To cite this version:Photon management has enabled a true revolution in the development of high-performance semiconductor materials and devices. Harnessing the highest amount of energy from photon relies on an ability to design and fashion structures to trap the light for the longer time inside the device for more electron excitation. The light harvesting efficiency in many thin-film optoelectronic devices is limited due to low photon absorbance. Here we demonstrate for the first time that slow photon circulation in sandwich-structured photonic crystals with two stopbands fine tuned are ideally suited to enhance and spectrally engineer light absorption. The sandwich-structured TiO 2 inverse opal possesses two stopbands, whose blue or red edge is respectively tuned to overlap with TiO 2 electronic excitation energy, thereby circulating the slow photons in the middle layer and enhancing light scattering at layer interfaces. This concept, together with the significantly increased control over photon management opens up tremendous opportunities for the realization of a wide range of highperformance, optoelectronic devices, and photochemical reactions.

and artificial photosynthesis systems, and to the design of light-emitting devices, respectively. [1] Light-matter interactions are intrinsically multiscale; therefore, hierarchically structured materials integrating features of nano-to microlength scales exhibit unique and often advantageous optical properties. [2] Photonic beads (PhBs), or spherical photonic crystals (PhCs), represent one of these hierarchical nanomaterials exhibiting intricate light-matter interactions. [3] While the periodic dielectrics contrast presented in photonic crystals induces a forbidden region for electromagnetic waves, namely the photonic bandgap (PBG), additional light-matter interactions observed on photonic beads arise from the spherical geometry. These interactions are 1) isotropic photonic bandgap properties versus the angledependent photonic bandgap in 2D and 3D photonic crystals, hence, representing materials with full bandgap; [4] 2) grating diffraction on curved surfaces; [3b] and 3) heterogeneous light-matter interactions from crystalline outer shell to internal random dense packing. [3b] In addition to photonic bandgap per se, the band edges have also been found to induce intricate light-matter interactions.Since the photons at the bandgap edges possess reduced group velocity, resulting in a light-trapping effect called the "slow photon," [5] the bandgap edge photons would have stronger Light management is of paramount importance to improve the performance of optoelectronic devices including photodetectors, optical sensors, solar cells, and light-emitting diodes. Photonic crystals are shown as an effective metamaterial for trapping light among their various photon management functions. Herewith, it is demonstrated that spherical photonic crystals, or in other words, photonic beads, possess a stronger light-trapping effect compared to the planar counterpart. The photonic beads are fabricated by colloidal self-assembly under microdroplet confinement employing microfluidic devices. The light-matter interactions are illustrated by the emission intensity and lifetime of the embedded emitters, namely carbon dots and upconversion nanoparticles (UCNPs). The bandgaps of the photonic beads are selected according to the emission and excitation peaks of the light emitters, whereby the emission or excitation peak overlaps the blue edge or red edge of the photonic bands, respectively. Significantly stronger emission and extended luminescence lifetime are observed in photonic beads ensemble in comparison to the planar photonic crystals, demonstrating enhanced light trapping owing to the spherical geometry, which introduces additional microcavity effect.

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