On-Chip Laser Breakthrough for Mid-Infrared Applications

Physicists at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have developed a small laser that produces exceptionally bright, brief pulses of light in a valuable yet challenging wavelength range, integrating the capabilities of larger optical systems onto a single chip. The study was published in Nature.

The research marks the first demonstration of a picosecond, mid-infrared laser pulse generator on a chip without external components. This device can generate an optical frequency comb, which is a spectrum of light with evenly spaced frequency lines (resembling a comb), and is currently used in high-precision measurements. This novel laser chip has the potential to accelerate the development of highly sensitive, broad-spectrum gas sensors for environmental monitoring, as well as new kinds of spectroscopy tools for medical imaging.

Federico Capasso, the Robert L. Wallace Professor of Applied Physics at SEAS and the Vinton Hayes Senior Research Fellow in Electrical Engineering, is the senior author of the paper.

The research, supported by the National Science Foundation and the Department of Defense, was a collaborative effort involving the Schwarz group at Vienna University of Technology (TU Wien), a group of Italian scientists led by Luigi A. Lugiato, and Leonardo DRS Daylight Solutions under the direction of Timothy Day.

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. What’s more, such devices can be readily produced at industrial laser foundries using standard semiconductor fabrication.

Federico Capasso, Robert L. Wallace Professor in Applied Physics, SEAS

The mid-infrared portion of the electromagnetic spectrum, invisible to the human eye, is currently a valuable tool in environmental applications. This is because many gas molecules, such as carbon dioxide and methane, readily absorb light in this wavelength range. Consequently, mid-infrared technology, particularly quantum cascade lasers (a technology pioneered by Capasso in the 1990s), has become an important method for monitoring environmental gases.

This new research shows a way to create a broad-spectrum light source capable of detecting, for instance, the unique absorption signatures of numerous different gases using 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. I think that’s a real possibility for the future of this platform.

Dmitry Kazakov, Study Co-First Author and Research Associate, SEAS

A key element in this new achievement of nanophotonic engineering is the quantum cascade laser. This laser produces focused beams of mid-infrared light by stacking different nanostructured semiconductor materials. While traditional semiconductor lasers have long used established techniques called mode-locking to generate their pulses, quantum cascade lasers are known to be particularly challenging to pulse due to their naturally ultra-fast internal processes.

Existing mid-infrared pulse generators that utilize quantum cascade lasers usually require intricate setups and numerous separate hardware components to achieve pulsed light emission. Furthermore, they often have limitations in terms of output power and the range of light frequencies they can produce (spectral bandwidth).

This novel pulse generator cleverly integrates several concepts from nonlinear integrated photonics and integrated lasers into a single device to create specific picosecond light pulses known as solitons. In designing their chip's architecture, the researchers drew inspiration from a seemingly unrelated light-modulating device called a Kerr microresonator. This innovative approach allowed them to bypass conventional pulse generation techniques, such as mode-locking.

“Our measurements were non-traditional when it came to quantum cascade laser research. We merged two types of fields and took what the Kerr resonator community does and applied it to our systems. That was an exciting process,” said Theodore Letsou, Study Co-First Author and Graduate Student at MIT and Research Fellow in Capasso’s group.

For me, the most significant impact of our new work – beyond the impressive physics – is the confidence it has given us in fabricating and operating multicomponent architectures, which is a capability that had remained a major challenge in mid-infrared integrated photonics until now. We’re already developing new architectures to enable functionalities previously thought impossible.

Benedikt Schwarz, Professor and Study Co-Author, TU Wien

The researchers built upon a fundamental theory from the 1980s that provided a basis for passive Kerr resonators. Luigi Lugiato, a co-author of this new paper, played a key role by adapting the original equation to describe the behavior of the mid-infrared laser system.

“This is an exciting culmination of a journey that began with the Lugiato-Lefever equation. What started as a model for passive systems has evolved into a unified framework for soliton frequency combs in all kinds of cavities. That path led us to predict solitons in optically driven quantum cascade lasers above the threshold – now confirmed by this experiment,” said Lugiato, Professor Emeritus at the University of Insubria, Italy.

This new mid-infrared laser demonstrates stable pulse generation over several hours. Importantly, it can be manufactured in large quantities using current industrial fabrication techniques, potentially accelerating its widespread use. The device comprises an externally controlled ring resonator, an integrated laser that powers the ring resonator, and a second active ring resonator that functions as a filter. The fabrication of these chips was carried out at TU Wien.

This technology promises to be a real game-changer in the field of mid-infrared spectroscopy. The ability to leverage existing fabrication processes to produce these devices in commercial volumes could really enable what’s next in several markets, including environmental monitoring, industrial process control, life sciences research, and medical diagnostics.

Timothy Day, Study Co-Author, Senior Vice President and General Manager, Business Unit, Leonardo DRS’ Daylight Solutions

The study was supported by funding from the National Science Foundation under Grant No. ECCS2221715. Additional financial support was provided by the Department of Defense through the National Defense Science and Engineering Graduate Fellowship Program and the European Research Council.

Harvard's Office of Technology Development has secured intellectual property protection for the innovations arising from this research and is currently investigating potential commercial applications.

The published paper's additional co-authors include Marco Piccardo, Lorenzo L. Columbo, Massimo Brambilla, Franco Prati, Sandro Dal Cin, Maximilian Beiser, Nikola Opačak, Pawan Ratra, Michael Pushkarsky, and David Caffey.

Journal Reference:

Kazakov, D., et al. (2025) Driven bright solitons on a mid-infrared laser chip. Nature. doi.org/10.1038/s41586-025-08853-y