A photonic chip built from silicon and a thin layer of lithium niobate has shown it can send light into two frequency channels in a controlled way—and, at a different setting, move almost all of that light into the partner channel. In the reported measurements, the device produced a balanced 50/50 split and near-complete frequency swapping, with more than 20 decibels of pump extinction—the reduction of the original driven frequency component—at a 10-gigahertz split between paired resonant states called supermodes.
The study evaluates whether a heterogeneously integrated thin-film lithium-niobate-on-silicon coupled-cavity modulator can make tunable, two-way transformations between frequency modes. The principal experimental sample was a designed, fabricated and characterized Si/TFLN coupled-cavity modulator.
Two resonators, one shared waveguide
The electro-optic TFLN layer was bonded onto a silicon photonics base fabricated in a CMOS-compatible foundry process. The device uses two racetrack resonators—ring-like waveguides with elongated straight sections—coupled through their near fields to a single bus waveguide. The paper describes the resulting paired resonant states as supermodes, the two frequency states tracked in the conversion measurements.
To test it, the researchers swept RF drive power and monitored the split optical output with an optical spectrum analyzer and a power meter. They fit the measured conversion spectra to a transfer-matrix model across RF amplifier output powers from 0 to 35 dBm, a standard unit for signal power. The fit was used to describe the device’s coupling and loss parameters.
The operating points
Thermal tuning near 1,620 nanometres produced an avoided crossing—an interaction in which two resonances split apart—of about 10 GHz. Passive transmission fits gave a free spectral range of about 55 GHz and quality factors of roughly 42,000 loaded, 130,000 intrinsic and 62,000 external, with α about 2.23.
Different reported RF settings corresponded to different transformations. At approximately 18.5 dBm of RF output power, the device acted as a 50/50 beam splitter. At approximately 30 dBm, it transferred nearly all the optical energy to the partner supermode.
Extra frequency components outside the two-supermode subspace—parasitic sidebands—were suppressed by more than 15 dB across all drive powers. In practical terms, the reported conversion remained concentrated in the two modes targeted by the experiment.
Fitting the TFLN conversion spectra estimated external coupling of about 1.85 GHz, intrinsic loss of about 1.51 GHz, α about 2.45 and VπL of about 8.28 V cm. VπL is the voltage-length figure used to describe the modulator’s electro-optic drive requirement; an independent linear-transmission fit gave α about 2.23.
A second material on the same structure
The researchers also tested the platform with a 600-nanometre TFLT die bonded to the coupled-cavity structure. During measured frequency-conversion characterization, that variant showed weaker bus and resonator couplings and slightly increased optical loss in the hybrid taper, the transition between the bonded layers.
The TFLT fit reported an 8 GHz splitting, external coupling of 1.22 GHz, intrinsic loss of 1.67 GHz, α of 1.46, Vπ of 275 volts and VπL of 9 V cm. The authors caution that the larger VπL alone does not establish inferior material performance, because modulation efficiency depends strongly on the thickness of the oxide layer between materials.
A laboratory result
The evidence remains a laboratory device result. The principal sample was a Si/TFLN coupled-cavity modulator, and its behavior was evaluated through RF-driven optical spectra and transfer-matrix fits; the study therefore does not demonstrate a complete integrated quantum circuit.
The document is an arXiv v1 preprint dated 26 August 2026. Its acknowledgments cite support from Sandia National Laboratories’ Laboratory Directed Research and Development program through the EPIQ project.
Paper data and sources
Original title: A heterogeneously integrated coupled-cavity frequency beam splitter
Authors: Lucas M. Cohen, Manuel H. Muñoz-Arias, Mohan Sarovar et al.
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-26
DOI: Not available
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