A Compact, Mid-Infrared Pulse Generator

Photonics & Imaging Technology INSIDER

Members of the research team in the Capasso lab. An optical table contains the experimental setup for characterization of the integrated laser sources reported in the paper. The screen shows a microscope image of an active ring resonator based on quantum cascade lasers, used for the demonstration of bright solitons on a laser chip. From left: Dmitry Kazakov, Pawan Ratra, Theodore Letsou, Marco Piccardo, Federico Capasso. (Image: Harvard University)

Physicists in 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.

Published in Nature , the research is the first demonstration of an on-chip, picosecond, mid-infrared laser pulse generator that requires no external components to operate. The device can make what’s called an optical frequency comb, a spectrum of light consisting of equally spaced frequency lines (like a comb), 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.

The paper’s senior author is Federico Capasso, the Robert L. Wallace Professor of Applied Physics at SEAS and the Vinton Hayes Senior Research Fellow in Electrical Engineering. Supported by the National Science Foundation and the Department of Defense, the research was a collaboration with the Schwarz group at Vienna University of Technology (TU Wien); a consortium of Italian scientists led by Luigi A. Lugiato; and Leonardo DRS Daylight Solutions led by 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,” Capasso said. “What’s more, such devices can be readily produced at industrial laser foundries using standard semiconductor fabrication.”

The mid-infrared 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-infrared light efficiently, this wavelength range has been an important tool in monitoring environmental gases, notably with quantum cascade laser technology that was pioneered by Capasso in the 1990s.

The new paper demonstrates 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. “I think that’s a real possibility for the future of this platform.”

Fundamental to the new feat of nanophotonic engineering is the quantum cascade laser, which generates coherent beams of mid-infrared 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 notoriously difficult to pulse due to their inherently ultra-fast dynamics. Existing mid-infrared 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 seamlessly 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 took inspiration from a seemingly unrelated type of light-modulating device called a Kerr microresonator. Their creative thinking allowed them to skirt traditional techniques, like mode-locking, for pulse generation.

The researchers drew on a foundational theory published in the 1980s that established a framework for passive Kerr resonators. One of the new paper’s co-authors is Luigi Lugiato, who worked on repurposing his original equation to describe the dynamics of the mid-IR laser system.

“This is an exciting culmination of a journey that began with the Lugiato-Lefever equation,” said Lugiato, Professor Emeritus at University of Insubria, Italy. “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 threshold – now confirmed by this experiment.”

The new mid-infrared 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 TU Wien.