Subwavelength lasing mode in gain-assisted low-index dielectric spherical resonators

We investigate subwavelength lasing mode in low-refractive-index dielectric spherical resonators. Different from using metals or high-index materials to confine light in the spherical resonator, we uncover that adding gain introduces a non-trivial eigenmode solution, which can significantly shrink the size of the resonators. We term this solution the zero-phase solution because the round phase shift of the eigenmode inside the sphere is zero. Because this solution relies mainly on the amount of gain introduced rather than the wavelength, it exhibits a very unconventional behavior of lasing different structured light in the same sphere at the same wavelength through selective gain tuning.