Fig. 4.

Microring resonator-assisted excitation of NaErF₄@NaYF₄ nanocrystals. a Schematic diagram of the set-up for the microring resonator-assisted excitation scheme. b Electrical field distribution in the waveguide structure when the incident light is in resonance (left) and out of resonance (right) with the microring resonator. c Emission spectra and optical micrographs (inset) recorded from an identical specimen in different resonance states. d Emission spectra of NaYF₄:Er (2−100%)@NaYF₄ nanoparticles as a function of Er³⁺ concentration under microring resonator-assisted excitation (1549.47 nm, 2300 kW cm⁻²). e Upconversion emission intensity at 382 nm as a function of excitation power density for NaYF₄:Er (2−100%)@NaYF₄ on microring resonator, demonstrating attenuation of UV emission with decreasing Er³⁺ contents