First Experiments on Solid-state HHG*

We reported the first observation of high-order harmonics from solid materials in 2011 [1]. These observations, at the time, were surprising because until then high-order harmonic generation (HHG) was mainly studied and utilized in isolated atoms and molecules, and the underlying mechanism was understood using a three step re-collision model. HHG from bulk ZnO crystals also showed a plateau feature much like in gas phase HHG, with the high-energy cutoff extending to around 25^th order when the applied peak field was ~0.6 V/Å. However, the detail analysis of the measured spectrum including the observed linear scaling of high-energy cutoff with the laser field (quadratic scaling is expected from re-collision model) indicated strongly that the microscopic mechanism in the solid-state is fundamentally different. Since then there has been a significant interest in exploring the detail mechanism, and in exploiting the potential of the solid-state. Our recent results are observation of multiple plateaus [2], strongly anisotropic high harmonic response in MgO [3], observation of high-harmonics from atomically thin isolated monolayer MoS₂ [4], strong carrier-envelope phase dependence in MgO [5], and observation of high-order harmonics in amorphous solids [6]. Implications of these results include probing atomic-scale structure (real-space picture) and driven dynamics involving entire Brillouin zone (momentum-space picture).

References:
[1] Ghimire, S. et al., Nat. Phys. 7, 138 (2011).
[2] Ndabashimiye, G. et al., Nature 534, 7608 (2016).
[3] You, Y. et al., Nat. phys. 13, 345 (2016).
[4] Liu, H. et al., Nat. phys., 13, 262–265 (2017).
[5] You, Y. et al., Optics Letters 42, 9, (2017).
[6] You, Y. et al., Nature Communications, 8,724 ( 2017).