Limits of Stable Near-Field Probing in Nanophotonic Traps

Near-fields around nanophotonic structures and waveguides can be used to optically interface particles ranging from atoms and molecules to microscopic biological and synthetic particles. Due to the strong, non-linear dependence of the near-field coupling strength on the particles' position, a change of the spread of the particles' position will change their mean coupling strength. When the particles are trapped, this position spread depends on their temperature, generally leading to temperature-dependent coupling. Here, we experimentally demonstrate that this effect renders optical probing of trapped particles with near fields an inherently transient process. Specifically, we trap cold atoms in a two-color dipole trap surrounding an optical nanofiber and probe them with the evanescent field of guided, resonant light. The scattering of this probe light heats up the atoms, leading to a decrease of the coupling strength as well as loss of atoms. We observe both effects via a concurrent decrease of the absorption signal. In addition, we demonstrate that the coupling strength can be recovered by cooling the atoms back to their initial temperature. Our findings are relevant for numerous situations where stable coupling of trapped particles to a nanophotonic structure is required.