Harmonic generation is a general phenomena of strongly-driven systems. The conversion of infrared laser light to the extreme ultraviolet in atomic-gases is a well known example that forms the basis of strong-field atomic physics and attosecond pulse generation. The process can largely be understood in terms of a three-step model that includes periodic field-ionization, acceleration and coherent recombination of electrons from isolated atoms. We have recently discovered that non-perturbative high harmonic generation (HHG) also occurs in crystalline solids[1]. The high-density of solids could lead to compact and efficient sources of coherent short wavelength radiation. The generation mechanism in solids a topic of intense debate and is fundamentally different from atomic-gases in that the it necessarily involves interactions with multiple atoms. In this talk, I will present our recent efforts to understand solid-state HHG, how it differs from gas phase harmonics and its dependence on electronic structure[2-4].

[1] S. Ghimire, A. D. DiChiara, E. Sistrunk, P. Agostini, L. F. DiMauro, and D. A. Reis. Observation of high-order harmonic generation in a bulk crystal. Nature Physics, 7(2):138–141, 02 2011.
[2] G. Ndabashimiye, S. Ghimire, M. Wu, D. A. Browne, K. J. Schafer, M. B. Gaarde, and D. A. Reis. Solid-state harmonics beyond the atomic limit. Nature, 534(7608):520–523, 06 2016.
[3] H. Liu, Y. Li, Y. S. You, S. Ghimire, T. F. Heinz, and D. A. Reis. High-harmonic generation from an atomically thin semiconductor. Nat Phys, advance online publication, 11 2016.
[4] Y. S. You, D. A. Reis, and S. Ghimire. Anisotropic high-harmonic generation in bulk crystals. Nature Physics, advance online publication, 11 2016.