Tunable bandpass imaging filter based on resonant tunneling through a ball lens assembly

We describe a ball lens assembly, which functions as a broadly tunable bandpass filter and polarizer with imaging capabilities. The physical basis is resonant tunneling of light through an air gap between two half-ball lenses symmetrically coated by few-layer (Si/SiO₂ or Ta₂O₅/SiO₂) admittance matching stacks. Tuning is achieved by simultaneous adjustments of the incident angle and the air gap thickness. Individual filters with operational ranges spanning visible (∼400-700nm) and near-infrared (∼1000-1800nm) wavelengths were assembled using 10 mm diameter lenses. We show that these filters, configured as a stand-alone scanning spectrometer, can provide a resolving power ∼100 and f-number ∼2.5 for a fiber-compatible input aperture <15µm in diameter. We also demonstrate that, with supplementary optics, the tunable ball filter might be used to implement a compact hyperspectral imaging system.