Strong-field Physics
Fundamental light-matter interactions We study the fundamental response of solid materials, atomic and molecular gases, as well as the vacuum to intense and super intense laser fields. Strong-field nominally refers to the low-frequency optical laser, where the strength of the laser field approaches or exceeds the interatomic binding strengths in crystals or molecules, which are on the order of volt per angstrom. This is the regime of non-perturbative laser-matter interaction, which includes nonlinear optical phenomena such as high-order harmonic generation (HHG) and above-threshold ionization (ATI). Such experiments are typically in table-top forms in PULSE laser laboratories. With the advent of modern x-ray free-electron lasers such as the LCLS, intense laser fields are now available in the x-ray wavelength range as well. In such a high-frequency regime, we study nonlinear multi-photon processes such as sequential and non-sequential x-ray absorption, nonlinear Compton scattering, and X-ray optical wave-mixing processes. Often, we perform pump-probe experiments, where the pump pulse initiates a novel ultrafast dynamics in materials and the probe pulse takes snap-shots of such rapidly evolving dynamics. Latest work on 2d-crystals In our recent study performed on atomically thin semiconductors of monolayer transition-metal dichalcogenides (specifically, WS2 and MoSe2), we characterized high-harmonic signals up to the 15th order (~3.8 eV). Systematic polarization analysis of the crystal-orientation dependence revealed that even-order harmonics exhibit sharp flipping in the polarization, whereas the odd-order harmonics continuously follow the direction of the driving field. Quantum-mechanical simulations are carried out and we attributed this observation to contributions from interband polarization. These results advance our knowledge of how crystal symmetry in two-dimensional crystals can be used to control polarization of the high-harmonic signals. Yuki Kobayashi,† Christian Heide,† Hamed K. Kelardeh, Amalya Johnson, Fang Liu, Tony F. Heinz, David A. Reis, and Shambhu Ghimire, Ultrafast Science 2021, 9820716 (2021). Latest work on probing topological phase transitions We demonstrate that circularly polarized laser-field-driven high-harmonic generation is distinctly sensitive to the non-trivial and trivial topological phases in the prototypical three-dimensional topological insulator bismuth selenide. We find strikingly different high-harmonic responses of trivial and non-trivial topological surface states that manifest themselves as a conversion efficiency and elliptical dichroism that depend both on the driving laser ellipticity and the crystal orientation. The origins of the anomalous high-harmonic response are corroborated by calculations using the semiconductor optical Bloch equations with pairs of surface and bulk bands. As a purely optical approach, this method offers sensitivity to the electronic structure of the material, including its nonlinear response, and is compatible with a wide range of samples and sample environments. Christian Heide, Yuki Kobayashi, Denitsa Baykusheva, Deepti Jain, Jonathan A. Sobota, Makoto H. Hashimoto, Patrick S. Kirchmann, Seongshik Oh, Tony F. Heinz, David Reis and Shambhu Ghimire, Nature Photonics 16, 620-624 (2022) Latest beam time at the LCLS We have been performing beam times at X-ray free-electron laser facilities such as the LCLS to measure X-ray optical wave-mixing from high quality bulk crystals such as diamond and silicon. The mixing offers a direct glimpse at the local electronic response of crystalline samples when excited by an optical pulse. In this way we are developing this novel nonlinear X-ray technique to better understand both linear and nonlinear light-matter interactions in crystalline solids. Strong Field Photoionization We have been studying the quantum interference patterns made by electrons when they are ripped out of an atom by laser fields with amplitudes of many volts per Angstrom. 
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