In a major breakthrough for optoelectronics, researchers at Columbia University have made the smallest yet integrated photonic circuit. In the process, they have managed to attain a high level of performance over a broad wavelength range, something not previously achieved.
The researchers believe their discovery is equivalent to replacing vacuum tubes in computers with semiconductor transistors—something with the potential to completely transform optical communications and optical signal processing.
The research community has been feverishly trying to build integrated photonic circuits that can be shrunk to the size of integrated circuits (ICs) used in computer chips. But there’s a big problem: When you use wavelengths of light instead of electrons to transmit information, you simply can’t compress the wavelengths enough to work in these smaller chip-scale dimensions.
In research described in the journal Nature Nanotechnology, the integrated photonic circuit fabricated by the Columbia researchers serves as a waveguide mode converter that can change one waveguide mode into another. Such waveguide mode converters are key enablers of a technology called “mode-division multiplexing,” which uses the same color of light but several different waveguide modes to transport independent channels of information simultaneously, all through the same waveguide. It is a strategy to increase the capacity of on-chip optical communications channels.
“The effect is like, for example, the George Washington Bridge suddenly has the capability to handle a few times more traffic volume, or a single football field can magically accommodate multiple teams to play simultaneously without interference,” explains Nanfang Yu, an assistant professor at Columbia and co-author of the research, told IEEE Spectrum.
The mode converters the Columbia researchers demonstrated have lengths as short as 1.7 times of the wavelength of light in open air. These mode converters can also convert an input waveguide mode to a pure output waveguide mode (rather than a mixture of output modes) over a wide range of wavelengths.
Such properties are not achievable in previously demonstrated devices, which typically have lengths ranging from a few times to a few hundred times of the wavelength of light in air and only tolerate wavelength variation of a few percent, according to the researchers.
The key to these achievements involved integrating metasurface structures onto the optical waveguides to shrink the device footprint and broaden the operating bandwidth. Metasurfaces are structured planes that are so thin that they are two-dimensional. Because of their structures, they manipulate light in unusual ways—the most notable being that they shorten the wavelengths of light.
The Columbia researchers fabricated metasurface structures consisting of arrays of nanoantennas with subwavelength spacings. These nanoantennas essentially pull light from inside the waveguide core, modify the light’s properties and release light back into the waveguides. If the nanoantennas are packed densely enough, they can convert the waveguide mode within a propagation distance that is no more than twice the wavelength.
The nanoantennas can be made of either metallic materials supporting plasmonic resonances or dielectric materials supporting the so-called Mie resonances. These resonances are oscillating, self-repeating waves that exist in close proximity to the antennas.
“Strong optical scattering at sub-wavelength intervals by these antennas provides the most efficient control of guided waves among all device configurations, which is the key to achieve small device footprint,” says Yu. “Furthermore, the nano-antennas form a ‘phased array’ that enables a one-way transfer of optical power from the incident waveguide mode to the output waveguide mode, which allows us to realize complete waveguide mode conversion over a distance no more than twice the free-space wavelength.”
This effectively reduces the size of the device for realizing waveguide mode conversion by a factor of 10 to 100 compared to current designs, according to Yu.
In addition to using the waveguide mode converters for mode-division multiplexing and increasing the capacity of on-chip optical communications channels, the technology could also be used to convert waveguide modes to surface waves.
To do this, the researchers used the nanoantenna arrays to draw light from deep inside the waveguide core to the surface of the waveguide. The resulting strong interactions between light and the nanoantennas could enable the fabrication of a few useful photonic integrated devices.
For instance, the large absorption of light by the antennas could enable broadband integrated perfect absorbers, which convert light into heat and prevent reflection of light back into photonic integrated circuits.
In addition, the ability to bring light from inside to outside waveguides also allows light to interact with any biological or chemical analyte placed in the vicinity of the waveguides.
Another possibility is that if the waveguide is made of a proper semiconductor and the nanoantennas are made of a proper metal, strong light at the surface of the waveguide could transfer electrons from the semiconductor waveguide to the metallic antennas, a mechanism called internal photoemission of hot electrons, which can enable us to create tiny integrated photodetectors.
For now, though, Yu and his team will continue to pursue mode-division multiplexing.
Yu adds: “We plan to realize system-level applications based on the devices, including mode-division multiplexing and de-multiplexing. We also plan to incorporate actively tunable optical materials into the photonic integrated devices to enable active control of light propagating in waveguides.”