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

TL;DR

This study investigates how quantum confinement in CdSe and CdS quantum dots influences intraband transitions and their role in enhancing high-order harmonic generation in solids, revealing size-dependent nonlinear optical effects.

Contribution

It demonstrates that increasing quantum dot size enhances intraband transitions and HHG efficiency by reducing subband gap energy and boosting photocarrier injection.

Findings

HHG intensity increases with quantum dot size

Larger QDs have reduced subband gaps, enhancing intraband transitions

Enhanced intraband transitions lead to higher HHG and carrier density

Abstract

Since the discovery of high-order harmonic generation (HHG) in solids, much effort has been devoted to understanding its generation mechanism and both interband and intraband transitions are known to be essential. 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 (QDs), where quantum confinement can be used to control the intraband transitions. We find that both the HHG intensity per excited volume and the generated carrier density increase when the average QD size is increased from about 2 nm to 3 nm. We show that the reduction of the subband gap energy in larger QDs enhances intraband transitions, and this in turn increases the rate of photocarrier injection by coupling with interband transitions, resulting in enhanced HHG.