Power/Performance Bits: Feb. 7

Stopping interference in integrated photonics
Researchers at EPFL and Purdue University combined integrated photonics and MEMS to develop an electrically driven optical isolator-on-a-chip that transmits light in only one direction.

Optical isolators are useful to prevent reflected light from other components compromising or interfering with an on-chip laser’s performance. They are often created using magnetic materials and magnetic fields, but this leads to issues with current foundry processes, not to mention getting them small enough in the first place.

Instead, the team built a magnet-free, electrically driven optical isolator that enables light routing on a chip using piezoelectric aluminum nitride (AlN) monolithically integrated on ultralow-loss silicon nitride (Si3N4) photonic integrated circuits.

Synchronously driving multiple piezoelectric MEMS actuators generated bulk acoustic waves electromechanically, which can couple to and deflect light propagating in the Si3N4 waveguide beneath them, the researchers said. This acousto-optic modulation mimics the effects of magnet-driven isolators.

A fabricated piezoMEMS-silicon nitride chips containing multiple optical isolators. (Credit: EPFL)

“Combining integrated photonics and MEMS engineering, we show a hybrid semiconductor fabrication technology that is fully CMOS-compatible and accessible via large-volume foundry processes,” said Dr. Junqiu Liu who leads the fabrication of Si3N4 chips at EPFL’s Center of MicroNanoTechnology (CMi).

The team achieved linear optical isolation of 10 dB, and experimental measurement of one-way, no-loss digital data transmission on an optical signal carrier.

Applications include chip-scale atomic clocks, lidar, photonic quantum computing, and on-chip spectroscopy, among others. One application the teams are working on is building quantum coherent microwave-to-optic converters.

Isolator for atomic sensors
Researchers from the University of Illinois Urbana-Champaign took another approach to prevent interference in photonic circuits, with a focus on controlling atoms in quantum sensing devices.

“Atoms are the perfect references anywhere in nature and provide a basis for many quantum applications,” said Gaurav Bahl, a professor in Mechanical Science and Engineering (MechSe) at the University of Illinois at Urbana-Champaign. “The lasers that we use to control atoms need isolators that block undesirable reflections. But so far the isolators that work well in large-scale experiments have proved tough to miniaturize.”

Magneto-optic isolators that let light exit the laser but prevent it from traveling backward and interfering with the laser have problems with miniaturization and can negatively affect nearby atoms.

“An isolator is a device that allows light to pass uninterrupted one way and blocks it completely in the opposite direction,” said Benjamin Sohn, a former graduate student and postdoctoral researcher at University of Illinois Urbana-Champaign who is now at NIST, Boulder. “This unidirectionality cannot be achieved using just any common dielectric materials or glasses, and so we need to be a little more innovative. We also want the isolator to operate at wavelengths of light tuned to atomic sensors, which can be hard even at large scales.”

The team’s design uses lithium niobate, a common optical material, and is adaptable for different wavelengths of light. In this case, the chips were optimized for use with 780 nanometer light, which is the wavelength needed to configure common rubidium-based sensors, with another for 1550 nm light.

“We wanted to design a device that naturally avoids loss, and the best way to do that is to have light propagate through nothing. The simplest bit of ‘nothing’ that can still guide photons along a controlled path is a waveguide, which is a very basic component in photonic circuits,” said Bahl.

The complete photonic isolator contains a waveguide and an adjacent ring resonator, shaped like an oblong racetrack. Normally, incoming light would pass from the waveguide into the resonator, irrespective of its direction, thus blocking all light flow. But when the team applied sound waves to the ring, the resonator only captured light that was moving backward through the waveguide. In the forward direction, light passed through the waveguide unimpeded, as if the resonator was not there.

In tests, photons only had a one-in-ten-thousand chance of making it through backward, reducing undesirable light absorption to nearly zero.

“The simplicity in fabrication is key—with our approach, you could print photonic isolators that work well for whatever wavelength you need, all on the same chip at the same time. This is just not possible with other approaches today,” said Ogulcan Orsel, a graduate student in electrical engineering at University of Illinois Urbana-Champaign.

One-way graphene
Researchers at Purdue University developed a “topological circulator” that takes advantage of a unique phase of graphene.

Graphene can support unidirectional electromagnetic waves on the edge. These “edge waves” are linked to a new topological phase of matter, called the optical-N invariant, and symbolize a phase transition in the material, not unlike the transition from solid to liquid, the researchers said.

In this new phase, light travels in one direction along the edge of the graphene and is robust to disorder, imperfections, and deformation. This nonreciprocal effect was harnessed to develop “topological circulators,” or one-way routers of signals that could be used in on-chip, all-optical processing.

The researchers added: “Circulators are a fundamental building block in integrated optical circuits but have resisted miniaturization because of their bulky components and the narrow bandwidth of current technologies. Topological circulators overcome this by being both ultra-subwavelength and broadband, enabled by a unique electromagnetic phase of matter. Applications include information routing and inter-connects between quantum and classical computing systems.”