Wavelength-scale noise-resistant on-chip spectrometer

Performant on-chip spectrometers are important for advancing sensing technologies, from environmental monitoring to biomedical diagnostics. As device footprints approach the scale of the operating wavelength, previously strategies, including those relying on multiple scattering in diffusive media, face fundamental accuracy constraints tied to limited optical path lengths. Here, we demonstrate a wavelength-scale, CMOS-compatible on-chip spectrometer that overcomes this challenge by exploiting inverse-designed quasinormal modes in a complex photonic resonator. These modes extend the effective optical path length beyond the physical device dimensions, producing highly de-correlated spectral responses. We show that this strategy is theoretically optimal for minimizing spectral reconstruction error in the presence of measurement noise. The fabricated spectrometer occupies a lateral footprint of only 3.5 times the free-space operating wavelength, with a spectral resolution of 10 nm across the 3.59-3.76 micrometer mid-infrared band, which is suitable for molecular sensing. The design of this miniaturized noise-resistant spectrometer is readily extensible to other portions of the electromagnetic spectrum, paving the way for lab-on-a-chip devices, chemical sensors, and other applications.