Assembly-Induced Photon Confinement and Recirculation in Upconversion Superparticles

Area of Science:

• Nanoscience and Materials Science
• Photonics and Optical Engineering

Background:

• Controlled self-assembly of nanoparticles into functional microscale architectures is a significant challenge.
• Upconversion nanoparticles (UCNPs) offer unique optical properties but require precise structural organization.

Purpose of the Study:

• To develop a novel emulsion-mediated strategy for assembling lanthanide sodium fluoride UCNPs into spherical superparticles.
• To investigate the influence of solvent polarity and UCNP shape on the self-assembly process and resulting superparticle properties.
• To demonstrate the capability of these superparticles for low-threshold stimulated emissions.

Main Methods:

• Utilizing an emulsion system with UCNP-containing cyclohexane droplets dispersed in an isopropanol/H2O medium.
• Controlling solvent diffusion to drive UCNP condensation into superparticles.
• Varying solvent polarity and UCNP morphology (quasi-spherical, cubic, hexagonal platelets) to study self-assembly outcomes.

Main Results:

• Achieved controlled self-assembly of UCNPs into smooth spherical superparticles by optimizing solvent polarity.
• Demonstrated that UCNP shape dictates superparticle surface roughness (smooth for quasi-spherical, rough for cubic/platelet).
• Observed low-threshold stimulated emissions (threshold < 4 W/cm², Q-factor > 10⁴) from spherical superparticles, with hybrid versions emitting across visible to NIR spectra.

Conclusions:

• The emulsion-mediated strategy offers a robust method for fabricating UCNP superparticles with tunable properties.
• Superparticle structure and composition directly influence optical functionalities, particularly stimulated emission.
• These UCNP superparticles serve as promising platforms for advanced integrated photonic devices.