The combination of electrical and photonic circuits is a major goal of the integrated photonics industry, but it also presents a problem. Electrical circuits are notorious for generating heat. If photonic devices become a little bit too hot, or a little bit too cold, their finely tuned photonic properties can be disrupted. It is possible to keep track of a photonic chip’s temperature. But, it’s been a complicated process, that has required external equipment. This is an impediment to shrinking photonic devices down to the sizes of the electronic chips underlying modern technologies. As it turns out, a thermometer has been part of photonic chips all along. Researchers at Columbia University have found that the thin-film metallic resistor routinely used to thermally tune photonic devices to the desired resonance frequency can also measure temperature. This finding could eliminate the need for bulky and costly external temperature sensors and help integrated photonics reach its full potential. “One of the key challenges for the broad adoption of silicon photonics in many applications is mitigating the high sensitivity of photonic devices to thermal variations,” said Alexander Gaeta, David M. Rickey Professor of Applied Physics and Materials Science and professor of electrical engineering at Columbia Engineering. “The technology we have developed here offers a straightforward approach that is foundry compatible and may find near-term applications in large-scale photonic integrated circuits for data communications and quantum information processing.” A thin-film metallic resistor with a temperature-dependent resistance placed directly above a high-Q microcavity acts as an on-chip resistance thermometer. Courtesy of Columbia University/Sai Kanth Dacha. For more than a decade, many in the field have been incorporating a thin film of platinum into their hardware. The platinum acts as a resistor, controlling the voltage applied to the resistor changes the resonant frequency. Platinum has also long been used, in its bulk form, as a temperature sensor in some of the most extreme environments, like the surface of Mars and the inside of nuclear reactors. A few years ago, when Sai Kanth Dacha joined Gaeta’s lab as a postdoctoral research scientist, he made the connection between these apparently unconnected uses for this material. “One day, we changed the heat source on one of our chips and decided to observe the resistance of the platinum,” said Dacha, who is the lead author on the work. “It changed — a lot.” Most bulk resistors are Ohmic, with a straight-line relationship between current and voltage over a large range of voltages. The thin-film platinum used in this work is not. Its behavior, Dacha said, is similar to that of a tungsten filament lamp, a well-known example of a thin film of metal with non-Ohmic behavior. “Eventually, tungsten filaments heat up so much that their properties change — that’s why they glow.” The researchers found that the integrated platinum resistors follow a similar voltage-current curve, which hinted at a strong temperature dependence of resistance and the ability to serve as a temperature readout. “It’s actually very simple. I’m surprised no one has seen it before,” Dacha said. “We can now directly measure temperature in real time and stabilize as needed.” The team documented the usefulness of this integrated thermometer as a means to stabilize microscopic photonic cavities. By frequency locking a commercial distributed feedback (DFB) laser to such a cavity, the researchers demonstrated a crucial component of optical communication networks that require compact light sources. They were able to keep the laser within a picometer of the desired wavelength for over two days. “That is better performance than some commercial telecommunication systems. And the beauty of it is that the cavity stabilization requires no photodetection at all,” said Dacha. They note the thermometer is platform-agnostic and should work with different materials and chip configurations. For example, it should help stabilize silicon ring modulators, a highly efficient method for switching light on and off that was pioneered by co-author Michal Lipson, Eugene Higgins Professor of Electrical Engineering and Professor of Applied Physics, and which is now used in commercial applications by companies such as NVIDIA. Keeping tabs on temperature is also critical for emerging quantum devices, which require extremely low temperatures; an integrated thermometer may help shrink the size of the necessary cryochambers. “So far, thermal issues have been a major unsolved problem in the field. We hope our work is one of the first big steps to realizing large-scale photonic devices capable of operating in real-world environments in a resource-efficient way,” said Dacha. The research was published in Nature Photonics (www.doi.org/10.1038/s41566-025-01789-9).