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

Abstract

We report the polarization-dependent\nand time-resolved photoluminescence\n(PL) properties of gold nanorods (AuNRs). AuNRs corresponding to three\ndifferent length-to-diameter aspect ratios (AR)1.86, 2.91,\nand 3.90were examined using single-nanorod spectroscopy and\nimaging; the nanorod volume was approximately constant over the three\nsample types. For each AuNR, an aspect ratio-independent transverse\nplasmon resonance (TSPR) was detected at 2.41 eV. Aspect-ratio-dependent\nlongitudinal surface plasmon resonances (LSPRs) were observed at 2.08\n± 0.19 eV, 1.76 ± 0.12 eV, and 1.53 ± 0.15 eV for the\n1.86-AR, 2.91-AR, and 3.90-AR samples, respectively. On the basis\nof both excitation and emission polarization-resolved two-photon photoluminescence\n(TPPL) measurements, AuNR PL emission proceeded by plasmon-mediated\nradiative electron–hole recombination. The resonant LSPR mode\nfrequencies of the nanorods were determined from interferometrically\ndetected TPPL signals. For these measurements, the interpulse time\ndelays of a spectrally broad laser pulse (1.48–1.65 eV) were\nchanged systematically with attosecond time resolution, and the TPPL\nsignal amplitude was recorded. The 1.86-AR AuNR did not support a\nplasmon mode that was resonant within the laser bandwidth, whereas\nthe 2.91-AR and 3.90-AR samples had LSPR frequencies that overlapped\nthe high- and low-energy components of the excitation pulse. The LSPR\nfrequencies were obtained by Fourier transformation of the time-domain\nTPPL data and compared to dark-field scattering spectra. The accuracy\nof the interferometric TPPL measurement for recovering plasmon resonance\nfrequencies was confirmed by polarization-dependent measurements;\nalignment of the laser electric field parallel to the nanorod major\naxis was LSPR resonant, whereas projection of the laser pulse into\nan orthogonal plane was not. Finally, dephasing times (<i>T</i>₂) for resonant plasmon modes were extracted from analysis\nof interferometric TPPL and second harmonic generation data. These\nresults showed that the dephasing time increased from 22 ± 4\nto 31 ± 9 fs as the LSPR resonance energy decreased from 1.76\nto 1.53 eV, as a result of less efficient plasmon dephasing due to\ninterband scattering for lower energy resonances. These results demonstrate\nthe capability of interferometric nonlinear optical imaging with single-nanostructure\nsensitivity for determining structure-specific dephasing times, which\ninfluence the efficiency of metal nanoparticle light-harvesting applications.\nTherefore, interferometric nonlinear optical (NLO) imaging is likely\nto make a significant impact on the rational design of photonic nanostructures.