Angle-tunable air-gap Fabry-Pérot cavities for inline spectroscopy and hyperspectral imaging

In conventional spectrometers and hyperspectral imaging systems, the angle-of-incidence dependence of the spectral response is typically regarded as undesirable, as it leads to unintended spectral shifts that require calibration across the field of view. Here, we demonstrate a spectral-scanning platform enabling both inline spectroscopy and hyperspectral imaging by harnessing the strong angular dependence of an air-gap Fabry-Pérot (FP) cavity as an intrinsic degree of freedom for spectral tuning. We show that engineering the refractive index of the cavity region to be equal to or lower than that of the incident medium significantly enhances the angular sensitivity of the FP resonance, enabling broadband spectral tuning from the short-wave infrared to the visible regime. By integrating the air-gap cavity with a photodetector, we demonstrate inline spectroscopy. We further demonstrate transmission hyperspectral imaging by placing the tunable filter in the illumination path before a patterned transmissive target and recording images with a monochrome camera. The air-gap cavity, fabricated using a simple SU-8-assisted wafer bonding process, provides a practical spectral-scanning element for compact spectroscopy and transmission imaging under controlled illumination.