Researchers at the Columbia University School of Engineering and Applied Science in New York City (US) have built photonic integrated devices with record-small footprints that can maintain optimal performance over an unprecedented broad wavelength range.
The engineers are using nano-antennas to control light propagating in optical waveguides with high efficiency. The innovation could revolutionize optical communications and optical signal processing.
“The technique is to control the momentum of light propagating along optical waveguides,” says Nanfang Yu, assistant professor of applied physics at Columbia. The expert explains that light propagates in waveguides in the form of waveguide modes: “An optical waveguide can support a fundamental waveguide mode and a set of higher-order modes similar to a guitar string that can support one fundamental tone and its harmonics. Each waveguide mode has its characteristic momentum. The control of the momentum of waveguide modes enables us to realize efficient conversion between any pair of waveguide modes, which is the basis of many device functionalities in photonic integrated circuits (ICs).”
Reportedly, Yu and his colleagues have built integrated nanophotonic devices with the smallest footprint and largest operating bandwidth ever. The professor explains the significance of this breakthrough: “Similar to the Moore’s law for electronic integrated circuits, there is a strong driving force to miniaturize photonic integrated devices, so as to increase the signal processing power of photonic integrated circuits.”
Smaller device footprints allow engineers to pack a higher density of devices on one unit area of the photonic IC chips. A larger operating bandwidth implies that the devices would be more robust against variations of the laser wavelength and varying operation conditions of the chip (e.g., temperature, humidity).
Significance of the optical nano-antennas
The expert says optical nano-antennas enable them to control light propagating in waveguides in the most efficient way. “The nano-antennas are patterned on the surface of the waveguides — these miniature antennas pull light from inside the waveguide core, modify the light’s properties and release light back into the waveguides,” he says, adding that without such antennas, the optical power of the guided light waves would be largely confined within the core of the waveguide, and could only be accessed via the small evanescent “tails” that exist near the waveguide surface.
“In our devices, the accumulative effect of a densely packed array of nano-antennas is so strong that they could achieve functions such as waveguide mode conversion within a propagation distance no more than twice the wavelength,” Yu says. “This represents a reduction of the device footprint by a factor of 10 to 100, compared with previously demonstrated devices.”
What is more, the compact footprint of these novel photonic integrated devices makes them broadband because there is an inverse relation between device dimension and their working bandwidth.
Photonic integrated devices for different applications
The team at Columbia has demonstrated different photonic integrated devices to address different applications: “The waveguide mode converters that we have demonstrated can be used in the so-called ‘mode-division multiplexing,’ which is a technique that uses the same color of light but several different waveguide modes to transport several independent channels of information simultaneously, all through the same waveguide,” Yu explains. “It is a strategy to increase the capacity of on-chip optical communications channels.”
Furthermore, the team has demonstrated devices that can efficiently convert guided lightwaves to strong surface waves that exist on the surface of the waveguides. “The resulting strong interactions between light and nano-antennas could enable us to build a few useful photonic integrated devices,” Yu points out. Namely, large absorption of light by the antennas could enable them to build broadband integrated perfect absorbers, which convert light into heat and prevent reflection of light back into photonic integrated circuits to interfere with the performance of other devices. Bringing light from inside to outside waveguides also allows light to interact with any biological or chemical analyte placed in the vicinity of the waveguides, which, as Yu explains, is the basis to create on-chip biochemical sensors.
Impact on advancement of photonic integration
Asked how his research work could impact the advancement of photonic integration, the professor replies: “It could potentially increase the integration density — the number of devices per unit chip area — of photonic ICs.”
Continuing with this research endeavor, Yu and his team aim to realize system-level applications based on the devices, including mode-division multiplexing and demultiplexing. “We also plan to incorporate actively tunable optical materials into the photonic integrated devices for active control of light propagating in waveguides, which could help us realize cool optical systems, such as optical radar (LIDAR) and augmented reality glasses,” the researcher concludes.
The work is detailed in the paper “Controlling propagation and coupling of waveguide modes using phase-gradient metasurfaces,” published in Nature Nanotechnology.
Written by Sandra Henderson, Research Editor, Novus Light Technologies Today