Design of Microlens Arrays for Thermal Imaging with Spintronic Poisson Bolometers

Abstract

Infrared (IR) detectors are widely used for their ability to sense thermal radiation. Recently, a room-temperature infrared detector known as the spintronic Poisson bolometer was introduced. Operating in a probabilistic regime governed by Poissonian counting statistics, it establishes a fundamentally different detection mechanism with the potential to beat conventional sensitivity limits. While offering fast digital readout, its sensitivity is currently limited by a small active area and array fill factor. In this work, we present design guidelines for spherical plano-convex microlens arrays that enhance light collection in spintronic Poisson bolometer arrays in the mid-wave infrared (MWIR). Guided by the simulations, we fabricate a microlens array sample to demonstrate that the chosen geometrical parameters are realistic and compatible with the fabrication process. A unique radiometric-stochastic model is used to quantify the resulting sensitivity improvements. Our work is the first systematic integration of microlens design with spintronic Poisson bolometer arrays, bridging microphotonics, spintronics, and thermal imaging.

Figures (5)

• Figure 1: Schematic of the imaging system and spintronic Poisson bolometer device structure.
• Figure 2: (a) Plano-convex spherical microlens geometrical parameters. (b) E-field distribution of a microlens with 4 lens sag and 30 diameter. The white dashed line marks the focal length. (c) Profile at the focus. (d) Focal length versus microlens diameter. (e) FWHM spot size versus microlens diameter.
• Figure 3: (a) Microscopic image of the microlens array sample. The bright spot is the alignment beam of the FTIR spectroscopy. (b) Profilometry of the microlens with 34 diameter. (c) Surface profile comparison with a perfect spherical shape. (d) Transmission spectra measured by FTIR spectroscopy in the MWIR.
• Figure 4: (a) Collection efficiency, (b) concentration factor, and (c) normalized power incident on sensor active area of Al$_2$O$_3$ spherical plano-convex microlenses in MWIR, as a function of microlens diameter and sensor active size.
• Figure 5: Spintronic Poisson bolometer full-image simulation with microlens arrays. (a) Thermal scene with 300 K temperature emits infrared photons in the 3-5 range. Here, we assume there is one pixel behind each microlens. (b) NEDT versus microlens diameter. (c) A better contrast is shown with a smaller NEDT.