Applied physicists in the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have discovered a new way to generate ultra-precise, evenly spaced "combs" of laser light on a photonic chip, a breakthrough that could miniaturize optical platforms like spectroscopic sensors or communication systems.
The research was led by Marko Lončar , the Tiantsai Lin Professor of Electrical Engineering and Applied Physics at SEAS, and published in Science Advances . The paper's first author is Yunxiang Song, a graduate student in Quantum Science and Engineering .
Optical frequency combs are laser sources whose colors are evenly spaced, like the teeth of a comb. They underpin many modern technologies requiring precision measurement, from atomic clocks to high-speed telecommunications. A landmark invention in optics and the subject of the 2005 Nobel Prize in Physics , conventional fiber-laser frequency combs are stable and reliable but can be limited by bulk and expense.
The Lončar lab is at the forefront of creating chip-scale optical frequency combs, or microcombs, by shrinking these laser sources to micron-sized photonic circuits. These microcombs offer many advantages, including requiring less power and having larger comb-line spacings suited for carrying high-bandwidth data.
The lab's platform of choice is thin film lithium niobate, a crystalline material with extraordinary light-modulation properties for integrated photonics — a technology that Lončar's group pioneered in the last decade . Functional microcombs require integration with a device called an electro-optic modulator, which manipulates the generated comb lines using electric signals. Smoothly combining the comb's generation and modulation onto one chip has been a long-sought goal for scientists.
While lithium niobate allows precise control of light with electricity, it's been so far difficult to achieve microcombs on this material due to a scattering of light caused by the material's crystal vibrations, known as the Raman effect. When a lithium-niobate micro-resonator is pumped with a laser, the Raman effect tends to take over, producing a single color instead of an evenly spaced microcomb.
In earlier work published last year in Optica , Song and the team developed a new, rotated racetrack-like resonator design that could suppress the Raman effect in lithium niobate. Working with a particular wafer orientation called X-cut lithium niobate, which has different Raman responses along different crystal axes, they demonstrated for the first time on this platform a class of microcombs called solitons.
In the new Science Advances paper, they used the same design strategy to produce yet another type of frequency comb, called a normal dispersion Kerr microcomb – the first such microcomb ever created on an X-cut lithium niobate chip. Such combs can convert a large fraction of laser power into a set of comb lines with spacing that is well matched for chip-scale optical communications and other applications.
"This work shows that normal dispersion combs can be implemented in technologically relevant thin-film lithium-niobate platform that features strong electro-optic modulation," Lončar said. "That's what you want for the next generation of microcomb-driven photonic systems."
The team was also surprised by an accidental discovery: despite designing their microresonator to suppress the nuisance Raman effect, a residual Raman scattering effect remained. But instead of wrecking the comb, this small effect interacted with the comb by locking to it and creating yet another new, hybrid microcomb, which is even broader and more versatile than the original.
"A broader, coherent frequency comb that leverages the Raman effect -- rather than being limited by it, as common wisdom suggests -- may be useful for covering spectral ranges that are nominally hard to generate combs in," Song said.
Theoretical modeling and simulations performed by collaborators at the University of Auckland confirmed this newly observed hybrid microcomb was indeed phase-coherent across its entire span, a critical property that must be satisfied by all frequency combs.
The new hybrid microcomb provides yet another way to generate frequency combs in spectral bands that are otherwise hard to reach. This capability could prove valuable for applications such as spectroscopy of gases and chemicals that absorb at specific wavelengths, or for sensing in the mid-infrared region of the electromagnetic spectrum.
Beyond any single application, the team's work is one of fundamental applied physics: the establishment of lithium-niobate as a uniquely powerful platform to make high-efficiency microcombs and high-speed electro-optical control that coexist on the same chip. Because the comb-generating resonator is a millimeter-scale ring on the surface of the chip, it can be fabricated seamlessly alongside other photonic building blocks that the Lončar group and others have pioneered.
The paper was co-authored by Zongda Li, Xinrui Zhu, Norman Lippok, and Miro Erkintalo. Federal funding sources for this research were the Air Force Office of Scientific Research, the National Science Foundation, and the Department of Defense.
