  Lanthanide-doped upconversion nanoparticles emerged recently as an attractive
material platform underpinning a broad range of innovative applications such as
optical cryptography, luminescent probes, and lasing. However, the intricate
4f-associated electronic transition in upconversion nanoparticles leads only to
a weak photoluminescence intensity and unpolarized emission, hindering many
applications that demand ultrabright and polarized light sources. Here, we
uncover a new strategy for achieving ultrabright and dual-band polarized
upconversion photoluminescence. We employ resonant dielectric metasurfaces
supporting high-quality resonant modes at dual upconversion bands enabling
two-order-of-magnitude amplification of upconversion emissions. We demonstrate
that dual-band resonances can be selectively switched on polarization, endowing
cross-polarization controlled upconversion luminescence with ultra-high degrees
of polarization, reaching approximately 0.86 and 0.91 at dual emission
wavelengths of 540 nm and 660 nm, respectively. Our strategy offers an
effective approach for enhancing photon upconversion processes paving the way
toward efficient low-threshold polarization upconversion lasers.
