Spectrometers are tools used to analyze the spectral composition of light governed by its origin, playing a crucial role in fields like astronomy. However, their size and complexity have often limited their use in space missions, which sometimes are the only way to study celestial bodies. 

A recent breakthrough might change all that. Researchers have unveiled a novel terahertz (THz) spectrometer that leverages cutting-edge technology, making it significantly smaller and lighter than traditional designs.

“The terahertz frequency region of the electromagnetic spectrum is crucial for understanding the formation and evolution of galaxies,” explained Wenye Ji, a doctoral student in the Optical Research Group at Delft University of Technology in the Netherlands. “However, current terahertz spectrometers face challenges such as low resolution, large volume, and complexity.”

Ji and his colleagues were able to create a centimeter-sized, low-weight terahertz spectrometer by taking advantage of metasurfaces. These are engineered materials with nanoscale structures that manipulate light in precise ways, enabling the miniaturization of optical devices without sacrificing performance. 

Spectrometers in astronomy

Spectrometers work by splitting incoming light into its individual wavelengths. Traditional designs achieve this using slits that deflect different wavelengths at varying angles. The separated light rays are then directed toward a detector, where their intensity is measured to construct a spectrum that can be used to determine vital information about its source.

In astronomy, terahertz radiation, with wavelengths about 1,000 times longer than visible light, holds unique importance. This radiation reveals critical details about the formation and evolution of stars, galaxies, and planets. For instance, the cool, dense regions of star-forming clouds emit terahertz radiation, providing insight into the physical and chemical conditions within these environments.

Furthermore, terahertz waves can penetrate cosmic dust that blocks shorter wavelengths, allowing astronomers to peer into the interiors of galaxies. However, Earth’s atmosphere is largely opaque to terahertz radiation, making it necessary to place spectrometers in space. 

But this is easier said than done, as traditional spectrometers require long optical paths for high resolution, resulting in bulky and heavy instruments that are unsuitable for space missions.

“The physical size and mass of optical components of a usual spectrometer, where the resolution is determined by the optical path length, are still an issue for space applications,” said Ji. “For example, four spectrometer modules for the future advanced space mission SAFARI-SPICA have a maximal physical size varying from 0.4 to 0.5 meters. The spectrometer becomes even more bulky and heavier when it aims to operate at longer wavelengths. 

“In order to meet the requirements of forthcoming space observatories operating in THz range, the development of a compact spectrometer with high resolution is imperative.”

Metasurfaces as a solution

To overcome these limitations, the research team applied metasurfaces to design a compact terahertz spectrometer. A metasurface is a two-dimensional material engineered to control electromagnetic waves. These surfaces consist of tiny structures called “meta-atoms,” which are smaller than the wavelength of light they interact with. Their subwavelength size allows them to manipulate light’s phase, amplitude, and direction more effectively than traditional optics, which rely on bulkier components like lenses and prisms

With the help of computer simulations that model how light interacts with various materials, the researchers designed a three-layer metasurface spectrometer for the 1.7–2.5 THz range. Their findings were published in the journal Laser and Photonics Reviews.

The core element of the device, the meta-atom, consists of three layers on a substrate. The bottom layer is a highly conductive, 300 nm-thick gold sheet. Above this is a 19 µm-thick dielectric layer made of non-conductive polyimide. The top layer is another gold sheet, 100 nm thick, patterned into double-anchor-shaped structures.

Fabricating the metasurface involved three main steps. First, a gold layer was deposited onto a silicon wafer. Next, a polyimide layer was spin-coated over the gold using a specialized centrifuge. Finally, a second gold layer was deposited onto the polyimide and patterned into periodic double-anchor shapes using photolithography and etching. The completed spectrometer featured a grid of 250 × 250 meta-atoms, each forming a square 45 microns across.

To validate the metasurface’s performance, the researchers irradiated it with laser beams operating at 2.10 THz, 2.11 THz, 2.20 THz, and 2.30 THz. The experimental results showed an almost exact match with the predictions from computer simulations. Minor discrepancies were attributed to differences in light coverage: simulations assumed the entire metasurface was illuminated, while the actual experiments used narrowly focused laser beams.

“Our results demonstrate for the first time a compact and light weight THz metasurface spectrometer concept,” said Ji. “This advance opens a new avenue for astronomical instruments for future space observatories.”

Future implications

The team envisions their metasurface spectrometer being integrated with advanced cryogenic detectors to create high-performance instruments for space missions. They are also exploring ways to extend the spectrometer’s frequency range by combining multiple metasurface devices into arrays, which could provide broader spectral coverage for studying a wider range of cosmic phenomena.

With its reduced size, weight, and complexity, this metasurface spectrometer could revolutionize space-based observations. By enabling more compact and cost-effective spacecraft, it has the potential to significantly advance our understanding of the Universe.

“The spectrometer system should be integrated with ultra-sensitive cryogenic detector arrays for space observatories, while with suitable room temperature 2D detector arrays for non-space applications,” Ji concluded.

Reference: Wenye Ji et al, Compact Metasurface Terahertz Spectrometer, Laser and Photonics Reviews (2024). DOI: 10.1002/lpor.202401290

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