Scalable on-chip diffractive speckle spectrometer with high spectral channel density.

The chip-scale integrated spectrometers are opening new avenues for a much wider range of applications than their conventional benchtop counterparts. While spectral reconstruction should be in command of both spectral resolution and bandwidth, a large number of spectral channels is among the key goals of the spectrometer design. However, the chip footprint eventually limits the spectral channel capacities of well-established spectral-to-spatial mapping structures like dispersive elements, filter arrays, random media, and so on. Here we suggest an alternative scheme by encoding the spectral information using on-chip diffractive metasurfaces. The in-plane metasurface is capable of producing intensity speckles to resolve the spectra. The spectral richness is greatly increased by scaling the architecture via three layers of cascaded metasurfaces. The readout of speckles is realized by two-dimensional imaging of the grating-diffracted pattern, enabling a large matrix for spectrum reconstruction. The spectrometer has a resolution of 70 pm over a bandwidth of 100 nm. Up to 1400 spectral channels were obtained within a compact chip area of only 150 μm × 950 μm. The on-chip diffractive spectrometer has a benchmark channel density of up to 10021 ch/mm, which compares favorably against other state-of-art waveguide structures.