Color Crosstalk in Two-Color Mid-Infrared LEDs With and Without Cavity Enhancement

Abstract

Mid-infrared (3– $5~\mu $ m) LEDs have assumed greater importance optical gas sensors and have been explored for use in midinfrared LED arrays, in both cases to replace thermal pixels. Compared to thermal pixels, mid-infrared LEDs have near instantaneous settling times, achieve higher radiance, can have multi-spectral output, and are safer. Multispectral output creates the possibility of emission into narrowed bands for either sensing multiple gas species or creating dual emission thermal pixel arrays. However, their adoption for these applications is hindered by spectral crosstalk from emission tails at room temperature, and additionally by low efficiency, problematic in dense LED arrays with strict power density requirements. This work explores three approaches to designing two-color mid-infrared LED arrays, targeting reduced spectral crosstalk and lower power requirements: 1) monolithic two-color LEDs; 2) monolithic two-color cavity LEDs; and 3) filtered single-color cavity LEDs combined spatially. Performance metrics, such as power-to-temperature efficiency and radiance-to-crosstalk ratios, are compared across designs. Incorporation of cavities narrows emission, improves spectral radiance by 5– $10\times $ and overlap with the emission band, lowers power requirements by ~2– $3\times $ , and can reduce crosstalk. In-band to cross-band radiance ratio is generally limited to around 10– $1000\times $ for monolithic two-color devices; two single color devices allow external filtering which improves the ratio to $10^{5}$ – $10^{7}$ . Results provide a framework for use of mid-infrared LEDs in multi-gas sensing and two-color mid-infrared LED arrays.