Spatiotemporal imaging of plasmonic fields near nanoparticles below the diffraction limit

Optically induced collective conduction-electron oscillations can generate intense plasmonic fields near metallic nanoparticles. We suggest a method for reconstructing such nanoplasmonic fields with nanometer spatial and sub-femtosecond temporal resolution from streaked photoemission spectra. Applying this imaging scheme to Au nanospheres, we demonstrate the accurate spatiotemporal reconstruction of the plasmonic near-field distribution in comparison with the directly calculated plasmonic field.

Figure 1

Spatiotemporal nanoplasmonic field imaging near isolated metallic nanospheres employing streaked photoemission spectroscopy. Electrons released by isolated XUV pulses are streaked by delayed nanoplasmonically enhanced visible or IR pulses upon leaving the particles' surface. $\tau$ designates the time delay between the phase-coherent streaking and XUV pulses. $\mathrm{\Omega }$ is the variable angle between the XUV and IR electric-field linear polarization directions. Red dots indicate points on the “$x=0$ longitude.”

Figure 2

Streaked spectra from 25-nm-radius Au nanospheres for different polarization directions $\mathrm{\Omega }$ and $\lambda =720$ nm (a)–(c) with and (d) without including ${\vec{E}}_{pl}$. (e) Streaked reference spectrum for $\mathrm{\Omega }=0$ and ${\lambda }_{\mathrm{ref}}=900$ nm including ${\vec{E}}_{pl}$. (f) Corresponding centers of energy.

Figure 3

Plasmonic (a) field enhancements and (b) phase shifts at the pole ($\theta =0$) of gold nanospheres with radii of 25 and 50 nm as a function of the incident streaking-field wavelength $\lambda$. The peak intensity of the streaking field is ${10}^{12}\text{W}/{\text{cm}}^2$. Exact results (solid and dashed lines) are calculated based on classical electrodynamics according to Eqs. (3) and (4). Symbols show values retrieved from simulated streaked photoelectron spectra. Values at the reference wavelength of 900 nm used for the plasmonic-field retrieval are circled.

Figure 4

Results for 25 nm – radius Au nanospheres for three XUV-IR pulse delays $\tau$. (a)–(c) Calculated exact and (d)–(f) retrieved total electric fields at the surface, normalized to the IR-laser-pulse amplitude $E_{IR,0}$. (g)–(i) Exact and retrieved total electric field versus the polar angle $\theta =\mathrm{\Omega }$ along the $x=0$ longitude ($\varphi =0$).

Figure 5

Exact and retrieved total electric near field, normalized to the IR-laser-pulse amplitude $E_{IR,0}$, versus the XUV-IR pulse delays $\tau$ for 25 nm – radius Au nanospheres for two different polar angles.