Compact Laser Chip Generates Mid-IR Picosecond Light Pulses

Physicists at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have created a compact laser that emits extremely bright, short pulses of light in a useful but difficult-to-achieve wavelength range, packing the performance of larger photonic devices onto a single chip. According to the researchers, the work is the first demonstration of an on-chip, picosecond, mid-IR laser pulse generator that requires no external components to operate.

The device can make what is called an optical frequency comb — a spectrum of light consisting of equally spaced frequency lines, used today in precision measurements. The new laser chip could one day speed the creation of highly sensitive, broad-spectrum gas sensors for environmental monitoring, or new types of spectroscopy tools for medical imaging.

(From left) Co-first author and research associate Dmitry Kazakov, research co-author Pawan Ratra, MIT graduate student and research fellow Theodore Letsou, research co-author Marco Piccardo, and senior author and Robert L. Wallace Professor of Applied Physics at SEAS Federico Capasso. Courtesy of Harvard.

“This is an exciting new technology that integrates on-chip nonlinear photonics to generate ultrashort pulses of light in the mid-infrared; no such thing existed until now,” said Federico Capasso, the Robert L. Wallace professor of Applied Physics at SEAS. “What’s more, such devices can be readily produced at industrial laser foundries using standard semiconductor fabrication.”

The mid-IR is an invisible section of the electromagnetic spectrum that is leveraged today in environmental applications. Because many gas molecules like carbon dioxide and methane absorb mid-IR light efficiently, this wavelength range has been an important tool in monitoring environmental gases. The researchers’ findings demonstrate a path to generating a broadband light source that could detect, for example, many different absorption fingerprints of gases in a single device.

“It’s a key step to creating what we call a supercontinuum source, which can generate thousands of different frequencies of light, all in one chip,” said Dmitry Kazakov, co-first author and research associate in Capasso’s group.

The quantum cascade photonic integrated chip contains two devices comprising of four components each, including a Fabry-Perot drive laser, a waveguide coupler, a resistive heater, and a racetrack resonator. Courtesy of Harvard.

Fundamental to the new feat of nanophotonic engineering is the quantum cascade laser, which generates coherent beams of mid-IR light by layering together different nanostructured semiconductor materials. Unlike other semiconductor lasers that have relied for decades on well-established techniques called mode-locking to generate their pulses, quantum cascade lasers remain difficult to pulse due to their inherently ultra-fast dynamics. Existing mid-IR pulse generators based on quantum cascade lasers typically require complex setups to achieve pulsed emission as well as many discrete hardware components. They are also generally limited to a certain output power and spectral bandwidth.

The new pulse generator combines, into a single device, several concepts in nonlinear integrated photonics and integrated lasers to make specific types of picosecond light pulses called solitons. In designing their chip architecture, the researchers drew on a foundational theory published in the 1980s that established a framework for passive Kerr resonators, integrating the framework into the device’s design.

The new mid-IR laser can reliably maintain pulse generation for hours at a time. Crucially, it can also be mass-produced using existing industrial fabrication processes, which could greatly increase the speed of its widespread adoption. The device is made of a ring resonator that can be externally driven, an on-chip laser that drives the ring resonator, and a second active ring resonator that acts as a filter. The chips were made at the Schwarz group at Vienna University of Technology.

The research was published in Nature (www.doi.org/10.1038/s41586-025-08853-y).