Size-controlled quantum dots reveal the impact of intraband transitions on high-order harmonic generation in solids

• Published in: Nature physics Vol. 18; no. 8; pp. 874 - 878
• Main Authors: Nakagawa, Kotaro, Hirori, Hideki, Sato, Shunsuke A., Tahara, Hirokazu, Sekiguchi, Fumiya, Yumoto, Go, Saruyama, Masaki, Sato, Ryota, Teranishi, Toshiharu, Kanemitsu, Yoshihiko
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.08.2022; Nature Publishing Group
• Subjects: 140/125; 639/624/399/354; 639/624/400/3923; 639/925/357/1017; Atomic; Carrier density; Carrier injection; Classical and Continuum Physics; Complex Systems; Condensed Matter Physics; Electronic structure; Harmonic generations; Infrared lasers; Injection; Letter; Mathematical and Computational Physics; Molecular; Optical and Plasma Physics; Physics; Physics and Astronomy; Quantum confinement; Quantum dots; Theoretical
• ISSN: 1745-2473; 1745-2481
• DOI: 10.1038/s41567-022-01639-3

Since the discovery of high-order harmonic generation (HHG) in solids 1 – 3 , much effort has been devoted to understand its generation mechanism and both inter- and intraband transitions are known to be essential 1 – 10 . However, intraband transitions are affected by the electronic structure of a solid, and how they contribute to nonlinear carrier generation and HHG remains an open question. Here we use mid-infrared laser pulses to study HHG in CdSe and CdS quantum dots, where quantum confinement can be used to control the intraband transitions. We find that both HHG intensity per excited volume and generated carrier density increase when the average quantum dot size is increased from about 2 to 3 nm. We show that the reduction in sub-bandgap energy in larger quantum dots enhances intraband transitions, and this—in turn—increases the rate of photocarrier injection by coupling with interband transitions, resulting in enhanced HHG. Both inter- and intraband transitions contribute to high-harmonic generation in solids, but their exact roles are not fully understood. Experiments with quantum dots show that enhanced intraband transitions lead to increased carrier injection and thus enhanced harmonic generation.