A new optical spectrometer is super-compact thanks to a metalens that focuses light at multiple wavelengths. The new device can detect light spectra with a resolution of 1 nm, and unlike its bulkier predecessors, it could be integrated onto a chip, with potential applications in security and information processing.
Imaging spectrometers that operate in the visible, near and short-wave infrared (VNIR/SWIR) regions of the electromagnetic spectrum are routinely deployed in fields such as atmospheric science, ecology, geology, agriculture and forestry. They work by recording a series of monochromatic images, then analysing their spectrum over a given imaging area.
One of the advantages spectrometers have over traditional cameras is that they are very good at controlling chromatic aberrations such as blurring or distortion that come about because light is spread over a region of space (dispersion) rather than focused to a point. They can do this because they contain additional optical elements such as a diffraction grating or a prism rather than just a lens and a detector.
The snag, however, is that these extra elements, together with the large volume over which the light must propagate, makes spectrometers relatively bulky. That means they can’t be carried on small satellites or drones.
A compact spectrometer
Researchers led by Xianzhong Chen from the Institute of Photonics and Quantum Sciences at Herriot-Watt University, UK, have now developed a compact spectrometer that can detect light spectra with a resolution of 1 nm over a broad range of wavelengths. It accomplishes this feat thanks to a planar nanostructure called an optical metasurface that can manipulate the amplitude, phase and polarization of incident light on a subwavelength scale.
The metasurface in the Herriot-Watt device consists of gold nanorods that are patterned atop an indium tin oxide-coated silicon dioxide substrate using standard electron beam lithography and lift-off processes. An individual device measures just 300 µm x 300 µm, and Chen explains that it is based on a novel lens design that accurately maps the wavelengths of an incident light beam to different positions on the flat focal plane of the lens. Using this design, it is possible to split and focus light with high-resolution control over its dispersion.
According to the researchers, who describe the new metalens spectrometer in Light Science & Applications, the compact and ultrathin nature of their device mean that it could be used in on-chip integrated photonics where spectral analysis and information are processed in a compact platform.
New metasurface lens banishes chromatic aberration
“Optical spectroscopy plays a very important role in various fields of science and technology with a wide range of practical applications,” Chen says. “The approach we propose is flexible and robust and provides a new scheme to control dispersion under illumination of both mono- and polychromatic incident light beams,” he tells Physics World.
The researchers are now working on improving the resolution and operating bandwidth of their metasurface spectrometer by increasing the sample size to millimetre scales. “This will increase the radius of the multi-foci ring and include more wavelength-dependent focal points,” Chen says.
