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.

Now, a collaboration between the labs of Tobias J. Kippenberg at EPFL and 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 Si₃N₄waveguide beneath them. This acousto-optic modulation – 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 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, 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. (Source: EPFL)

Reference: H. Tian et al.: Magnetic-free silicon nitride integrated optical isolator, Nat. Phot. 15, 828 (2021); DOI: 10.1038/s41566-021-00882-z

Link: Laboratory of Photonics and Quantum Measurements, Institute of Physics, Swiss Federal Institute of Technology Lausanne EPFL, Lausanne, Switzerland