Researchers at MIT have filled in a crucial piece of the puzzle that could enable the creation of photonic chips on the standard silicon material that forms the basis for most of today’s electronics The new component is a “diode for light,” says Caroline Ross, the Toyota Professor of Materials Science and Engineering at MIT. It is analogous to an electronic diode, a device that allows an electric current to flow in one direction but blocks it from going the other way; in this case, it creates a one-way street for light, rather than electricity.
Fabrication process flow of a nonreciprocal racetrack resonator on SOI. The silicon racetrack resonator is initially fabricated by electron beam lithography on SOI. Atomic force microscopy was measured between step 1 and 2 to verify the exposure of silicon resonator surface. PLD: pulsed laser deposition, RTA: rapid thermal annealing.
Non-reciprocal photonic devices, including optical isolators and circulators, are indispensible components in optical communication systems. However, the integration of such devices on semiconductor platforms has been challenging because of material incompatibilities between semiconductors and magneto-optical materials that necessitate wafer bonding, and because of the large footprint of isolator designs. Here, we report the first monolithically integrated magneto-optical isolator on silicon. Using a non-reciprocal optical resonator on an silicon-on-insulator substrate, we demonstrate unidirectional optical transmission with an isolation ratio up to 19.5 dB near the 1,550 nm telecommunication wavelength in a homogeneous external magnetic field. Our device has a small footprint that is 290 µm in length, significantly smaller than a conventional integrated optical isolator on a single crystal garnet substrate. This monolithically integrated non-reciprocal optical resonator may serve as a fundamental building block in a variety of ultracompact silicon photonic devices including optical isolators and circulators, enabling future low-cost, large-scale integration.
This is essential, Ross explains, because without such a device stray reflections could destabilize the lasers used to produce the optical signals and reduce the efficiency of the transmission. Currently, a discrete device called an isolator is used to perform this function, but the new system would allow this function to be part of the same chip that carries out other signal-processing tasks.
To develop the device, the researchers had to find a material that is both transparent and magnetic — two characteristics that rarely occur together. They ended up using a form of a material called garnet, which is normally difficult to grow on the silicon wafers used for microchips. Garnet is desirable because it inherently transmits light differently in one direction than in another: It has a different index of refraction — the bending of light as it enters the material — depending on the direction of the beam.
The researchers were able to deposit a thin film of garnet to cover one half of a loop connected to a light-transmitting channel on the chip. The result was that light traveling through the chip in one direction passes freely, while a beam going the other way gets diverted into the loop.
The whole system could be made using standard microchip manufacturing machinery, Ross says. “It simplifies making an all-optical chip,” she says. The design of the circuit can be produced “just like an integrated-circuit person can design a whole microprocessor. Now, you can do an integrated optical circuit.”
That could make it much easier to commercialize than a system based on different materials, Ross says. “A silicon platform is what you want to use,” she says, because “there’s a huge infrastructure for silicon processing. Everyone knows how to process silicon. That means they can set about developing the chip without having to worry about new fabrication techniques.”
This technology could greatly boost the speed of data-transmission systems, for two reasons: First, light travels much faster than electrons. Second, while wires can only carry a single electronic data stream, optical computing enables multiple beams of light, carrying separate streams of data, to pass through a single optical fiber or circuit without interference. “This may be the next generation in terms of speed” for communications systems, Ross says.