Frequency Upconversion by Ultrafast Plasmonic Coupling Between Metal and Semiconductor Nanoparticles

Abstract

Nonlinear frequency conversion at the nanoscale offers a pathway to compact, ultrafast light sources, but conversion efficiency is often constrained by weak nonlinearities and the lack of phase matching. This dissertation describes ultrafast plasmonic coupling between metal and semiconductor nanoparticles as a strategy to enhance harmonic generation in ultrathin metal–semiconductor nanoparticle heterostructures, where phase-matching requirements become negligible. I first demonstrate cascaded high-harmonic generation, in which second-order nonlinearities — second harmonic generation followed by sum-frequency generation — produce efficient upconversion, with the intermediate sum-frequency step strongly amplified when resonant with a localized surface plasmon. I then show that efficient harmonic generation requires a close harmonic relationship between the excitation wavelength and the plasmon resonances of the constituent nanoparticles; when this condition is not satisfied, the dominant upconversion pathway shifts from coherent harmonic generation to incoherent multiphoton photoluminescence. To elucidate the underlying mechanism, I perform distance-dependent and time-resolved measurements on Au/CuS nanoparticle heterostructures, which reveal surface-dipolar coupling and an ultrafast, harmonic-induced plasmonic resonant energy-transfer pathway. The heterostructures developed in this work leverage the broad and tunable plasmonic resonances of metal and semiconductor nanoparticles to generate multiple wavelengths from a single excitation. These findings establish a versatile design framework for families of multi-plasmonic heterostructures with enhanced nonlinear response at multiple wavelengths, enabling potential applications in bioimaging, environmental sensing, and quantum communication.