Since the discovery of high-order harmonic generation (HHG) in solids^1,2,3, much effort has been devoted to understand its generation mechanism and both inter- and intraband transitions are known to be essential^{1,2,3,4,5,6,7,8,9,10}. 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.

自从发现固体中的高次谐波产生 (HHG) ^1,2,3以来，人们一直致力于了解其产生机制，并且已知带间和带内跃迁都是必不可少的^{1,2,3,4, 5,6,7,8,9,10}. 然而，带内跃迁受固体电子结构的影响，它们如何促进非线性载流子的产生和 HHG 仍然是一个悬而未决的问题。在这里，我们使用中红外激光脉冲研究 CdSe 和 CdS 量子点中的 HHG，其中量子限制可用于控制带内跃迁。我们发现，当平均量子点尺寸从大约 2 nm 增加到 3 nm 时，每个激发体积的 HHG 强度和产生的载流子密度都会增加。我们表明，较大量子点中子带隙能量的降低增强了带内跃迁，而这反过来又通过与带间跃迁耦合增加了光载流子注入速率，从而增强了 HHG。