Near field excited state imaging via stimulated electron energy gain spectroscopy of localized surface plasmon resonances in plasmonic nanorod antennas

Abstract

Continuous wave (cw) photon stimulated electron energy loss and gain spectroscopy (sEELS and sEEGS) is used to image the near field of optically stimulated localized surface plasmon resonance (LSPR) modes in nanorod antennas. An optical delivery system equipped with a nanomanipulator and a fiber-coupled laser diode is used to simultaneously irradiate plasmonic nanostructures in a (scanning) transmission electron microscope. The nanorod length is varied such that the m = 1, 2, and 3 LSPR modes are resonant with the laser energy and the optically stimulated near field spectra and images of these modes are measured. Various nanorod orientations are also investigated to explore retardation effects. Optical and electron beam simulations are used to rationalize the observed patterns. As expected, the odd modes are optically bright and result in observed sEEG responses. The m = 2 dark mode does not produce a sEEG response, however, when tilted such that retardation effects are operative, the sEEG signal emerges. Thus, we demonstrate that cw sEEGS is an effective tool in imaging the near field of the full set of nanorod plasmon modes of either parity. The localized surface plasmon resonances (LSPR) sustained in noble metal nanostructures have inspired many new concepts in fields such as photovoltaics 1-3 , photocatalysis 4-6 , biosensing 7-9 , readout strategies for quantum computing 10,11 , and terahertz optical 12-14 and magnetic meta atoms/materials 15-18. While standard far field optical scattering techniques are used to probe the resonance conditions of individual nanostructures as well as nanostructure ensembles, probing the resultant near field is often more challenging. Several techniques such as scanning near field optical microscopy (SNOM) 19-23 , photoemission electron microscopy (PEEM) 24,25 , and electron energy loss spectroscopy (EELS) 26-29 have been used to probe the near field distribution of LSPRs. Of the near field techniques, EELS is unique in that the swift electron acts like a white (spectrally broad) evanescent field and thus can excite the full plasmonic spectrum of both bright and dark modes with atomic scale resolution. To this end, EELS has been utilized to characterize individual nanoparticle LSPRs as well as surface plasmon polaritons (SPP) and in particular the LSPR modes in nanorods 30-41. Beyond standard EELS, photoinduced near field electron microscopy (PINEM) is used to image the near field of optically excited nanostructures 42-47. In PINEM, a pulsed laser photo-ejects electron beamlets or single electrons from the cathode, which are accelerated and arrive at the specimen synchronously to a second laser pulse that interacts with the sample. Thus PINEM enables the study of photoinduced near field phenomena at the nanoscale and the intense sample laser pulse (~ 1 × 10 15 W/m 2) induces photon stimulated electron energy loss (sEEL) and gain (sEEG) peaks. In addition to experimental demonstrations, several theoretical papers have described the sEEG and sEEL processes 48-51. Additionally, by...

Visualizing Electric and Magnetic Field Coupling in Au-Nanorod Trimer Structures via Stimulated Electron Energy Gain and Cathodoluminescence Spectroscopy: Implications for Meta-Atom Imaging

“…To better elucidate the electric and magnetic optical field coupling stimulating the electric and magnetic dipole modes, we further explore the sEEGS tilt dependence of the two modes. As previously discussed, − the sEEG intensity is proportional to the product of the spontaneous EELS intensity, the laser irradiance, and the extinction cross section of the LSPR mode at the laser energy. As shown in ref , the spontaneous EELS intensity for rods aligned parallel to the tilt axis slightly decreases with increasing θ and remains constant for rods aligned perpendicular to the tilt axis.…”

“…Clearly, the lithographically patterned structures have slight differences in the rod lengths, widths, heights, and gaps, which could result in complicated in-plane and out-of-plane near-field distributions. Second, our previous model ignores substrate effects, which also perturb the optically induced and electron-induced LSP near-field distribution. , Thus, the observed behavior of the normalized electric mode could be due to either, or a combination, of these effects. To elucidate these contributions, various future studies are proposed: (1) comparing oligomers on substrates with different indices of refraction, (2) intentionally patterning various oligomer asymmetries, and (3) exploring different laser energies to probe different spectral regions.…”

“…Clearly, the lithographically patterned structures have slight differences in the rod lengths, widths, heights, and gaps, which could result in complicated in-plane and out-of-plane near-field distributions. Second, our previous model 33 ignores substrate effects, which also perturb the optically induced and electron-induced LSP near-field distribution. 44,45 Thus, the observed behavior of the normalized electric mode could be due to either, or a combination, of these effects.…”

“…As previously discussed, − the sEEG intensity is proportional to the product of the spontaneous EELS intensity, the laser irradiance, and the extinction cross section of the LSPR mode at the laser energy. As shown in ref , the spontaneous EELS intensity for rods aligned parallel to the tilt axis slightly decreases with increasing θ and remains constant for rods aligned perpendicular to the tilt axis. The spontaneous EELS intensity is also sensitive to the aloof position, which can vary slightly during experimentation.…”

“…It can also be used to photoexcite the sample and thus enables photoinduced near-field imaging/spectroscopy of materials in a conventional microscope. For instance, we have demonstrated continuous wave (cw) stimulated EEL (sEEL) and stimulated EE gain (sEEG) signatures in silver nanostructures and have characterized the longitudinal localized surface plasmon resonance (LSPR) modes ( m = 1, 2, and 3) of gold nanorods as well as coupled dimers . While ultrafast time-resolved photoinduced near-field electron microscopy has been developed − and has elucidated interesting plasmonic , and biological processes, the cw excitation of materials in a conventional microscope is useful for revealing many excited-state phenomena.…”