  In order to construct a dielectric analog of spaser, we study theoretically
and experimentally several configurations of cluster-based unit cells for an
all-dielectric metasurface characterized by resonant conditions of the trapped
mode excitation. Excitation of the trapped mode is realized by either specific
displacement of particles in the cluster, or perturbation of the equidistantly
spaced particles by off-centered round holes or coaxial-sector notches. It
turns out that the latter approach is more advantageous for enhancement of
electric near-field with homogeneous distribution in-plane of the structure and
its strong localization outside the high-refractive-index dielectric particles.
This feature opens prospects for realization of highly desirable subwavelength
flat lasing structures based on strong near-field interaction with substances
exhibiting pronounced nonlinear characteristics and properties of gain media.
