Time-resolved high harmonic generation

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tr-HHG experimental geometry. An 80 fs, 10 micron driver pulse is used to generate harmonics from a 100 nm thick epitaxial VO2 sample. The transmitted harmonic spectrum is then collected and recorded using a spectrometer (or photodiode). A 50 fs, 1.5 micron pump at variable delay, 𝜏, from the driver is used to photoexcite the IMT. The fluence of the pump can be modulated using a half-waveplate, polarizer energy throttle.
Figure adapted from: Mina R. Bionta, Elissa Haddad, Adrien Leblanc, et al., “Tracking ultrafast solid-state dynamics using high harmonic spectroscopy,” Phys. Rev. Research 3, 023250 (2021).
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Comparison of change in optical IR transmittance (top) and fifth harmonic yield (bottom). The time dependent change in optical IR transmittance (top), from traditional pump-probe spectroscopy, or the fifth harmonic yield (bottom) driven by 10 microns (tr-HHG) following the photoexcited IMT in VO2. Negative delays indicate the harmonic generating driver pulse arrives before photoexcitation. The various phases of VO2 are annotated on the curves. No revival of the IR transmittance is observed for long time-scales in a or short, in b. c Long-term recovery and revival of the harmonics as the VO2 transitions to the ℳ phase. The dashed rectangles are expanded upon with higher time-resolution in b and d. The shaded green rectangles show the region of the fast dynamics for the M1٭ → M1٭,b transition. In both c and d the suppression of the harmonics in the R phase can be seen for high fluences.
Figure from: Mina R. Bionta, Elissa Haddad, Adrien Leblanc, et al., “Tracking ultrafast solid-state dynamics using high harmonic spectroscopy,” Phys. Rev. Research 3, 023250 (2021).
 
 

Since the first observation of high harmonic generation (HHG) from solids (S. Ghimire et al., Nat. Phys. 7, 138 (2011)), this topic has become a highly active research field in ultrafast laser science (e.g. G. Vampa et al. Nature 522, 462 (2015)). In recent years, most efforts have been focused on understanding the physical mechanisms of HHG by investigating the nonlinear processes involved in various solids with a wide variety of bandgaps and different laser wavelengths to improve the generation process. To date, HHG has not been used to track dynamics in solids nor has it been used to interrogate strongly correlated materials.

In my research, I establish time-resolved high harmonic generation (tr-HHG) as a powerful spectroscopy method for tracking photoinduced dynamics in materials through a detailed investigation of the photoexcited insulator-to-metal phase transitions in the strongly correlated material vanadium dioxide. These complex and rich dynamics are ideal to benchmark the capabilities of our new technique. I take an experimental approach to test this hypothesis by investigating how the tr-HHG yield changes during these photoinduced phase transitions.

We show that time-resolved high harmonic spectroscopy is also sensitive to all these phases, and completely unexpectedly, can distinguish between the two different metallic phases. This is in strong contrast to traditional IR reflectivity (or any other spectroscopy we are aware of) which we show is incapable of discriminating between the phases. Furthermore, for the first time, we show that time-resolved high harmonic spectroscopy can resolve coherent phonon dynamics in a solid, and even more important, a dynamic that was never observed for VO2 for the range of pump fluence used. Given the simplicity and broad applicability of our technique (no vacuum setup – a femtosecond laser system coupled to an optical parametric amplifier and a spectrometer), this demonstration is a jumping off point for the future use of the method in a variety of materials and is likely to stimulate significant theoretical efforts aimed at understanding the mechanisms associated with HHG in strongly correlated materials.

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