On the possibility of detecting local refractive index changes in optically transparent objects by means of a point nanoantenna attached to a fibre microaxicon

It is shown theoretically that the use of the spectral registration of the dipole local plasmon resonance (DLPR) displacement in a single spherical gold nanoantenna, placed near the surface of a homogeneous dielectric medium, allows the mapping of extremely small variations (to 5 × 10^-4) of the refractive index (RI) of this medium. Using the quasi-static approximation, we have developed an analytic model that allows evaluation of the spectral displacement of the nanoantenna DLPR depending on the variation in the medium refractive index. The point probe based on a fibre microaxicon with a gold spherical nanoantenna attached to its top is proposed that allows practical implementation of the developed RI scanning method. Numerical calculations of the probe characteristics using the time-domain finite-difference method are presented, and it is shown that for the case of a gold spherical nanoantenna of small size, comparable with the skin layer thickness in gold, the relative spectral shift value is in good agreement with the results obtained by using the developed analytic model.