The new hybrid nanophotonic system, consisting of an atomic vapor with a sharp (dipole-allowed) resonance at frequency atom and well-known dispersion (center). The vapor is infiltrated in a 3-D opal that displays a photonic gap (left). As a result, researchers predicted a narrow resonance in or near the gap, shown as the narrow blue trough (right), where atomic properties appear to be turned upside down. [Image: MESA+ Institute for Nanotechnology, University of Twente]
Opals—naturally occurring photonic crystals—get their colorful “fire” from the interplay between light and the gemstones' internal structure. Researchers in the Netherlands have hybridized such crystals with cesium atoms to change their internal reflectivity and even to flip their expected reflection spectrum upside down (Phys. Rev. B, doi: 10.1103/PhysRevB.91.045123).
Inside opals, the periodic array of silica molecules creates a photonic band gap at a certain frequency. Philip Harding and colleagues at the MESA+ Institute of Nanotechnology at the University of Twente changed the gap relative to the resonances of cesium atoms inside the photonic crystal. Such manipulations could play an important role in optical signal controls in future photonic circuits.
First, the team grew an opal photonic crystal by depositing layers of silicon dioxide directly onto the viewport window of a vacuum chamber. During the experiment, the chamber was filled with a gas of heated cesium-133 atoms that infiltrated the photonic crystal. The cesium isotope has a strong resonance, and its vapor changes the refractive index of the voids in the photonic crystal—changes that evolve with gradual heating of the cell, from 370 K to 420 K.
Periodic measurements of the reflectivity spectra during the heating revealed that the characteristic resonance peaks shifted and the photonic band gap changed due to the interactions between the cesium atoms and silica units. By the time the highest temperature was reached, the peaks that had existed at 370 K had “flipped” and become troughs.
Tiny variations during fabrication make it difficult to fine-tune all the resonators in a photonic crystal, according to the authors. Tuning the photonic crystal with a gas could help make future photonic sensors or switches possible.