Spectrometers typically use optical elements such as diffraction gratings and prisms to achieve the dispersion necessary to measure the properties of light over a specific portion of the spectrum. The use of free-space optical elements, along with free-space propagation volume, results in bulky devices that are incompatible with on-chip integration technology. Instead of using dispersive optical components, researchers at Heriot-Watt University tailored and controlled dispersion using a single metasurface based on a multifoci metalens design. The researchers developed a compact, high-resolution spectrometer that could enable spectral analysis and information processing with an on-chip, integrated photonics platform. The researchers’ metalens design is based on wavelength and phase multiplexing, and it can simultaneously realize wavelength splitting and light focusing. The metasurface spectrometer can split and focus light beams of different wavelengths at predesigned positions on a focal plane, accurately mapping wavelength information into its focal points located on the same plane. Further, it can do so with a high-resolution dispersion control, and under the illumination of both monochromatic and polychromatic light beams. The single-layer metasurface consists of gold nanorods, with spatially variant orientations, sitting on a glass substrate. Upon the illumination of left circularly polarized light, the right circularly polarized light is converged at a ring that is designed to incorporate multiple focal points. Each focal point corresponds to a different incident wavelength. According to the researchers, the group first considered the phase profile of a metalens that can generate a single focal point at the desired position governed by Fermat’s principle. Individual phase profiles of different metalenses with a single focal point can be integrated into one combined phase profile, the researchers said. Once this is achieved, the wavelength of each focal point is considered a design variable, and it is added to the phase profile of the multifoci metalens. The researchers said that the larger samples of the metasurface enable accurate linewidth detection and continuous spectrum recognition. The increase in the sample size increases the number of wavelength-dependent focal points. This in turn boosts resolution and the working bandwidth of the instrument. The ultracompact, ultrathin metasurface spectrometer demonstrated nanometer spectral resolution over a broadband visible domain at a working distance of 300 μm. To the best of the researchers’ knowledge, this is the first experimental demonstration of multifoci dispersion engineering using a single metasurface for the implementation of an ultrathin, compact spectrometer. A combination of design flexibility and the device’s ultrathin nature make the spectrometer an attractive choice for monolithic on-chip integration with sensor technology, the researchers said. Applications include information security and information processing, they said. Due to the easy fabrication of such ultrathin devices and their potential for integration with electronics and sensors, the metasurface spectrometer could lead to the development of compact spectrometry applications for fields such as material characterization, disease diagnostics, and quality control, where stringent weight and volume constraints exist. The research was published in Light: Science & Applications (www.doi.org/10.1038/s41377-023-01148-9).
