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, previous strategies, including those relying on multiple scattering in diffusive media, face fundamental accuracy constraints due to limited optical path lengths and noise-sensitive random transmission matrices. Here, we demonstrate a wavelength-scale, CMOS-compatible on-chip spectrometer that overcomes the challenges 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 decorrelated spectral responses. We show that this strategy is theoretically optimal for minimizing spectral reconstruction error in the presence of 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 3590-3760 nm mid-infrared band, which is suitable for various 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.