  The donor-acceptor interaction of a charge-coupled heterostructure
encompassing a metal and an amorphous semiconductor subjected to a laser field
has many potential applications in the realm of nonlinear optics. In this work,
we fabricate an electron donor gold nanoparticle (AuNP) and acceptor amorphous
Ge24Se76 heterostructure on a quartz substrate using a sequential thermal
evaporation technique. In this charge-coupled heterostructure, we demonstrate
the ultrafast third-order nonlinear absorptive and refractive response and
their sign reversal compared to pristine Ge24Se76. Enhanced optical
nonlinearity in these heterostructures of varying plasmonic wavelengths is due
to charge transfer, verified by the Raman spectroscopy. Further, the ultrafast
transient absorption measurements support the thesis of charge transfer in the
AuNP/Ge24Se76 heterostructure. These findings open up exciting opportunities
for developing novel device technologies with far-reaching applications in
nonlinear optics.
