A highly desirable goal in photonics is the integration of nanophotonics with atomic systems. Although much progress has been made in this area using cavity quantum electrodynamics and nanoscale dielectric waveguides, researchers at California Institute of Technology in the USA have now presented another exciting possibility — the combination of atomic physics with photonic-crystal waveguides (Appl. Phys. Lett. 104, 111103; 2014). Su-Peng Yu and Jonathan D. Hood together with co-workers used silicon nitride (Si3N4) nanowires to fabricate dispersion-engineered photonic-crystal waveguides that are capable of trapping single atoms and generating strong light–matter interactions.
The key component of their waveguide is an 'alligator' photonic-crystal waveguide region consisting of two parallel Si3N4 waveguides whose outer edges are sinusoidally modulated and whose straight inner edges are separated by a constant-width gap (see image). These waveguides are designed such that the lower (dielectric) and upper (air) band edges lie close to the D1 and D2 transitions of caesium atoms, respectively. This configuration allows caesium atoms to be trapped in the waveguide gap by using a dielectric-band blue-detuned from the D1 line as a trapping beam and the air-band mode as a probe on the D2 line of the trapped atoms. When this is done, caesium atoms become trapped in areas of the x–y plane where the intensity of the dielectric-band mode is zero.
The researchers report that these waveguides satisfy five criteria for hybrid atom–photonic systems suitable for use in experiments in quantum optics and atomic physics involving optically trapped ultracold atoms. Specifically, they can be fabricated with a high enough precision to permit photonic bandedges to be reliably produced near electronic transitions of atoms, can stably trap atoms while simultaneously realizing a strong atom–field interaction, enable efficient coupling to and from guided modes of nanophotonic elements, provide sufficient optical access for laser cooling and trapping, and have a low optical absorption and a high thermal conductivity thereby enabling 1 mK trap depths.
The team considers that this waveguide technology represents a significant advance towards performing experiments using ultracold atoms and nanophotonic chip-based optical circuits. They anticipate that further enhancement of the waveguide performance through reducing the optical absorption and scattering loss in the nanowire waveguides should permit atoms to be trapped using the fields of far-off-resonance guided modes.
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Pleasants, S. Trapping single atoms. Nature Photon 8, 427 (2014). https://doi.org/10.1038/nphoton.2014.131