  Third-order harmonic generation (THG) plays a vital role in microscopy,
optical communications etc. Conventional methods of obtaining efficient THG in
macroscopic crystal is already mature; however, they will finally limit the
miniaturization and integration of on-chip laser sources. To date, THG from
either photonic crystals or metamaterials provide compact photonic platforms,
however selection of materials remains elusive. Herein, we experimentally
demonstrate a giant enhancement of THG efficiency from an air/high index
Ge2Sb2Te5 (GST225) /gold multi-layered Fabry-Perot cavity. At cavity resonant
wavelength in near-infrared regime, the efficiency of THG from a 50 nm thick
amorphous GST225 planar film is boosted by 422 times compared to that of
nonresonant conditions. Interestingly, the THG efficiency has a dramatic
decrease of three orders when the structural state of GST225 is transformed
from amorphous to crystalline. Our findings have a potential for achieving
ultra-compact nonlinear optical source with high efficiency and switchable
functionality.
