Optical waveguides can be used to tailor the characteristics of electromagnetic waves. Nanostructuring these devices can push their refractive indices to extreme limits, at which exotic physical phenomena emerge. Waveguides with a zero-index, for example, have an infinite wavelength and light can propagate through them freely, without accumulating phase. This phase is what normally creates the “ripples” in a light wave and slows it down.

Such zero-index waveguides might be used to make low-index media for applications in areas such as light supercoupling, quantum optics and beam-steering. Until now, however, it has been difficult to make platforms that are compatible with conventional silicon photonic fabrication processes.

Phase-free light propagation in the telecoms regime

A team of researchers led by Eric Mazur has now succeeded in making a zero-index platform in a corrugated silicon waveguide that boasts phase-free light propagation in the telecoms regime (at a wavelength of 1625 nm). Thanks to a technique called on-chip interferometry, they were able to measure this index by directly imaging stationary light waves using a standard objective lens. More strikingly still, they observed coherent oscillations of light waves that spanned the entire length of the waveguide at the zero-index wavelength.

Mazur and colleagues fabricated their silicon waveguide (which is based on the standard 220 nm thick silicon-on-insulator slab) using conventional silicon photonics fabrication processes. They use e-beam lithography to define the structure and then reactively etch it to form the waveguide (that is, remove the parts of the silicon where the waveguide isn’t). The final structure consists of a single row of a zero-index metamaterial with a lattice constant of 760 nm and a cylindrical hole with a radius of 212 nm.

Pulses of light don’t ‘stop’ when they enter the waveguide

“In contrast to previous such waveguides, the ‘phase-advance-free-modes’ in our structure also possess finite group indices, which means that pulses of light don’t ‘stop’ when they encounter it,” explains team member and lead author of the study Orad Reshef. “This means that it can be used to transfer energy.”

"Aside from making a zero-index silicon waveguide, our work also features, to the best of our knowledge, the first direct observation of an infinite optical standing wave,” he tells nanotechweb.org. “We are extremely excited by this, but it being a purely physics demonstration, it will not have the same broad applicability as a new type of engineered structure, which the zero-index silicon waveguide is.”

It is not all plain sailing though. The optical losses in the new waveguide are still relatively high when compared to other standard silicon waveguides, admits Reshef. “These losses are a direct consequence of a zero-index mode, which easily couples to unbound plane light waves that escape the structure. Fortunately, since these losses do not come from light absorption we have a few tricks up our sleeve that we can use to reduce and eventually completely eliminate these losses,” he reveals.

The research is detailed in ACS Photonics DOI: 10.1021/acsphotonics.7b00760.