Fig. 1. Schematic diagrams of different gratings. (a) Offner convex grating imaging system^[18]; (b) grating in the YZ and YX planes of OAC^[24]

Fig. 2. Optical path diagram of +1 diffraction convex grating imaging spectrometer^[27]

Fig. 3. Optical path diagram of typical concave grating spectrometer^[28]

Fig. 4. Schematic diagram of the vacuum imprinting process for fabricating VLS concave grating^[30]

Fig. 5. Structures of different spectrometers^[34]. (a) Flat-field concave holographic grating structure; (b) crossed C-T structure

Fig. 6. Schematic diagram of a cross-dispersed broadband spectrometer^[37]

Fig. 7. Echelle grating^[39]. (a) Main interface of echelle grating; (b) crossed dispersion principle of echelle grating spectrometer

Fig. 8. Optical system of long slit echelle grating spectrometer^[42]

Fig. 9. Typical optical path diagram of Fourier transform spectrometer^[51]

Fig. 10. Schematic diagrams of optical systems for typical miniature spectrometers^[57]. (a) Based on traditional grating; (b) based on Fresnel grating

Fig. 11. Schematic diagram of optical path for solid immersion grating microspectrometer^[61]

Fig. 12. Optical system of microspectrometer for Chl-α fluorescence detection^[63]

Fig. 13. Common filter arrangement schemes^[64]. (a) Schematic diagram of microspectrometer scheme based on filter array; (b) filtering strategy based on waveguide ring resonator

Fig. 14. Schematic diagram of the architecture composed by four interferometers^[68]

Fig. 15. Schematic diagram of principle for single nanowire spectrometer^[70]

Fig. 16. Schematic diagram of principle for on-chip spatial-heterodyne Fourier transform spectrometer^[73]

Fig. 17. Schematic diagram of spectrally tunable interleaved nanophotonic metasurface lenses based on compound eyes of long-legged flies^[74]