Reveal plasmonic coupling and energy transport at nanoscale

Surface plasmons, referred as collective electron oscillations in metal surface, show unique electromagnetic field enhancement and light localization properties, greatly promoting the efficiency of light-matter interactions. With the breakthrough of optical diffraction limit, surface plasmons show many potential applications in the fields of nonlinear optics, nanophotonics and information processing, such as plasmonic nanoantennas, plasmonic waveguides and plasmonic photodetector.
    Due to the subwavelength localization property of surface plasmons, traditional optical microscopy cannot fulfill the requirement of plasmonic characterization at nanoscale. To provide better guidelines for the design of nanophotonic devices, precisely revealing plasmonic properties at nanoscale is needed. Photoemission electron microscopy (PEEM) has been demonstrated as a promising approach to investigate the plasmonic properties of surface plasmons. PEEM allows direct imaging of the near field of plasmonic nanostructures. By tuning the excitation wavelength, the near-field spectral properties can also be studied. Together with time-resolved pump-probe techniques, the ultrafast dynamics of surface plasmons can be further acquired. All these studies by PEEM pave the way for getting insights into the fundamental physics of surface plasmons and better designing of nanophotonic devices.

PEEM images of Au nanochains and plasmonic energy transport

    The research group of Prof. Hiroaki Misawa from the Research Institute for Electronic Science at Hokkaido University studies on the plasmonic properties by using PEEM. They developed an ultrafast PEEM system, which allows to characterize the near-field properties of surface plasmons in multiple (spatial, spectral, and temporal) domains.
    Recently, they experimentally investigated the plasmon coupling in finite one-dimensional (1-D) gold nanoparticle chains by PEEM using near infrared femtosecond laser pulses as the excitation source. They obtained the near-field mapping exhibiting the local field enhancement sites with ~ 10 nm spatial resolution. By further tuning the excitation wavelength, they could obtain the near-field spectra of the 1-D nanochains, revealing the energy splitting and the corresponding near-field mode distributions. The evolution of the near-field surface plasmon peak wavelengths with the chain length and the gap distances gave the same tendency as that observed in the far field, providing the direct proof of the near-field coupling in the nanochains. Moreover, the energy transport along the gold nanochains in the near field was observed under oblique wide-field illumination. Together with numerical simulations, it was found that the near-field coupling and sub-radiant plasmonic modes induced by the retardation effect are responsible for the directional energy transport. These findings can deepen the understanding of plasmon coupling and energy transport in metallic nanostructures and promote the potential applications in plasmonic waveguides and biosensing.

About the Group
Prof. Hiroaki Misawa and Dr. Quan Sun are corresponding authors of this paper. Prof. Misawa is a professor and former director at the Research Institute for Electronic Science at Hokkaido University, Japan. His recent research interest is plasmonic chemistry especially in the plasmon-induced artificial photosynthesis. He was the leader of programs such as Grant-in-Aid for Specially Promoted Research. He has authored more than 300 research papers. He obtained the Commendation for Science and Technology by the Minister of Education, Culture, Sports, Science and Technology, Prizes for Science and Technology in 2015 and CSJ (The Chemical Society of Japan) Award in 2016. Since 2015, he has held an additional post as a chair professor at National Chiao Tung University, Taiwan. The first author and co-corresponding author of this paper is Dr. Quan Sun, who is an assistant professor at Prof. Misawa’s group (Hokkaido University) and whose main research interests include near-field properties of plasmonic structures probed by PEEM and femtosecond laser micro/nanofabrication. Dr. Sun also holds a visiting professorship at Jilin University.

Sun Q, Yu H, Ueno K, Zu S, Matsuo Y et al. Revealing the plasmon coupling in gold nanochains directly from the near field. Opto-Electron Adv 2, 180030 (2019).