Hot-Electron Intraband Luminescence from Single Hot Spots in Noble-Metal Nanoparticle Films

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Hot-Electron Intraband Luminescence from Single Hot Spots in Noble-Metal Nanoparticle Films

Tobias Haug, Philippe Klemm, Sebastian Bange, and John M. Lupton

Phys. Rev. Lett. 115, 067403 – Published 7 August 2015

Abstract

Disordered noble-metal nanoparticle films exhibit highly localized and stable nonlinear light emission from subdiffraction regions upon illumination by near-infrared femtosecond pulses. Such hot spot emission spans a continuum in the visible and near-infrared spectral range. Strong plasmonic enhancement of light-matter interaction and the resulting complexity of experimental observations have prevented the development of a universal understanding of the origin of light emission. Here, we study the dependence of emission spectra on excitation irradiance and provide the most direct evidence yet that the continuum emission observed from both silver and gold nanoparticle aggregate surfaces is caused by recombination of hot electrons within the conduction band. The electron gas in the emitting particles, which is effectively decoupled from the lattice temperature for the duration of emission, reaches temperatures of several thousand Kelvin and acts as a subdiffraction incandescent light source on subpicosecond time scales.

Received 16 March 2015

DOI:https://doi.org/10.1103/PhysRevLett.115.067403

Authors & Affiliations

Tobias Haug, Philippe Klemm, Sebastian Bange^*, and John M. Lupton

Institut für Experimentelle und Angewandte Physik, Universität Regensburg, 93051 Regensburg, Germany

^*Corresponding author. sebastian.bange@ur.de

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Vol. 115, Iss. 6 — 7 August 2015

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Images

Figure 1
Localized continuum emission from a single hot spot in a disordered silver nanoparticle film grown by the Tollens reaction, excited by 86 fs laser pulses at 770 nm wavelength and 80 MHz repetition rate. (a) Microscope image of the emission, detected in the 818–1050 nm spectral range. (b) Typical emission spectrum (black) of a single luminescent site for an irradiance of $1 kW / {cm}^{2}$ with scattered laser radiation (shown in green) blocked by a filter. (c) Irradiance dependence of integrated visible (blue) and infrared (red) emission, with power-law exponent $p$ stated.
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Figure 2
Spectroscopy of a single luminescent site in a silver nanoparticle film, excited at 770 nm (1.61 eV) by either 86 fs laser pulses at 80 MHz repetition rate (black), or continuous-wave illumination (red). (a) Emission spectra at $100 W / {cm}^{2}$ (solid) and $1 kW / {cm}^{2}$ (dashed) irradiance, filtered in the range 1.52–1.72 eV to suppress elastically scattered light. Raman bands around $\pm 170 meV$ relative to the excitation are marked by gray vertical lines. (b) Spectrally resolved power-law exponent for the irradiance dependence of the emitted photon flux, with fit errors indicated for selected data points. The same irradiance range was used for all photon energies. The gray guideline crosses the origin. Two representative fits, marked 1 and 2, are shown in the inset.
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Figure 3
Nonlinearity analysis for a single luminescent hot spot on an evaporated gold film of nominal thickness $\sim 40 nm$ . (a) Emission spectra at $100 W / {cm}^{2}$ (solid) and $1 kW / {cm}^{2}$ (dashed) irradiance with 86 fs pulses at 770 nm (1.61 eV). Raman bands around $\pm 170 meV$ relative to the excitation are marked by gray vertical lines. (b) Spectrally resolved power-law exponent. The gray diagonal is a line through the origin.
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Figure 4
Extraction of effective temperature $T_{e}^{*}$ of the electron gas from the irradiance dependence of single hot spot emission in silver nanoparticle films. (a) Logarithmic plot of emission photon flux versus irradiance for an emission window of $2.25 \pm 0.04 eV$ . Circles (red) and squares (black) show two consecutive sweeps in laser power. (b) Logarithmic plot of the irradiance dependence of power-law exponents calculated from the slope of the data in panel (a). A linear fit (red) determines the thermal coefficient $a$ . (c) Fit results for coefficient $a$ as a function of the energy of emitted photons. The weighted mean of 4.57 is indicated in red. (d) Electron gas temperature $T_{e}^{*}$ calculated from the experimental data as a function of irradiance $E^{*}$ . The red curve corresponds to ${(T_{e}^{*})}^{a} \propto E^{*}$ using the average $a = 4.57$ .
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