Max-Planck progresses UV spectroscopy with high-resolution dual-comb method…

13 Mar 2024

…and Stanford launches “micro” frequency comb, as basis for mass-market adoption of such devices.

Scientists in the group of Nathalie Picqué at the Max-Planck Institute of Quantum Optics, Garching, Germany, say they have made a significant leap in the field of ultraviolet spectroscopy by successfully implementing high-resolution linear-absorption dual-comb spectroscopy in the UV spectral range. This achievement “opens up new possibilities for performing experiments under low-light conditions, paving the way for novel applications in various scientific and technological fields.”

The results are described in Nature.

Dual-comb spectroscopy, a powerful technique for precise spectroscopy over broad spectral bandwidths, has been mainly used for infrared linear absorption of small molecules in the gas phase. It relies on measuring the time-dependent interference between two frequency combs with slightly different repetition frequencies.

A frequency comb is a spectrum of evenly spaced, phase-coherent laser lines that acts like a ruler to measure the frequency of light with extreme precision. The dual-comb technique does not suffer from the geometric limitations associated with traditional spectrometers and offers great potential for high precision and accuracy.

Low light intensities

However, dual-comb spectroscopy typically requires intense laser beams, making it less suitable for scenarios where low light levels are critical. The MPQ team has demonstrated that dual-comb spectroscopy can be effectively employed in starved-light conditions at power levels more than a million times weaker than those typically used.

This achievement was achieved using two distinct experimental setups with different types of frequency-comb generators. The team developed a photon-level interferometer that accurately records the statistics of photon counting, showcasing a signal-to-noise ratio at the fundamental limit. This opens up the prospect of dual-comb spectroscopy in challenging scenarios where low light levels are essential.

The MPQ researchers precisely controlled the mutual coherence of two comb lasers with one femtowatt per comb line, demonstrating an optimal build-up of the counting statistics of their interference signal over times exceeding one hour.

“Our innovative approach to low-light interferometry overcomes the challenges posed by the low efficiency of nonlinear frequency conversion, and lays a solid foundation for extending dual-comb spectroscopy to even shorter wavelengths,” said Bingxin Xu, the post-doctoral scientist who led the experiments.

One promising future application is the development of dual-comb spectroscopy at short wavelengths to enable precise vacuum- and extreme-ultraviolet molecular spectroscopy over broad spectral spans. Currently, broadband extreme-UV spectroscopy is limited in resolution and accuracy and relies on unique instrumentation at specialized facilities.

“Ultraviolet dual-comb spectroscopy, while a challenging goal, has now become a realistic one as a result of our research. Importantly, our results extend the full capabilities of dual-comb spectroscopy to low-light conditions, unlocking novel applications in precision spectroscopy, biomedical sensing, and environmental atmospheric sounding,” said Picqué herself.

Stanford announces new type of frequency comb

Announced at the same time, researchers at Stanford University have unveiled a new type of frequency comb – a “microcomb” – which, they say, “could be the basis for mass-market adoption of the devices in everyday electronics.”

The Stanford group has integrated two different approaches for miniaturizing frequency combs into one easily producible, microchip-style platform.

Among the many applications the researchers envision for their versatile technology are powerful handheld medical diagnostic devices and widespread greenhouse gas monitoring sensors.

“The structure for our frequency comb brings the best elements of emerging microcomb technology together into one device,” said Hubert Stokowski, a postdoctoral scholar in the lab of Amir Safavi-Naeini, and lead author of the study, also published in Nature.

“We can potentially scale our new frequency microcomb for compact, low-power, and inexpensive devices that can be deployed almost anywhere, Stokowski added.

“We’re very excited about this new microcomb technology that we’ve demonstrated for novel sensors that are both small and efficient enough to be in someone’s phone, someday,” said Safavi-Naeini, associate professor in the Department of Applied Physics at Stanford’s School of Humanities and Sciences and senior author of the study.

This new device is called an Integrated Frequency-Modulated Optical Parametric Oscillator, or FM-OPO. The tool’s complex name indicates that it combines two strategies for creating the range of distinct frequencies, or colors of light, that constitute a frequency comb.

One strategy, optical parametric oscillation, involves bouncing beams of laser light within a crystal medium, wherein the generated light organizes itself into pulses of coherent, stable waves. The second strategy centers on sending laser light into a cavity and then modulating the phase of the light – achieved by applying radio-frequency signals to the device – to ultimately produce frequency repetitions that similarly act as light pulses.

Strategic combination

These two strategies for microcombs have not been used widely because both come with drawbacks. These issues include energy inefficiency, limited ability to adjust optical parameters, and suboptimal comb “optical bandwidth” where the comb-like lines fade as the distance from the center of the comb increases.

The researchers fashioned the components at the heart of the new frequency comb using integrated lithium niobate photonics. These light-manipulating technologies build upon advances in the related, more established field of silicon photonics, which involves fabricating optical and electronic integrated circuits on silicon microchips. “Lithium niobate has certain properties that silicon doesn’t, and we couldn’t have made our microcomb device without it,” said Safavi-Naeini.

Next, they brought together elements of both optical parametric amplification and phase modulation strategies. The team expected certain performance characteristics from the new frequency comb system on lithium niobate chips – but what they saw proved far better than they anticipated.

Overall, the comb produced a continuous output rather than light pulses, which enabled the researchers to reduce the required input power by approximately an order of magnitude. The device also yielded a conveniently “flat” comb, meaning the comb lines farther in wavelength from the center of the spectrum did not fade in intensity, thus offering greater accuracy and broader utility in measurement applications.

The new microcombs, with further honing, should be readily manufacturable at conventional microchip foundries with many practical applications such as sensing, spectroscopy, medical diagnostics, fiber-optic communications, and wearable health-monitoring devices.