Speckle-Engineered Upconversion Amplification in Nanoemulsion-Templated Hydrogel Microdomes

Abstract

Hydrogel-based photonic systems integrating luminescent emitters offer promise as soft, reconfigurable optical platforms, yet most designs lack internal optical engineering to control light propagation and confinement. Here, we present a lithographically programmable softphotonic platform in which upconversion nanocrystals (UCNs) encapsulated within fluorocarbon nanoemulsion droplets are embedded in a poly(ethylene glycol) diacrylate (PEGDA) hydrogel microdome. Upon drying, strong refractive index contrast between the PEGDA matrix and fluorocarbon droplets creates a cooperative optical microenvironment that structures the near-infrared (NIR) excitation beam into a speckle-like field with localized hot spots while extending the photon dwell time within the microdome via internal reflection-based waveguiding. These effects yield a fully reversible, greater than sevenfold enhancement of upconversion luminescence—well beyond simple concentration or mechanical densification. This optical gain originates from multiple-scattering-assisted speckle excitation activated only in the contracted microdome state. Because UCNs are pumped by invisible NIR speckle illumination that rapidly varies in 3D across the microdome height, the incoherent sum of the photoluminescence manifests as a homogeneous filter-free visible brightness increase. The hydrogel microdomes, fabricated via a customized digital micromirror device (DMD)-based microlithography, enable high-resolution patterning of moisture-responsive displays, multicolor emission motifs, and reversible QR-code encryption, establishing a scalable route toward speckle-engineered soft photonic systems.