Asymmetric Dielectric Metasurfaces for Upconversion Lasers

Upconversion lasers capable of lasing at low pump intensities would enable interesting applications including passive night vision goggles. Fernandez-Bravo et al. demonstrated such a laser based on upconverting nanoparticles (UCNPs) and a plasmonic nanostructure in which the lattice plasmon mode was coupled to the emission from UCNPs [1]. In this work, we present an asymmetric dielectric metasurface with resonances at both the emission and absorption wavelengths of UCNPs to further enhance the performance of these lasers. This titanium dioxide (TiO 2 )-based metasurface, as shown in Fig. 1(a), consists of a periodic arrangement of nanocylinders with an asymmetric unit cell (three cylinders with radius $\mathrm{r}_{1}$ and one cylinder with radius $\mathrm{r}_{2}$ ). Such an asymmetry distorts the symmetry-protected bound states in the continuum and results in high-Q resonances [2]. Fig 1. (b) shows the tilted SEM image of a sample, which we fabricated following the process introduced in Ref. [3]. The transmission spectra of bare metasurface (no UCNP coating) obtained via simulation and experiment are plotted in Fig 1. (c). The resonances at 832 nm and 606 nm (in experiment) have quality factors of $\lambda/\Delta\lambda=92$ and 65, respectively. Our simulations show that these resonances red shift to the absorption wavelength (980 nm) and emission wavelength (660 nm) of UCNPs, respectively, after the addition of UCNPs.