Resonant secondary light emission from plasmonic Au nanostructures at high electron temperatures created by pulsed-laser excitation

Abstract

Plasmonic nanostructures are of great current interest as chemical sensors, in vivo imaging agents, and for photothermal therapeutics. We study continuous-wave (cw) and pulsed-laser excitation of aqueous suspensions of Au nanorods as a model system for secondary light emission from plasmonic nanostructures. Resonant secondary emission contributes significantly to the background commonly observed in surface-enhanced Raman scattering and to the light emission generated by pulsed-laser excitation of metallic nanostructures that is often attributed to two-photon luminescence. Spectra collected using cw laser excitation at 488 nm show an enhancement of the broad spectrum of emission at the electromagnetic plasmon resonance of the nanorods. The intensity of anti-Stokes emission collected using cw laser excitation at 785 nm is described by a 300 K thermal distribution of excitations. Excitation by subpicosecond laser pulses at 785 nm broadens and increases the intensity of the anti-Stokes emission in a manner that is consistent with electronic Raman scattering by a high-temperature distribution of electronic excitations predicted by a two-temperature model. Broadening of the pulse duration using an etalon reduces the intensity of anti-Stokes emission in quantitative agreement with the model. Experiments using a pair of subpicosecond optical pulses separated by a variable delay show that the timescale of resonant secondary emission is comparable to the timescale for equilibration of electrons and phonons.gold nanorods | electron-hole pairs | surface-enhanced Raman scattering background

Enhanced Third-Order Optical Nonlinearity Driven by Surface-Plasmon Field Gradients

“…Our measured incoherent emission spectra therefore undergo a significant transformation with increasing incident power. In agreement with previous work [24,25,31], we can approximately describe the spectra by a Boltzmann distribution I IE (δω) ∝ e −δω/k B T e , with the electron temperature T e increasing from 410 ± 30 K to 1700 ± 300 K for excitation powers between 0.4 mW and 2.2 mW. The extracted electron temperature is shown in Fig.…”

“…Its power dependence is close to quadratic (1.98 ± 0.06) for the spectrally integrated signal as shown in Fig. 1c (black), and exhibits a frequency-dependent power law exponent due to varying spectral shape (see Supplement) in agreement with previous studies [24,25].…”

“…The broad background is due to incoherent emission (IE) derived from electronic excitations within the sp-band of Au [24]. Its power dependence is close to quadratic (1.98 ± 0.06) for the spectrally integrated signal as shown in Fig.…”

Cubic Nonlinearity Driven Up-Conversion in High-Field Plasmonic Hot Carrier Systems

“…6,28 Previous theories also propose measurements of surface plasmon polariton collisions. [29][30][31] More fundamentally, since surface plasmons are electromagnetic modes (photons) confined to metal surfaces, multi-plasmon decay can also be explored using conventional two-photon measurements on plasmonic systems, [32][33][34] with the distinction between plasmons and photons being on-versus off-resonance. The limit in exploring such measurements currently is that predictions of intrinsic nonlinear optical properties of plasmonic metals and theories of multiplasmon decay or the associated carrier dynamics are not yet available.…”

Plasmonic nano-resonator enhanced one-photon luminescence from single gold nanorods

“…The strong polarization-dependence of PL indicates a coherent mechanics in light emission. Very recently, the thermal occupation of electron-hole excitations in Raman scattering has been suggested to understand the anti-Stokes emission [10]. However, the theory is difficult to consistently explain both the anti-Stokes and Stokes components of the OPL from the gold nanorods excited by a cw laser (see Fig.…”