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Concentrated laser-like illumination that spans a broad spectrum of frequencies is greatly sought after for numerous scientific investigations and various applications, such as quality assessment in the production of semiconductor electronic chips. However, generating such wideband and coherent light has proven challenging to accomplish with anything other than sizable, energy-intensive tabletop systems.
Currently, a Caltech group led by Alireza Marandi, an educator in electrical engineering and applied physics at Caltech, has developed a minuscule device capable of emitting an exceptionally vast array of laser-light frequencies with remarkable efficiency—all on a microchip. This advancement holds promise in fields ranging from communication and imaging to spectroscopy, where the light would facilitate the identification of atoms and molecules in diverse environments.
The scientists detail their innovative nanophotonic device and methodology in a publication that appears in the journal Nature Photonics. The principal author of this paper is Ryoto Sekine (PhD ’25), who carried out the research as a graduate student in Marandi’s laboratory.
“We are demonstrating that with a singular nanophotonic device and minimal input energies in the femtojoule range, it is indeed possible to encompass a wide spectrum of the electromagnetic spectrum, ranging from visible wavelengths to the mid-infrared. This feat had never been accomplished before,” says Marandi.
The Caltech device employs a technology that has existed since 1965: an optical parametric oscillator (OPO). Essentially, an OPO functions as a resonator— a diminutive engineered light trap that takes incoming laser light at a specific input frequency and utilizes a unique nonlinear crystal—specifically, lithium niobate—that can generate light at different frequencies with precise engineering.
Generally, OPOs originate from a laser source with a limited frequency range and produce outputs at varying frequencies yet still within a narrow spectrum. Traditionally, they have been utilized as laser-like sources with widely adjustable output frequencies.
A Light Comb
In contrast, in this research, Marandi and his associates have refined their OPO at the nanoscale on a chip to create what is referred to as a frequency comb, a spectrum of uniformly spaced laser-like light over an extensive frequency range with minimal input energy. The frequency comb spans a remarkably wide spectral range, delivering sharp, stable lines from visible light all the way to the longer mid-infrared wavelengths.
Two researchers were awarded a share of the 2005 Nobel Prize in Physics for their contributions to the development of the frequency comb technique. Unlike standard lasers, which emit a singular color of light, frequency combs serve as a ruler for light across a variety of frequencies. These combs have been employed to enhance many aspects, from the accuracy of atomic clocks and light-based measurements to environmental monitoring.
However, Marandi notes, “there have been two primary obstacles with frequency combs: One is that the sources are excessively large, and the second is that creating them in various required spectral windows is challenging. Our research provides a pathway toward addressing both issues.”
The pivotal advancements of the new device are what Marandi refers to as dispersion engineering—contouring the way different wavelengths of light navigate through the device, ensuring they remain cohesive rather than dispersing—and a meticulously crafted resonator structure. Collectively, these features enable the device to effectively widen the spectrum while sustaining coherence with an exceptionally low threshold, or energy level at which it starts functioning.
A Surprisingly Broad Coherent Spectrum
Marandi expresses that he and his team were astonished by the device’s capabilities. “We activated it and increased the power, and upon inspecting the spectrum, we discerned that it was extraordinarily broad. We were particularly surprised that the super-broad spectrum was indeed coherent. This contradicted the conventional descriptions of how OPOs function,” he states.
This prompted the researchers to return to their simulations and theoretical work to unravel how this phenomenon could occur. In their simulations, elevating the energy of incoming light above the threshold led to an incoherent spectrum—that is, composed of various wavelengths and not phase-locked, which indicates no frequency comb is formed. Yet, back in the laboratory, the spectrum remained coherent when operating well above threshold.
“It took us about six months to realize that there exists this new operational regime of OPOs in which the OPO operates far above its threshold and coherence is reinstated,” Marandi notes. “Given that the threshold of this OPO is orders of magnitude lower than previous OPOs, and the dispersion and resonator are designed differently from earlier realizations, we could witness this extraordinary spectral broadening, which is orders of magnitude more energy efficient than other spectral broadening techniques.”
The researchers assert that their work could transform how frequency comb-based technologies, currently confined to tabletop setups, could progress to integrated photonic devices. One of the primary techniques required to generate stable frequency combs necessitates significantly broadening their spectrum. The energy required for such broadening has been a significant hurdle preventing the integration of frequency comb technologies onto chips.
Moreover, most photonic technologies, including the majority of well-established lasers and detectors used for molecular measurements, function within the near-infrared or visible spectrum. OPOs that commence from near-infrared lasers as the input frequency and then efficiently convert the light, producing coherent light in the mid-infrared region, could enable researchers, such as those in spectroscopy, to tap into a wealth of information at lower frequencies. Simultaneously, such a device could allow access to the higher-frequency range for atomic spectroscopy.
The publication is titled “Multi-Octave Frequency Comb from an Ultra-Low-Threshold Nanophotonic Parametric Oscillator.” Other authors include former Caltech graduate students Robert M. Gray (PhD ’25) and Luis Ledezma (PhD ’23), along with current Caltech graduate student Selina Zhou and former postdoctoral researcher Qiushi Guo. The device’s nanofabrication was accomplished at the Kavli Nanoscience Institute at Caltech. This work received support from grants provided by the Army Research Office, the National Science Foundation, the Air Force Office of Scientific Research, DARPA, the Center for Sensing to Intelligence at Caltech, and JPL, which is overseen by Caltech for NASA.
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