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.
Highlights
Continuous wave photon stimulated electron energy loss and gain spectroscopy is used to image the near field of optically stimulated localized surface plasmon resonance (LSPR) modes in nanorod antennas
The laser-off spectrum has a dipole resonance at 1.62 eV and a peak at 2.25 eV, which is attributed to the higher order LSPR modes
The laser-on EEL point spectrum is similar to the laser-off spectrum except a small stimulated electron energy loss (sEEL) peak and sEEG peak emerges at ± 1.58 eV, respectively
Summary
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. 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)[19,20,21,22,23], photoemission electron microscopy (PEEM)[24,25], and electron energy loss spectroscopy (EELS)[26,27,28,29] 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 can excite the full plasmonic spectrum of both bright and dark modes with atomic scale resolution. We have studied the recrystallization, grain growth, phase separation, and dewetting of an A g0.5Ni0.5 film[58], and resonant cw sEEG and sEEL in nanostructures resulting from a dewet silver film[59]
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