Spatiotemporal imaging of few‐cycle nanoplasmonic fields using photoemission electron microscopy

Abstract

Surface plasmons are capable of concentrating light on both a nanometre spatial and femtosecond temporal scale, thus serving as a basis for nanotechnology at optical frequencies. However, the simultaneously small and fast nature of surface plasmons leads to new challenges for spatiotemporal characterization of the electric fields. An especially successful method for this purpose is photoemission electron microscopy (PEEM) in combination with ultrashort laser pulses. This method uses the high spatial resolution offered by electron microscopy together with the temporal resolution offered by femtosecond laser technology. By combining PEEM with state‐of‐the‐art sources of ultrashort bursts of light, we have contributed to two pathways towards the ultimate goal: the full spatiotemporal reconstruction of the surface electric field at arbitrary nanostructures. The first approach is based on extending interferometric time‐resolved PEEM (ITR‐PEEM) [1] to the few light cycle regime by using two synchronized pulses from an ultra‐broadband oscillator. Because the photon energy (1.2‐2.0 eV) is well below the material work function, photoemission occurs through a multiphoton process. The measurement is performed by scanning the delay between two identical, sub‐6 fs pulses and measuring the local photoemission intensity (Fig. 1a). We have applied this method to a variety of nanostructures, including rice‐shaped silver particles, nanocubes, and gold bow‐tie nanoantennas. As an example, results from the rice‐shaped silver nanoparticles are shown in Fig. 1. We excited multipolar surface plasmons at grazing incidence, and imaged the photoelectrons emitted from the two ends of the nanoparticle (Fig. 1b). Upon scanning the delay between the two pulses, the interference fringes measured from the two ends of the nanoparticle are shifted with respect to each other (Fig. 1c). We show that these shifts correspond to locally different instantaneous frequencies of the near‐field within the same nanoparticle, and that these differences occur due to a combination of retardation effects and the excitation of multiple surface plasmon modes [2]. The second approach is based on using high‐order harmonic generation (HHG) to produce attosecond pulses in the extreme ultraviolet (XUV) region. Attosecond XUV pulses have been proposed to enable a direct spatiotemporal measurement of nanoplasmonic fields with a temporal resolution down to 100 as [3]. However, PEEM imaging using HHG light sources has turned out to be a major challenge due to numerous issues such as space charge effects, chromatic aberration, and poor image contrast [4‐6]. To address these issues, we perform HHG using a new optical parametric chirped pulse amplification system delivering 7 fs pulses at 200 kHz repetition rate. We show how the XUV pulses generated by this system allow for PEEM imaging with both higher resolution and shorter acquisition times. For comparison, Fig. 2 shows PEEM images of silver nanowires on a gold substrate, imaged using high‐order harmonics at 1 kHz repetition rate (Fig. 2a, acquisition time is 400 s) and at 200 kHz repetition rate (Fig. 2b, acquisition time is 30 s). The image quality is clearly improved (Fig. 2c). We also show how the higher repetition rate allows for PEEM imaging using only primary (“true”) photoelectrons, whereas previous studies have acquired images using secondary electrons [4‐6].