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 (<i>T</i>₂) 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