-Harmonic In-Situ d-scan). Its in-situ nature can account for the nonlinear phase accumulated by the high intensity pulse travelling to the experiment, and within the experiment itself, unlike conventional ex-situ techniques that require sampling a small fraction of the pulse for diagnostic purposes. Experiments were conducted in argon gas targets, where the retrieved spectrum and spectral phase compare very well with the ones obtained through an independent ex-situ SPIDER technique. The achieved low retrieval error and the excellent agreement between THIS:d-scan and SEA-F-SPIDER measurements provide further evidence for a perturbative cubic dependence of the TH on the laser field, even in a situation where laser intensities are clearly in the non-perturbative regime and can excite strong-field processes such as HHG. In neon targets and at higher pulse intensities, the THIS:d-scan measurement already shows signs of pulse reshaping within the target, which cannot be observed by standard ex-situ diagnostics.
THIS:d-scan can be implemented with minor modifications to existing strong-field setups. The fact that the TH emission occurs in gases, solids, and plasmas over a broad range of target densities, laser pulse durations, central wavelengths and intensities extending well into the relativistic regime, makes THIS:d-scan a very promising technique for the in-situ temporal characterization of ultrashort laser pulses during strong-field laser–matter interaction at the extreme conditions presently attainable with ultra-high power lasers.
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