Recent experiments demonstrate a new technique for confining light in nanoscale waveguides.
(Left to right) Lawrence Berkeley National Laboratory's Xiang Zhang, Ziliang Ye and Volker Sorger have demonstrated the first true nanoscale waveguides for next generation on-chip optical communication systems.
Photonic components of future communications systems will require tight confinement of optical modes over significant propagation distances. The recent experiments of a team at Lawrence Berkeley National Laboratory (U.S.A.) demonstrate a new technique for confining light in nanoscale waveguides (Nature Comm. 2, 331, doi:10:1038/ncomms1315).
The Berkeley team conceptualized a new kind of quasiparticle, the hybrid plasmon polariton, capable of confinement to volumes much smaller than the optical diffraction limit without the loss of strength that occurs when surface plasmon polaritons cross a metal-dielectric interface.
To demonstrate the concept, researchers Volker Sorger, Ziliang Ye and their colleagues built a tiny waveguide device: a layer of the semiconductor zinc sulfide evaporated onto a silver film, separated by a 10-nm-thick layer of low-dielectric magnesium fluoride. The light originated from the back side of the device’s quartz substrate. In separate experiments, the team used laser light with wavelengths of 633, 808 and 1,427 nm.
Unlike in surface plasmon polariton experiments, the semiconductor waveguide and the metal carrier are not quite touching. The introduction of the low-dielectric layer into the tiny gap between the conductor and insulator strongly confines the plasmonic energy within the gap, Sorger said. It acts almost like an optical capacitor.
Imaging with near-field scanning optical microscopy revealed that the modes were confined to regions of 50 to 60 square nm. According to Sorger, the optical field of the hybrid modes is comparable to the gate length of a transistor on a modern integrated circuit.
Sorger, who just completed his Ph.D. in mechanical engineering at the University of California at Berkeley under OSA Fellow Xiang Zhang, is now studying the same kind of platform with different materials: silicon instead of zinc sulfide, silica instead of magnesium fluoride and aluminum instead of silver. These materials are compatible with current semiconductor/CMOS and silicon-on-insulator fabrication techniques.
Patricia Daukantas is a freelance science writers who specialize in optics and photonics.