Recent advances in the deposition of patterned thin film spectral filters have enabled a new class of radically miniaturized spectral sensors. This new technology enables numerically large arrays of spectral bandpass filters with unprecedented manufacturing economy. For example, a 64-channel array occupying two square millimeters and spanning 400-900 nm can be deposited with as few as eight coating steps. Mating this filter array to a photodiode array yields a tiny multispectral sensor with diverse applications.
The bandpass filters are single-cavity Fabry-Perot designs with common top and bottom mirrors. The dielectric spacer layer between them determines the passband wavelength and is patterned to differing thicknesses using a binary scheme, i.e., each successive “sub-spacer” layer is half the thickness of the previous one. The technical challenge is uniformly patterning and depositing thinner and thinner sub-spacers, which can be only a few nanometers thick. We have demonstrated 64-channel arrays covering the spectral range of 400-900 nm and 775-1075 nm.
These arrays have been mated to high-responsivity 2D silicon detectors, in much the same way that linear variable filters are mated to linear detector arrays. The resulting sensor is less than 3 x 3 x 1 mm in size and ideal for integration into mobile devices, wearable electronics, autonomous aerial vehicles, and countless industrial applications. Sensor performance is currently being evaluated for food quality and freshness measurement, drug identification, fuel quality measurement, explosives detection, colorimetry and illumination measurement, solar flux monitoring, remote sensing, and myriad other applications.