Open Access

Otto HaulerORCiD, Lukas A. Jakob, Kai BraunORCiD, Florian LaibleORCiD, Monika FleischerORCiD, Alfred MeixnerORCiD, Frank WackenhutORCiD

• The plasmon ruler has drawn substantial attention for measuring small separations between two plasmonic nanoparticles (NPs), which can be as sensitive as to observe the bending of DNA. Usually, the plasmon ruler is based on the spectral shift of the localized surface plasmon resonance (LSPR) caused by the distance-dependent dipole–dipole coupling between two plasmonic NPs. Here, we present an approach to the plasmon ruler by detecting the phase of the scattered light. We have developed a combination of a confocal microscope and a Michelson interferometer, which allows us to measure the phase change caused by a single nanoparticle in a diffraction-limited focal volume. The plasmon ruler is realized by a gold nanoparticle dimer (GND) deposited on a flexible polydimethylsiloxane substrate, where the same GND can be investigated with variable gaps by applying strain by stretching of the substrate. In this experimental configuration, the GND is located at a low reflecting interface leading to a large phase difference of more than 100° relative to the substrate, which allows us to easily detect a single GND. Interestingly, an increase of the gap by 11 nm leads to a large phase shift of 94°, even though there is only a 11 nm spectral shift of the LSPR. We find that both the spectral and phase shifts have the same r–3 dependence on the particle gap, which is expected for pure dipole–dipole coupling.