Polycrystallinity of Lithographically Fabricated Plasmonic Nanostructures Dominates Their Acoustic Vibrational Damping

Abstract

The study of acoustic vibrations in nanoparticles provides unique and unparalleled insight into their mechanical properties. Electron-beam lithography of nanostructures allows precise manipulation of their acoustic vibration frequencies through control of nanoscale morphology. However, the dissipation of acoustic vibrations in this important class of nanostructures has not yet been examined. Here we report, using single-particle ultrafast transient extinction spectroscopy, the intrinsic damping dynamics in lithographically fabricated plasmonic nanostructures. We find that in stark contrast to chemically synthesized, monocrystalline nanoparticles, acoustic energy dissipation in lithographically fabricated nanostructures is solely dominated by intrinsic damping. A quality factor of Q = 11.3 ± 2.5 is observed for all 147 nanostructures, regardless of size, geometry, frequency, surface adhesion, and mode. This result indicates that the complex Young's modulus of this material is independent of frequency with its imaginary component being approximately 11 times smaller than its real part. Substrate-mediated acoustic vibration damping is strongly suppressed, despite strong binding between the glass substrate and Au nanostructures. We anticipate that these results, characterizing the optomechanical properties of lithographically fabricated metal nanostructures, will help inform their design for applications such as photoacoustic imaging agents, high-frequency resonators, and ultrafast optical switches.

Acoustic Far-Field Hypersonic Surface Wave Detection with Single Plasmonic Nanoantennas

“…Although acoustic damping of particles on a substrate seems to be dominated by internal crystalline defects, as recently reported [25], in this letter we report that vibrational modes in single nanostructures can significantly couple to SAWs on the underlying substrate and thereby probed in the acoustic far-field. These SAW excitations induce coherent vibrations in a second nanoantenna at distances much larger than the characteristic amplitude of modes of the source, which are estimated to be on the order of sub-nanometric and even sub-atomic scales [26,27].…”

“…However, the use of periodic arrays has limitations such as the generation of pseudo-SAWs due to scattering into the substrate [21, 23] and inhomogeneous damping caused by the size dispersion of nanostructures. Thus, measurements in single nanostructures are required to further elucidate the acoustic wave damping of the generated coherent phonons [24].Although acoustic damping of particles on a substrate seems to be dominated by internal crystalline defects, as recently reported [25], in this letter we report that vibrational modes in single nanostructures can significantly couple to SAWs on the underlying substrate and thereby probed in the acoustic far-field. These SAW excitations induce coherent vibrations in a second nanoantenna at distances much larger than the characteristic amplitude of modes of the source, which are estimated to be on the order of sub-nanometric and even sub-atomic scales [26,27].…”

Strong vibrational coupling in room temperature plasmonic resonators

“…In general, the total quality factor for a given vibrational mode can be expressed as , where is the intrinsic damping quality factor, and corresponds to the environment damping. Normally intrinsic damping of chemically synthesized nanoparticles is relatively small 17,39 , although it may become dominant with lithographically fabricated plasmonic nanostructures where the crystal defects were abundant 18 . The measured quality factor for Au nanoplates on Lacey carbon films is remarkable, especially considering that molecular capping ligands were presented, as can be seen from the TEM measurements in Supplementary Fig.…”

“…However, these plasmonic nanoresonators suffer from both intrinsic and environmental energy dissipation mechanisms that reduce the vibrational quality factors. The intrinsic damping effect can be reduced by using single crystal nanoparticles created by chemical synthesis, rather than the polycrystalline particles produced by lithography 17,18 . Environmental damping for plasmonic nanoresonators predominantly occurs by radiation of acoustic waves into the surroundings 19 .…”

Elastic Purcell Effect

“…We note that while neither silicon nor gold are isotropic materials [42] (characterized by elastic anisotropic ratios A Si ≈ 1.6 and A Au ≈ 0.6), their simplified description does not incur significant shifts of resonant frequencies nor changes in Purcell factors. Additionally, we have verified by full numerical calculations that when viscous damping in gold is included through the complex Young's modulus [43] the quality factors and the Purcell factors characterizing higher-order elastic modes are significantly quenched. While further studies of viscous damping are required to understand its exact magnitude, this mechanism will likely promote the use of lowest-order elastic modes in nanostructures for enhancing radiative emission from localized emitters.…”