Shaking Light with Sound

Integrated silicon nitride photonic chips with aluminium nitride actuators. (Source: J. He & J. Liu, EPFL)

Piezoelectric materials can convert electrical voltage to mechanical displace­ment and vice versa. They are ubiquitous in modern wireless communi­cation networks such as in cellphones. Today, piezoelectric devices, including filters, trans­ducers and oscil­lators, are used in billions of devices for wireless communi­cations, global positioning, navi­gations, and space appli­cations. Now, a collaboration lead by Tobias J. Kippenberg at EPFL and Sunil A. Bhave at Purdue University has combined piezo­electric aluminium nitride (AlN) technology – used in modern cellphones’ radio frequency filters – with ultralow-loss silicon nitride inte­grated photonics, demonstrating a new scheme for on-chip acousto-optic modu­lation.

The hybrid circuit allows wideband actua­tion on photonic wave­guides with ultralow electrical power – a feat that has been so far challenging. The circuit itself was manu­factured using CMOS-compatible foundry processes, which are widely used to construct micro­processors, micro­controllers, memory chips, and other digital logic circuits. To build the circuit, the scientists used silicon nitride, which has emerged as a leading material for chip-scale, micro­resonator-based optical frequency combs. Microcombs are used in a range of precision-demanding appli­cations, including coherent communi­cations, astronomical spectro­meter cali­bration, ultrafast ranging, low-noise microwave synthesis, optical atomic clocks, and most recently, parallel coherent LiDAR.

The researchers fabricated piezo­electric AlN actuators on top of the ultralow-loss silicon nitride photonic circuits, and applied a voltage signal on them. The signal induced bulk acoustic waves electro­mechanically, which can modulate the generated microcomb in the silicon nitride circuits. In short, sound shakes light. A key feature of this scheme is that it maintains the ultralow loss of silicon nitride circuits. “This achieve­ment represents a new milestone for the micro­comb tech­nology, bridging inte­grated photonics, micro­electro­mechanical systems engineering and nonlinear optics,” says Junqiu Liu, who leads the fabri­cation of silicon nitride photonics chips at EPFL’s Center of MicroNano­Technology (CMi). “By harnes­sing piezo­electric and bulk acousto-optic inter­actions, it enables on-chip optical modulation with unpre­cedented speed and ultralow power consumption.”

Using the new hybrid system, the researchers demonstrated two inde­pendent appli­cations: First, the optimi­zation of a microcomb-based massively parallel coherent LiDAR, based on their previous work. This approach could provide a route to chip-based LiDAR engines driven by CMOS micro­electronic circuits. Second, they built magnet-free optical isolators by spatio-temporal modulation of a silicon nitride micro­resonator. “The tight vertical confine­ment of the bulk acoustic waves prevents cross-talk and allows for close placement of the actuators, which is challenging to achieve in p-i-n silicon modu­lators,” says Hao Tian.

The new technology could provide impetus to microcomb applications in power-critical systems, e.g. in space, datacenters and portable atomic clocks, or in extreme environments such as cryogenic temperatures. “As yet unforeseen applications will follow up across multiple communities,” says Kippenberg. “It’s been shown time and again that hybrid systems can obtain advantages and functionality beyond those attained with individual constituents.” (Source: EPFL)

Reference: J. Liu et al.: Monolithic piezoelectric control of soliton microcombs, Nature 583, 385 (2020); DOI: 10.1038/s41586-020-2465-8

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

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