Plasmon Dephasing in Gold Nanorods Studied Using Single-Nanoparticle Interferometric Nonlinear Optical Microscopy

We report the polarization-dependent and time-resolved photoluminescence (PL) properties of gold nanorods (AuNRs). AuNRs corresponding to three different length-to-diameter aspect ratios (AR) - 1.86, 2.91, and 3.90 - were examined using single-nanorod spectroscopy and imaging; the nanorod volume was approximately constant over the three sample types. For each AuNR, an aspect ratio-independent transverse plasmon resonance (TSPR) was detected at 2.41 eV. Aspect-ratio-dependent longitudinal surface plasmon resonances (LSPRs) were observed at 2.08 ± 0.19 eV, 1.76 ± 0.12 eV, and 1.53 ± 0.15 eV for the 1.86-AR, 2.91-AR, and 3.90-AR samples, respectively. On the basis of both excitation and emission polarization-resolved two-photon photoluminescence (TPPL) measurements, AuNR PL emission proceeded by plasmon-mediated radiative electron-hole recombination. The resonant LSPR mode frequencies of the nanorods were determined from interferometrically detected TPPL signals. For these measurements, the interpulse time delays of a spectrally broad laser pulse (1.48-1.65 eV) were changed systematically with attosecond time resolution, and the TPPL signal amplitude was recorded. The 1.86-AR AuNR did not support a plasmon mode that was resonant within the laser bandwidth, whereas the 2.91-AR and 3.90-AR samples had LSPR frequencies that overlapped the high- and low-energy components of the excitation pulse. The LSPR frequencies were obtained by Fourier transformation of the time-domain TPPL data and compared to dark-field scattering spectra. The accuracy of the interferometric TPPL measurement for recovering plasmon resonance frequencies was confirmed by polarization-dependent measurements; alignment of the laser electric field parallel to the nanorod major axis was LSPR resonant, whereas projection of the laser pulse into an orthogonal plane was not. Finally, dephasing times (T₂) for resonant plasmon modes were extracted from analysis of interferometric TPPL and second harmonic generation data. These results showed that the dephasing time increased from 22 ± 4 to 31 ± 9 fs as the LSPR resonance energy decreased from 1.76 to 1.53 eV, as a result of less efficient plasmon dephasing due to interband scattering for lower energy resonances. These results demonstrate the capability of interferometric nonlinear optical imaging with single-nanostructure sensitivity for determining structure-specific dephasing times, which influence the efficiency of metal nanoparticle light-harvesting applications. Therefore, interferometric nonlinear optical (NLO) imaging is likely to make a significant impact on the rational design of photonic nanostructures.