All-Optical Photonic Crystal Bolometer with Ultra-Low Heat Capacity for Scalable Thermal Imaging

Abstract

High-speed thermal imaging in the long-wave infrared (LWIR) is critical for applications from autonomous navigation to medical screening, yet existing uncooled detectors are fundamentally constrained. Resistive bolometers are limited by electronic noise and the parasitic thermal load of wired readouts, while state-of-the-art nanomechanical resonators typically rely on vacuum packaging to maintain the mechanical $Q$ needed for sensitivity. Here, we introduce and demonstrate an uncooled thermal detector that addresses these challenges via an all-optical transduction mechanism. The heterogeneously integrated pixel is engineered for minimal thermal mass, combining pyrolytic carbon absorbers for broadband LWIR absorption, hollow zirconia structures for ultra-low-conductance thermal isolation, and a silicon photonic crystal cavity that serves as a high-$Q$ optical thermometer. Operating at ambient temperature and pressure, we measure a specific detectivity of $1.1\times10^{7}$ Jones and a thermal time constant of $27~μ\mathrm{s}$, corresponding to a speed that surpasses typical high-sensitivity uncooled technologies by an order of magnitude. Based on this detectivity measurement, which is limited by the noise floor of the external optical detection electronics, a physics-based model predicts a $>25\text{-fold}$ performance enhancement to its fundamental thermorefractive-noise-limited value ($3\times10^{8}$ Jones). The optical readout remains functional across ambient and vacuum environments. We expect this architecture to provide a general route toward scalable, high-performance thermal imaging systems.