Fabrication of suspended metal–dielectric–metal plasmonic nanostructures

Dipole nano-antennas have predominantly been investigated in their lateral orientation with their long axes in plane with a supporting substrate. However, the response of coupled dipole antennas oriented vertically to a supporting substrate has so far been out of experimental reach. Here, we present a self-aligned electron-beam lithography technique for fabricating such antennas consisting of metal nanostructures on both sides of a suspended silicon nitride membrane. This 30 nm thick membrane provides an ultra-smooth metal/dielectric interface and uniformly defines the antenna feed-gap size in an array of antennas. It is also a suitable substrate for probing the nano-antenna response with monochromated electron energy-loss spectroscopy (EELS) in a transmission electron microscope. We provide details of this double-sided patterning process, and show the excitation of hybridized plasmon modes in EELS with electrons directed along, and at an angle to, the antenna axis.

Zhaogang Dong, Michel Bosman, Di Zhu, Xiao Ming Goh and Joel K W Yang

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