Fig. 1. Main regulation methods of rare-earth-doped upconversion luminescence^[38-44]

Fig. 2. Research results of UCNPs luminescence regulated by plasmonic nanocavities. (a) Luminescence intensities and lifetimes of UCNPs regulated by different plasmonic nanocavities; (b) various plasmonic nanocavities, (i) tilted silver nanocube-silver film nanocavity^[45], (ii) silver nanocube-gold film nanocavity^[46], (iii) gold nanorod nanocavity^[47], (iv) gold nanocrystalline nanocavity arrays^[32], (v) gold nanodisc-gold film nanocavity arrays^[48], (vi) monocrystal silver microplate structure^[36], (vii) gold microplate-silver microplate nanocavity^[49], (viii) gold nanofilm coated fiber cavity^[50], (ix) silver nanocube^[51]

Fig. 3. Design of a tilted-nanocavity-coupled UCNP^[45].(a) Schematic of the tilted-nanocavity-coupled UCNP; (b) TEM image of a cut through a nanoscale-thick foil containing the optimized tilted-nanocavity-coupled UCNP; (c) an enlarged view of the area outlined by the white box in Fig.(b); (d) energy-level diagram of Er³⁺ showing the major transitions of the upconversion process; (e) UCL spectra of the tilted-nanocavity-coupled UCNP, UCNPs on the silver plate and UCNPs on the glass slide (red curve shows the measured dark-field scattering spectrum); (f) normalized time-resolved luminescence spectrum of the tilted-nanocavity-coupled UCNP (red points) and instrument-response function (IRF) curve of the avalanche photodiode detector (grey curve)

Fig. 4. Upconversion process is significantly accelerated and enhanced using the tilted-plasmonic nanocavity^[45]. Simulated (a) x-y plane and (b) y-z plane spatial maps of the radiative-rate enhancement (γ_r/γ₀) of the tilted-nanocavity-coupled UCNP; (c) simulated x-y plane spatial map of the quantum-efficiency enhancement (η/η₀) of the tilted-nanocavity-coupled UCNP; (d) ESA enhancement factor induced by the local near-field in the gap area; (e) dependence of luminescence intensity on laser excitation power density at the 653 nm emission wavelength for the UCL from the nanocavity (red dots) and UCNPs on glass (blue dots); (f) experimental and simulated luminescence enhancement factors relative to glass as references (error bar represents the standard deviations of the experimental enhancement factors from five randomly selected nanocavities. The mean value and the standard deviation are 6.2×10⁴ and ±7.1×10³, respectively. The inserted three-dimensional histogram represents the simulated enhancement factor, which is considered the product of the quantum-efficiency enhancement f_em, excitation enhancement f_ex, and enhanced light extraction α)

Fig. 5. Far-field directional emission enhanced by nanocavity^[45]. Measured back-focal-plane images of the UCL (a) in the tilted-nanocavity and (b) on a glass substrate (insets: schematics of the structures); (c) simulated (lines) and experimental (area fill diagrams) angular θ distribution radiation patterns of the UCL in the nanocavity (red) and on glass (green) (red and green lines are the simulated results of UCL in the nanocavity and on the glass, respectively. The red and green area fill plots are experimental results. The blue area fill plot represents the experimentally detectable range of angular θ)

Fig. 6. Chiral control for UCL^[45]. (a) Spectra of LCP and RCP UCL, and the corresponding UCL chirality g factor for the nanocavity (blue) and for UCNPs on glass (light grey); (b) square of the electromagnetic-field distribution in the x-y plane for LCP-RCP (excitation-DCP) for the 650 nm and 550 nm emission from the tilted-nanocavity; (c) simulated emission-DCP of chiral radiative-rate enhancement at 650 nm in the tilted-nanocavity (square dotted outlines represent the silver nanocube boundary, and the dotted circle represents the UCNP)