We study, both experimentally and theoretically, the modification of
Er
3
+
photoluminescence properties in
Si
dielectric nanoslots. The ultrathin nanoslot (down to 5-nm thickness), filled with
Er
in
Si
O
2
, boosts the electric and magnetic local density of states via coherent near-field interaction. We report an experimental 20-fold enhancement of the radiative decay rate with negligible losses. Moreover, via modifying the geometry of the all-dielectric nanoslot, the outcoupling of the emitted radiation to the far field can be strongly improved, without affecting the strong decay-rate enhancement given by the nanoslot structure. Indeed, for a periodic square array of slotted nanopillars an almost one-order-of-magnitude-higher
Er
3
+
PL intensity is measured with respect to the unpatterned structures. This has a direct impact on the design of more efficient CMOS-compatible light sources operating at telecom wavelengths.