Critical dimension where the macroscopic definition of refractive index can be applied at a nanometric scale

This Brief Report reports a measurement of the critical dimension where the macroscopic definition of refractive index could be applied at nanometric scale. A fluorescent nanolayer is used as a local probe of the refractive index via its radiative lifetime. By covering the emitting layer with nanofilms of high refractive index material the sphere radius R in which the effective-medium theory has to be applied is estimated as λ/4 in air.