"We have proposed a method that should allow a single atom to be trapped near a metallic nanotip within a region just a few nanometres in size," team member Darrick Chang of Caltech told nanotechweb.org. "And this trapped atom can be brought to within tens of nanometres of other surfaces."
The technique could allow a trapped atom to be directly coupled to nanosystems, such as micro and nano-photonic devices, or charged or magnetized quantum systems. The atom could also be optically detected or manipulated with high efficiency using optical waves (surface plasmons) guided along the nanotip surface, says Chang.
The method is based on a common trapping technique for atoms that relies on the fact that light causes atoms to polarize or acquire a dipole moment. The energy associated with this atomic polarization is minimized when the atom moves to a region where the light field is weakest or strongest, depending on the conditions. Chang and colleagues use a laser beam to illuminate a sharp metallic nanotip that acts as a "lightning rod", which strongly enhances the laser field and causes large field variations near the end of the tip.
Vanishing field
"In fact, there is a small region near the tip where the variations cause the field to nearly vanish and where atoms can be trapped," explained Chang. "The lightning rod effect makes the trap tight and robust, which means that the trap persists even when the nanotip is brought close to other surfaces."
The technique could be used to trap individual atoms near devices like photonic crystal cavities or micro-toroidal resonators for realizing strong interactions between the atom and single photons confined in these devices. Until now, methods to trap atoms near such systems have remained elusive, says Chang.
Other applications include using the trapped atoms to detect weak magnetic fields near a surface with high spatial resolution. "More ambitiously, such a system might be exploited to explore novel many-body physics, where the small distances between atoms enable very large interactions between them."
The team is now setting up an experiment to trap an atom near a nanotip. Initially, the researchers hope to observe the predicted nanoscale confinement of the atom as well as efficient optical coupling between the atom and guided surface plasmons propagating along the nanotip – relatively unexplored territory for trapped atoms. "If this is successful, we will then develop some of the applications mentioned above," said Chang.
The work was reported in arXiv.