Near-field single photon detection in a scattering SNOM

A conical tip made out of good conductive metal can be used to efficiently localize the optical field at the apex of the tip. For a tip of finite length both a field singularity (lightning rod effect) and a surface plasmon resonance contribute to the E-field enhancement. A strongly absorbing superconducting nanodetector placed in the optical near-field of the tip shows enhanced optical absorption. The design of an optimal tip-detector system is non-trivial because the strong damping by the detector shifts the resonance wavelength of the tip and significantly lowers the quality factor of the resonance. We compare calculations of the field enhancement of a bare tip to the absorption enhancement in the detector in the presence of the tip as a function of tip length, apex radius and semi-angle of the cone. The resonance of a 225 nm long gold tip in the presence of a detector occurs at similar to 1000 nm and is red-shifted by 150 nm compared to the resonance of a bare tip.