Spatiotemporal control of nanooptical excitations

Abstract

The most general investigation and exploitation of light-induced processes require simultaneous control over spatial and temporal properties of the electromagnetic field on a femtosecond time and nanometer length scale. Based on the combination of polarization pulse shaping and time-resolved two-photon photoemission electron microscopy, we demonstrate such control over nanoscale spatial and ultrafast temporal degrees of freedom of an electromagnetic excitation in the vicinity of a nanostructure. The time-resolved cross-correlation measurement of the local photoemission yield reveals the switching of the nanolocalized optical near-field distribution with a lateral resolution well below the diffraction limit and a temporal resolution on the femtosecond time scale. In addition, successful adaptive spatiotemporal control demonstrates the flexibility of the method. This flexible simultaneous control of temporal and spatial properties of nanophotonic excitations opens new possibilities to tailor and optimize the light-matter interaction in spectroscopic methods as well as in nanophotonic applications.

Fig. 1.

(A) Experimental scheme. Polarization-shaped pump and circularly polarized probe laser pulses illuminate the surface (incidence angle of 65°, slightly different azimuth angles with difference angle δ) with adjustable time delay τ. The 3D pulse representations show amplitude envelopes of the electric field vector as a function of time, and the color indicates momentary frequency. The inset shows those two-photon excitation pathways for pump (red arrow) and probe interactions (blue arrow) that contribute to a τ-dependent signal and that promote electrons from below the Fermi level E_F via intermediate states to above the vacuum energy E_vac. (B) Scanning electron microscopy provides an image of an individual sun-shaped planar silver nanostructure. (C) One-photon-induced PEEM with UV light from a mercury vapor lamp shows emission from an individual nanostructure (gray outlines). (D–E) Normalized two-photon PEEM patterns are shown for excitation with the pump pulse (fluence 30 nJ cm^-2) only (D) and the probe pulse (fluence 3 nJ cm^-2) only (E), for pulse shapes as they are schematically represented in A.

Fig. 2.

Ultrafast switching of nanoscale excitation. (A) Time-dependent field amplitude for the TE (dashed black line) and TM (solid purple line) components of the polarization-shaped pump pulse that is also shown in 3D representation in Fig. 1A. The vertical gray bars indicate the delay for which cross-correlation patterns are shown in parts C and D. (B) Time-dependent cross-correlation signals are obtained after subtraction of the delay-time-independent background for the integrated photoemission yield from the whole nanostructure (black) and two different regions of interest (ROI 1 green dashed and ROI 2 solid red border) as defined in parts C and D. To facilitate the comparison the yield is displayed as average number of counts per CCD pixel. The curve representing the total emission is scaled by a factor of seven. (C–D) Normalized cross-correlation emission patterns after background subtraction are shown for the two different delays τ = 13 fs (C) and τ = 213 fs (D). The regions of interest for the time-dependent cross-correlation signals of B are indicated as solid red and dashed green rectangles in relation to the nanostructure (gray outlines).

Fig. 3.

Adaptive ultrafast spatiotemporal control. (A) The time-dependent field amplitude is shown for the TE (dashed black line) and TM (solid purple line) components of the adaptively optimized polarization-shaped pump pulse. Optimization time windows are indicated as gray-shaded bars. (B) Time-dependent cross-correlation signals after subtraction of the delay-independent background are shown for the total integrated photoemission yield (black, scaled by a factor of 55) and the two regions of interest (red and green). (C–D) Normalized cross-correlation emission patterns are shown after background subtraction for delays of τ = -507 fs (C) and τ = 493 fs (D), and the regions of interest are marked by colored rectangles.