  Conventional cameras, such as in smartphones, capture wideband red, green and
blue (RGB) spectral components, replicating human vision. Multispectral imaging
(MSI) captures spatial and spectral information beyond our vision but typically
requires bulky optical components and is expensive. Snapshot multispectral
image sensors have been proposed as a key enabler for a plethora of MSI
applications, from diagnostic medical imaging to remote sensing. To achieve
low-cost and compact designs, spatially variant multispectral filter arrays
(MSFAs) based on thin-film optical components are deposited atop image sensors.
Conventional MSFAs achieve spectral filtering through either multi-layer stacks
or pigment, requiring: complex mixtures of materials; additional lithographic
steps for each additional wavelength; and large thicknesses to achieve high
transmission efficiency. By contrast, we show here for the first time a
single-step grayscale lithographic process that enables fabrication of bespoke
MSFAs based on the Fabry-Perot resonances of spatially variant
metal-insulator-metal (MIM) cavities, where the exposure dose controls
insulator (cavity) thickness. We demonstrate customizable MSFAs scalable up to
N-wavelength bands spanning the visible and near-infrared with high
transmission efficiency (~75%) and narrow linewidths (~50 nm). Using this
technique, we achieve multispectral imaging of several spectrally distinct
target using our bespoke MIM-MSFAs fitted to a monochrome CMOS image sensor.
Our unique framework provides an attractive alternative to conventional MSFA
manufacture, by reducing both fabrication complexity and cost of these
intricate optical devices, while increasing customizability.

Applied Physics;Instrumentation and Detectors;Optics