Nanophotonic devices are typically made from either one of two different types of components: plasmonic metal nanostructures or conventional photonic components made from semiconductors. Plasmonic structures have the advantage of being able to concentrate light into very small volumes — much smaller than the wavelength used — but suffer from high transmission losses. Conventional photonic nanostructures, on the other hand, function with lower losses, but do not provide the same level of photon confinement.

Researchers from Zhejiang University in China1 have now investigated hybrid structures made from a combination of photonic and plasmonic components. Their aim was to fabricate devices with low-loss light propagation making use of the highly confined optical fields in plasmonic circuits. “The hybrid scheme combines advantages of both photonics and plasmonics, which could lead to applications in signal processing, including optical sensors and modulators,” comments Limin Tong from the research team.

Fig. 1: Hybrid photonic waveguides. Light from a silica nanofiber is coupled into a photonic ZnO semiconductor nanowire with a plasmonic silver nanowire attached at the end.

Their ‘photonic nanowires’ were prepared from zinc oxide (ZnO) semiconductors and plasmonic silver nanowires attached end-on-end. Laser light coupled into the ZnO nanowire efficiently excites surface plasmons in the silver nanowire and vice versa (Fig. 1), and the light propagates along the ZnO–Ag nanowire. This shows that a plasmon–photon conversion occurred with good efficiency. This coupling between the photonic and plasmonic modes is all the more remarkable in view of the size mismatch: the ZnO nanowire is about twice the diameter of the silver nanowire.

Based on this design, the researchers fabricated a variety of complex structures, including hybrid beam splitters that divide light into adjacent plasmonic and photonic circuits. When light from the two circuits recombines, typical interference patterns between the beams are produce from both arms, indicating that the laser light maintained its coherence. The group also constructed closed-loop microcavities, which showed good confinement of light. According to Xin Guo from the research team, this further underlines the potential of hybrid circuits. “It demonstrates that these structures offer ultra-tight confinement of light and thus ultra compactness without high loss.”

In future research, these circuits may benefit from the longstanding know-how in semiconductor devices. For example, gain media could be integrated into the photonic nanowires to compensate for any losses in the circuits. In that way, the respective advantages of plasmonic and photonic components can be used for the benefit of these hybrid circuits.