Acoustic vibrations in plasmonic nanoparticles can be monitored through all-optical means, offering non-invasive ways to probe mechanical properties of nanoscale structures and their environment. Lithographically prepared nanoparticles are ideal for applications as their vibrational behavior can be tuned through size and shape control. Unfortunately, these polycrystalline particles exhibit low vibrational quality factors Q due to vibrational damping. The relative contributions of intrinsic and extrinsic damping, caused by lattice defects and energy transfer at the particle-substrate interfaces, are still debated but are critical to understand for the design and fabrication of nanoparticles with large Q. Using single particle-transient extinction microscopy, we investigate the acoustic vibrations of individual lithographically prepared gold nanodisks on glass as a function of their aspect ratio and particle-substrate binding strength, facilitated through titanium adhesion layers. We find that strongly bound particles possess multiple acoustic modes that can exhibit greatly enhanced Q factors for specific aspect ratios and modes, while weakly bound particles support a single mode with constant Q. We theoretically assign these findings to a substrate-induced effect that only occurs for strong particle-substrate binding and leads to new acoustic modes which can experience minimized extrinsic damping at the particle-substrate interface. We also generally observe much larger theoretical Q factors. With lattice defects not being theoretically considered, this discrepancy implies large intrinsic damping in lithographically prepared nanoparticles, explaining the constant Q values for weakly surface-bound particles. Our work provides agreement for previously contradicting reports and offers control of acoustic behavior through the particle-substrate interface.

Controlling the acoustic behavior of single lithographically prepared gold nanodisks through the particle-substrate binding strength

ACS Spring 2022

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