Spectro­meters work by breaking down light waves into their different colors to provide information about the makeup of the objects being measured. UC Santa Cruz researchers are designing new ways to make spectro­­meters that are ultra-small but still very powerful, to be used for anything from detecting disease to observing stars in distant galaxies. Their inexpensive production cost makes them more accessible and customizable for specific appli­cations. The team of researchers is led by an inter­disciplinary colla­boration between Holger Schmidt and Kevin Bundy.

The researchers demonstrate a novel, extremely high-performance spectrometer that can measure light with a 0.05 nanometers wavelength resolution. That’s the same resolution that can be achieved on a device 1,000 times bigger. “That's essentially as good as a big, standard, expensive spectrometer,” said Schmidt. “That’s really pretty impressive and very competitive.”

Miniaturi­zing spectrometers is an active area of research, as spectro­meters are used in many fields but can be as big as a three-story building and extremely expensive. However, miniaturized spectro­meters often do not perform as well as bigger instruments, or they are very difficult and expensive to manu­facture because they require extremely precise nano­fabrication. UC Santa Cruz researchers have created a device that is able to achieve high performance without such costly manu­facturing. Their device is a miniature, high-powered waveguide which is mounted on a chip and used to guide light into a specific pattern, depending on its color. 

Information from the chip is fed into a machine learning algorithm that reads the patterns created by different wavelengths of light in order to reconstruct the image with extremely high accuracy and precision – an approach of recon­structive spectrometry. This technique produces accurate results because the machine learning algorithms don’t require highly precise input to be able to distinguish the light patterns, and can constantly improve upon their own performance and optimize themselves to the hardware. 

Because of this, the researchers can make the chips with relatively easy and inexpensive fabrication techniques, in a process that takes hours rather than weeks. The lightweight, compact chips for this project were designed at UCSC, and fabricated and optimized at Brigham Young University in partnership with Schmidt’s longtime colla­borator Aaron Hawkins and his undergraduate students. “Compared to more sophisticated chip design, this only requires one photo­lithography mask which makes the fabrication much easier and much faster,” Hawkins said. “Someone with some basic capa­bilities could reproduce this and create a similar device tuned to their own needs.”

The researchers envision that this technology can be used for a wide range of applications, though their preliminary focus is to create powerful instruments for astronomy research. Because their devices are relatively inexpensive, astro­nomers could specialize them to their specific research interests, which is practically impossible on much larger instruments that cost millions of dollars. The research team is working to make the chips functional on the UC-operated Lick Observatory telescope, first to take in light from a star and later to study other astro­logical events.

With such high accuracy on these devices, astronomers could start to understand phenomena such as the makeup of atmospheres on exoplanets, or probing the nature of dark matter in faint dwarf galaxies. The compara­tively low cost of these devices would make it easier for scientists to optimize them for their specific research interests, something nearly impossible on tradi­tional devices. Leveraging long standing expertise at UC Santa Cruz in adaptive optics systems for astronomy, the researchers are colla­borating to figure out how to best capture the faint glimmers of light from distant stars and galaxies and feed it through into the minia­turized spectro­meter.

“In astronomy, when you try to put something on a telescope and get light through it, you always discover new challenges – it’s much harder than just doing it in the lab. The beauty of this colla­boration is that we actually have a telescope, and we can try deploying these devices on the telescope with a good adaptive optics system,” Bundy said.

Beyond astronomy, the research team shows that the tool is capable of fluorescence detection, which is a noninvasive imaging technique used for many medical applications, such as cancer screening and infectious disease detection. In the future, they plan to develop the technology for Raman scattering analysis. This is a technique that uses light scattering for the detection of any unique molecule, often used as a specialized test to look for a specific chemical substance, such as the presence of drugs in the human body or toxic pollutants in the environ­ment. Because the system is so straight­forward and does not require the use of heavy instrumentation or fluidics like other techniques, it would be convenient and robust for use in the field. 

The researchers also demonstrate that the compact waveguides can be placed alongside each other to enhance the performance of the system, as each chip can measure a different spectra and provide more infor­mation about whatever light it is observing. So, the researchers demonstrate the power of four waveguides working together, but Schmidt envisions that hundreds of chips could be used at once. This is the first device shown to be able to use multiple chips at once in this way. The researchers will continue to work to improve the sensi­tivity of the device to get even higher spectral resolution. (Source: UCSC)

Reference: M. N. Amin et al.: Multi-mode interference waveguide chip-scale spectrometer, APL Photon. 9, 100802 (2024); DOI: 10.1063/5.0222100

Link: Applied Optics Group, ECE Dept., UC Santa Cruz, Santa Cruz, USA