A traffic light for light-on-a-chip

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

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

Combining integrated photonics and micro-electromechanical systems, a collaboration between EPFL and Purdue University has developed an electrically driven optical isolator-on-a-chip that transmits light in only one direction. This technology opens new path for chip-based applications such as LiDAR, quantum interconnects and spectroscopy.

Integrated photonics allow us to build compact, portable, low-power chip-scale optical systems used in commercial products, revolutionizing today’s optical datacenters and communications. But integrating on-chip optical gain elements to build lasers or to amplify optical power runs the risk of reflected light from other components compromising or interfering with the laser’s performance.

The solution is to increase on-chip optical isolation. Typically, optical isolation is achieved with magnetic materials and magnetic fields, but these are not compatible with current semiconductor foundry processes; meanwhile, creating strong external magnetic fields on the chip’s micrometer scale is challenging in itself. Consequently, electrically driven, magnet-free optical isolators are highly desired in the field.

In an article published in Nature Photonics, a collaboration between the labs of Professor Tobias J. Kippenberg at EPFL and Professor Sunil A. Bhave at Purdue University showcases such a magnetic-free, electrically driven optical isolator that enables light routing on a chip.

Combining integrated photonics and micro-electromechanical systems (MEMS)technology, the device is made using piezoelectric aluminium nitride (AlN) monolithically integrated on ultralow-loss silicon nitride (Si3N4) photonic integrated circuits.

By synchronously driving multiple piezoelectric MEMS actuators, bulk acoustic waves are generated electromechanically, which can couple to and deflect light propagating in the Si3N4 waveguide beneath them. This acousto-optic modulation, known as “spatio-temporal modulation”, mimics the effects of magnet-driven isolators. By replacing magnetic materials with piezoelectric thin-film transducers, the requirement of magnetic field is entirely avoided.

While magnetic-free optical isolators have been shown before, this is the first one that is driven electrically and operated in the linear optical regime. The study reports linear optical isolation of 10 dB, and experimental measurement of one-way, no-loss digital data transmission on an optical signal carrier.

“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,” says Dr Junqiu Liu who leads the fabrication of Si3N4 chips at EPFL’s Center of MicroNanoTechnology (CMi).

The new optical isolators can seed new applications including chip-scale atomic clocks, light detection and ranging (LiDAR), photonic quantum computing, and on-chip spectroscopy, among others. One particular application that the two teams are working on is building quantum coherent microwave-to-optic converters that could conquer efforts in quantum interconnects between distant superconducting qubits, which require conversion of single quanta of the microwave field to the optical domain and vice versa.


US National Science Foundation (NSF)

Swiss National Science Foundation (SNF)

Defense Advanced Research Projects Agency (DARPA)

Air Force Office of Scientific Research (AFOSR)

European Union Horizon 2020


H. Tian, J.Liu, A. Siddharth, R. N. Wang, T. Blésin, J. He, T. J. Kippenberg, S. A. Bhave. Magnetic-free silicon nitride integrated optical isolator. Nature Photonics 21 October 2021. DOI: 10.1038/s41566-021-00882-z