Optical frequency combs — laser sources that emit evenly spaced colors of light — are foundational, ubiquitous tools for precision measurement, found in optical clocks, gas-sensing spectrometers, and instruments that detect the light signatures of exoplanets. Traditionally, frequency combs are produced by large, fiber-laser systems ranging from the size of a shoebox to a refrigerator.
Engineers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) are at the forefront of shrinking these powerful laser sources onto photonic chips to make "microcombs" at millimeter to micron scales, useful not only for their smaller size, but in next-generation telecommunications applications, such as generating multiple data carriers over a single optical fiber.
New research led by Marko Lončar, the Tiantsai Lin Professor of Electrical Engineering and Applied Physics, describes a new, generalized model for how to design so-called resonant electro-optic microcombs on thin-film lithium niobate, a material featuring a strong electro-optic effect, or the ability to efficiently mix electronic signals with optical ones.
The research demonstrates that a single thin-film lithium niobate chip can host electro-optic microcomb generators that are highly programmable and compact, and tailorable to diverse tasks simply by reprogramming a microwave input. The work could make electro-optic microcombs more practical, easier to design, and energy efficient.
The work is published in Nature Physics .
Over the last decade, Lončar's group at Harvard has pioneered the development of chip-scale electro-optic microcombs that are efficient, stable, and highly controllable with microwaves. These devices work through the combination of a single-frequency laser and a microwave electrical signal that interact via a resonator-enhanced electro-optic modulator to produce a broadband frequency comb.
Because of thin-film lithium niobate's strong electro-optic effect, the platform can support on-chip electro-optic microcomb generation that exhibits qualitatively different behavior than older, bulkier electro-optic microcombs. High-efficiency, low-voltage modulation can drive extreme changes in the light signals that aren't well understood by available models.
Loncar's team decided to try and push electro-optic microcombs into new regimes while mapping the precise physics of what happens when the combs are produced. To do so, they engineered long, racetrack-shaped resonators out of lithium niobate with equally long electro-optic modulators embedded. This design allowed the team to explore modulation depths that are difficult to achieve in bulkier systems.
They observed a rich variety of new comb behaviors and corresponding laser pulse patterns inside their optical cavity. By systematically mapping all of these new behaviors in lab experiments, the team arrived at a physically intuitive, detailed, quantitative model for describing resonant electro-optic microcombs.
"Using this on-chip platform, we were able to dive deep into the high modulation depth regime of EO microcombs, and comprehensively explore the types of combs, and correspondingly, pulses, that can be supported," said first author Yunxiang Song, a graduate student in Lončar's lab.
These insights allowed the team to expand the operation of their electro-optic microcombs into a new place: multiple microwave signal inputs that can generate a broadband frequency comb and cover a greater range of light spectra. This breakthrough could lead to microcombs that can reach larger ranges of unknown optical frequencies, useful for metrology and other applications.
The device fabrication in this work was performed at the Harvard University Center for Nanoscale Systems, a member of the National Nanotechnology Coordinated Infrastructure Network, which is supported by the National Science Foundation under NSF award No. ECCS-2025158.
