Researchers have developed a new ultrafast laser platform that generates ultra-broadband ultraviolet (UV) frequency combs with an unprecedented one million comb lines, providing exceptional spectral resolution. The new approach, which also produces extremely accurate and stable frequencies, could enhance high-resolution atomic and molecular spectroscopy.
Optical frequency combs — a type of laser that emits thousands of regularly spaced spectral lines — have transformed fields like metrology, spectroscopy and precision timekeeping via optical atomic clocks, earning the 2005 Nobel Prize in Physics.
The initial frequency combs operated within the visible to near-infrared range. Shortly after their introduction, their spectral range was extended to the UV region through optical harmonic generation, unlocking a new spectral domain for precision laser spectroscopy. “Nevertheless, achieving both broadband coverage and high spectral resolution in the UV range has remained a considerable challenge,” said research team leader Konstantin Vodopyanov from CREOL, The College of Optics & Photonics, at the University of Central Florida.
In Optica, Optica Publishing Group’s journal for high-impact research, the researchers describe their high-resolution dual-comb spectroscopy system which generates light across two ultra-broad UV spectral regions. With a line spacing of just 80 MHz, the frequency combs exhibit a resolving power of up to 10 million.
“Broadband, high-resolution UV spectroscopy provides unique insights into electronic transitions in atoms and molecules, making it invaluable for applications such as chemical analysis, photochemistry, atmospheric trace gas sensing and exoplanet exploration, where the simultaneous detection of numerous absorption features is essential,” said Vodopyanov.
Dual-Comb Spectroscopy
To utilize UV frequency combs containing a million closely spaced spectral lines for spectroscopy applications, the researchers needed a method capable of achieving high spectral resolution — beyond the capabilities of existing spectrometers. They turned to dual-comb spectroscopy, a powerful new technique that combines two frequency combs with slightly different line spacings on a single detector, producing interferograms. By applying a Fourier transform, the entire spectrum can be reconstructed with exceptionally high spectral resolution and rapid data acquisition.
"Although, over the past decade, dual-comb spectroscopy has made significant progress in the mid-infrared and terahertz regions, a notable gap remains in the UV spectral range, where existing demonstrations fall short in terms of resolution, bandwidth or both," said Vodopyanov.
To address this challenge, the researchers developed a laser platform that generates highly coherent ultrafast infrared pulses at a wavelength of 2.4 µm. Using a nonlinear crystal, they produced the 6th and 7th harmonics, resulting in two UV bands: the 6th harmonic covering approximately 1,000,000 spectrally resolved comb lines and the 7th harmonic containing around 550,000. This yielded two UV spectral ranges spanning 372–410 nm and 325–342 nm. To enable dual-comb spectroscopy, they replicated the broadband UV frequency comb system, allowing for further refinement of the UV comb's structure
Precision Spectral Lines
By referencing the spectral lines to an atomic clock, the researchers ensured that they could perform highly precise spectroscopic measurements, suitable for the most demanding applications.
As a demonstration, they used the dual-comb spectroscopy system to measure the narrow reflection spectrum of a volume Bragg grating mirror made by IPG/OptiGrate. The new system achieved a resolving power of 10,000,000, which the researchers say is far superior to existing grating and Fourier spectrometers.
Next, the researchers aim to extend the technology to even deeper UV regions, potentially down to a wavelength of 100 nm.