Surprisingly large fluorescence enhancements via lossless all-dielectric spherical mesocavities

Nano- and micro- particles are popular media to enhance optical signals, including fluorescence from a dye proximal to the particle. Here we show that homogeneous, lossless, all-dielectric spheres with diameters in the mesoscale range, between nano- ($\lesssim 100$~nm) and micro- ($\gtrsim 1$ $\mu$m) scales, can offer surprisingly large fluorescence enhancements, up to $F\sim 10^4$. Our numerical simulations reveal that the enhancement originates from multipolar ($4 \lesssim \ell \lesssim 10$) resonances, which induce strong electric field enhancement within spheres without requiring the engineering of sophisticated shapes, precise nanogaps, generation of hot spots, or designer metasurfaces. The order, $\ell$, of these resonances is larger than conventionally utilized dipolar or quadrupolar Mie resonances in nanoparticles and smaller than of typical whispering gallery modes ($\ell \gtrsim 20$) in microparticles. With the absence of non-radiative Ohmic losses inherent to plasmonic particles, we show that $F$ can increase, decrease or even stay the same with increasing intrinsic quantum yield $q_0$, for suppressed, enhanced or intact radiative decay rates of a fluorophore, respectively. Further, the fluorophore may be located inside or outside the particle, providing additional flexibility and opportunities to design fit for purpose particles. The presented analysis with simple dielectric spheres should spur further interest in this less-explored scale of particles and experimental investigations to realize their potential for applications in imaging, molecular sensing, light coupling, and quantum information processing.