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Abstract

High-harmonic generation (HHG) is one of the nonlinear optical phenomena in strong laser-matter interaction. In contrast to HHG in gas and liquid, solid-state HHG can be applied in various aspects, such as XUV source production, carrier dynamics study in real time, band structure modification^1,2,3. Unlike in conductors, the electrons in semiconductors can only move relatively freely once they are excited to certain energy levels. One can study their electronic structures through light-matter interactions. In this project, the HHG spectroscopy of photoexcited bulk ZnO crystal will be studied in femtosecond resolution with sophisticated, high temporal resolution optical pump-probe spectroscopy. Studying the dynamics of excited charge carriers in semiconductors may help us to understand the structure of these materials, and how light propagates through materials, and if possible, how one can make use of this light-matter interaction.

Although bulk materials serve as effective media for HHG, they are not suitable for the emerging need of nanometer-scale on-chip nano-photonics. Two-dimensional (2D) nanostructures consist of single layers of atoms have better particle confinement than bulk materials and display broadband ultrafast optical response and strong excitonic effects^4,5,6,7. Theoretical and experimental works have showed stronger nonlinear optics effects can be realised in the doped graphene^8,9,10. However, as HHG from graphene is still one of the advanced topics in the ultrafast community, very few experimental studies have been conducted¹¹ and no published report on charge carrier dynamics in graphene have been studied using all-optical methods. In this project, HHG from unexcited and excited 2D materials will be explored, in particular graphene, at the resolution down to femtosecond scale. The dynamics of electrons in 2D materials will be revealed via all-optical method for the first time.

¹Garg et al., Nat. Photonics 12 291–296 (2018).
²Silva et al., Nat. Photonics 12 266–270 (2018).
³Uzan-Narovlansky et al., Nat. Photonics 16 428–432 (2022).
⁴Ullah et al., Adv. Optical Mater. 10 2101860 (2022).
⁵Ceballos, Zhao, Adv. Funct. Mater. 27 1604509 (2017).
⁶Sun et al., Nat. Photonics 10 227 (2016).
⁷Xiao et al., Nanophotonics 6 1309 (2017).
⁸Cox et al., Nat Commun 8 14380 (2017)
⁹Yang, Nano Lett. 11 3844–3847 (2011).
¹⁰Zhu et al., Phys. Rev. Applied 14, 064049 (2020).
¹¹Yoshikawa et al., Science 356 6339, 736-738 (2017).