Ultralow-threshold upconversion infrared vision via a microsphere-mediated directional photofield

Constructing micro-/nanostructure-modulated photofields in upconversion devices to absorb low-energy photons and emit high-energy light is revolutionary for bioimaging, lasers, and photovoltaics, with proven capability to boost upconversion luminescence (UCL) by orders of magnitude. However, photoenergy dissipation and inadequate absorption result in excitation thresholds exceeding 1 mW/cm², which exceeds retinal safety limits and hinders wearable upconversion optics. Here, we report the use of upconversion core-shell microsphere-induced infrared field convergence, NaYF₄:Yb,Er shell-based resonant cavities for multiple reflection-absorption-upconversion and photonic crystal amplifiers to improve UCL intensity three orders of magnitude, and achieve ultralow threshold (0.0025 mW/cm²). The 500 nm upconversion core-shell microspheres generated 1200-fold stronger electric field through concentrated photofield and attained 8-fold infrared absorption with a forward/backward emission ratio of 150. Fabricated upconversion contact lenses significantly improved dark-light imaging clarity and vision restoration in retinal degeneration rabbits. Microsphere-mediated directional upconversion strategy maximizes photoenergy utilization, paving the way for high-performance wearable upconversion devices.