New frequency comb spots molecules in 20 nanoseconds

31 Oct 2023

NIST and Toptica Photonics develop system for atmospheric chemistry, combustion science, quantum physics.

Laser frequency combs are valuable analytical tools for observing chemical phenomena or detecting the presence of particular species at low concentrations.

The key to these applications is the precision with which a laser output can be divided up into discrete frequency lines to create the comb structure, and creating combs able to illuminate certain ultrafast phenomena has remained a challenge.

A team at NIST, working with Toptica Photonics and the University of Colorado Boulder, has now developed a frequency comb system that can detect the presence of specific molecules in a sample every 20 nanoseconds, and published the findings in Nature Photonics.

The work builds on previous work at NIST into optimizing laser frequency combs for different applications. In 2021 it demonstrated how a single frequency comb instrument could detect several greenhouse gases, and put the principle to work on its own campus.

Before that in 2020 NIST developed semiconductor-chip-based micro-combs intended to allow precision optical frequency measurements for telescopes and atomic clocks.

The new device is a dual-frequency comb, an arrangement in which two distinct frequency combs with different repetition rates are incorporated into the same output, usually using two femtosecond lasers emitting pulses in lockstep. NIST used a simpler and cheaper setup referred to as electro-optic combs, in which a single continuous beam of light is split into two beams and an electronic modulator then alters each one, shaping it into the individual teeth of a frequency comb.

Each tooth is a specific frequency of light that can then be absorbed by a molecule of interest. The high power, mutual coherence and repetition rates of the combs made this way enabling fully resolved spectral transitions to be recorded in very short timescales, according to NIST.

Frequency combs to order

"Whereas conventional frequency combs can have thousands or even millions of teeth, this electro-optic comb only had 14 in a typical experimental run," commented NIST. "However, as a result each tooth had much higher optical power and was far apart from others in frequency, resulting in a clear, strong signal that enabled the researchers to detect changes in the absorption of light at the 20-nanosecond time scale."

NIST tested its system by measuring the supersonic pulses of carbon dioxide emerging from a small nozzle in an air-filled chamber and monitoring the motion of the pulse, using an optical parametric oscillator to shift the comb teeth from their natural near-IR wavelengths to the mid-IR absorbed by carbon dioxide molecules.

These transient vapor details are hard to accurately obtain even with the most sophisticated computer simulations, according to the project, but could be vital for improving the design of combustion engines or understanding how greenhouse gases interact with the atmosphere.

"In a more complicated system like an aircraft engine we could use this approach to look at a particular species of interest, such as water or fuel or carbon dioxide, to observe the chemistry," said NIST's David Long. "We can also measure things such as pressure, temperature or velocity by looking at changes in the signal."

Future work will include tuning the comb to other regions of the mid-infrared to detect other molecules. NIST believes that its approach could make frequency comb technology more widely available across many research fields and industries, allowing researchers to essentially generate any comb they wished.

"What is truly special about this work is that it substantially lowers the barrier to entry for researchers who would like to use frequency combs to study fast processes," said Greg Rieker from the University of Colorado Boulder's Precision Laser Diagnostics Lab.

• In other Toptica Photonics news, the company has introduced two new wavelengths to its TopMode single-frequency diode laser platform, adding 445 and 447 nanometers to the available outputs.

The new frequencies "push the boundaries of high-resolution Raman spectroscopy, microlithography, multi-wavelength digital holography and interferometry," according to Toptica.