Zuerch Lab
ULTRAFAST MATERIALS CHEMISTRY AT BERKELEY
Zuerch Lab
ULTRAFAST MATERIALS CHEMISTRY AT BERKELEY

Solid State Higher Harmonic Generation (sHHG)
Scheme of solid-state high harmonic generation (sHHG) (a,b) compared to gas phase HHG (c,d), both in the real space and momentum space picture. In sHHG the electron wave packet is bound to the quantum states permitted in the condensed-phase system allowing the emitted sHHG radiation to use as a proxy for material properties. Figure from [Ghimire & Reis, Nat. Phys., 15 (2019)]

Solids can be used as a medium to generate harmonics of an intense laser pulse. While gas-phase higher harmonic generation (HHG) is widely used in generating attosecond pulses, recently, solid state higher harmonic generation (sHHG) has gained attention. Compared to HHG, sHHG has richer dynamics due to the periodic structure of the dense medium. Rather than the well-established semi-classical tunnel ionization description that explains HHG (c-d), sHHG process involves tunnel ionization followed by interactions of the charge carriers with the periodic lattice potentials. Therefore, sHHG contains valuable information about the band structure of solids (a-b) and is sensitive to additional material properties such as symmetry.

Example of a sHHG spectrum driven by a 3.9 µm laser pulse in a monolayer flake (microscope image in inset) of MoS2 measured in our lab. Even and odd order high order harmonics generated are generated due to the broken inversion symmetry in the monolayer. The right panels show the sHHG (5th order) and photoluminescence (PL) in a 100µm thick c-cut ZnO crystal upon a time delayed photo-doping using an ultrashort 800 nm pump pulse. While the sHHG process is strongly damped, the generation of electrons (indicated by the PL signal) is enhanced by the pre-excited carriers.

Uniquely equipped with a CEP stabilized tunable source and synchronized OPA’s, we are aiming to elucidate quantum dynamics and phase transitions in solids using sHHG emission as a proxy. In our experimental endstation we can investigate samples at cryogenic temperatures. A broadband THz pulse can be combined with the ultrafast optical probes in the same experiment. Using these capabilities, we are interested in studying material dynamics such as phase transitions and charge transport in several classes of materials such as semiconductors, atomically-thin materials, heterostructures, and intercalated layered materials.

[1] R. Hollinger, et al., “The role of free carrier interaction in strong field excitations in semiconductors”, Physical Review B 104, 035203 (2021)- doi: https://doi.org/10.1103/PhysRevB.104.035203

[2] G. Zograf, et al., “High-harmonic generation from metasurfaces empowered by bound states in the continuum”, Arxiv:2008.11481 (2020) – doi: https://arxiv.org/abs/2008.11481

[3] R. Hollinger, et al., “Polarization Dependent Excitation and High Harmonic Generation from Intense Mid-IR Laser Pulses in ZnO”, Nanomaterials 11, 4 (2021)- doi: https://doi.org/10.3390/nano11010004

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