Use of photonic jets produced by dielectric microspheres for increasing sensitivity and angle-of-view of MWIR detectors

Abstract

Recently, it was experimentally demonstrated (K.W. Allen et al., APL 108, 241108 (2016)) that microspheres can be used as contact microlenses to enhance the efficiency of collection of light by individual pixels in mid wave infrared (MWIR) focal plane arrays (FPAs). In this work, using finite difference time domain (FDTD) modeling, we optimized the designs of such FPAs integrated with microspheres for achieving maximal angle of view (AOV) as a function of the index of refraction and diameter of the spheres. We also designed structures where the spheres are partly immersed in a layer of photoresist. Our designs are developed for both front-side and back-side illuminated structures. Compared to standard microlens arrays, our designs provide much larger angle of view reaching ~15 degrees for front-illuminated and ~4 degrees for back-illuminated structures. Our designs allow decreasing the sizes of photosensitive mesas down to wavelength-scale dimensions determined by the minimal waists of the focused beams produced by the dielectric microspheres, so-called photonic jets. This opens a principle possibility to reduce the dark current and increase the operating temperature of MWIR FPAs. We also discuss the techniques of fabrication of such FPAs integrated with a large number of microspheres and show that suction assembly of microspheres is a promising method of obtaining massive-scale integration of microspheres onto the individual pixels with very small concentration of defects.