Dual coherent and incoherent two-photon luminescence in single gold nanorods revealed by polarization and time-resolved nonlinear autocorrelation

Abstract

It is well known that gold nanorods (AuNRs) readily emit two-photon luminescence (TPL) when excited by a broad bandwidth laser pulse that is tuned to the AuNRs’ localized surface plasmon resonance. The nature of the mechanism (i.e., especially its degree of coherence) is under active debate. In this work, we measured the TPL emission from single nanorods while varying the angle θ between the linearly polarized laser electric field and the nanorod’s orientation. Data were best fit with a linear combination of cos4 θ and cos2 θ functions. While the former function may represent TPL signals arising from both coherent and incoherent processes, the later function is indicative of a purely incoherent process. To further validate this assessment, we measured TPL emission from single nanorods in a time-resolved collinear autocorrelation setup. The autocorrelation signal exhibited a large peak at zero delay, which is characteristic of coherent two-photon absorption and two lower intensity wings extending to a few picoseconds, which demonstrates the existence of a long-lived intermediate state that contributes to a two-step incoherent absorption process. We conclude that TPL in AuNRs can result from a combination of coherent and incoherent absorption processes.