Analysis of nanogap-induced spectral blue-shifts of plasmons on fiber-integrated gold, silver and copper nanowires

We analyze the properties of HE2 surface plasmon polaritons on fiber-integrated gold, silver and copper nanowires and find a systematic blue-shift of the measured resonance wavelengths which we attribute to the emergence of a narrow nanometer-sized gap between the nanowire surface and the surrounding silica cladding. Our analysis relies on the determination of the nanogap width from the experimentally measured phase-matching wavelength by comparison with numerical simulations, revealing that these gaps are much smaller than expected from the bulk material considerations. This implies a diminished coefficient of thermal expansion along the radial direction that we believe results from the domination of interfacial van der Waals forces at high temperatures. These results are important for future fiber designs involving nanowires fabricated by pressure-assisted melt filling. (C) 2017 Optical Society of America.