  One-dimensional (1D) metal-dielectric (MD) periodic structures take advantage
of large refractive index contrast between metal and dielectrics to invoke
extremely high nonlinear ultrafast responses of metal. These structures are
also special due to their extremely high laser damage threshold. The Bragg like
1D MD structure (Ag/SiO2)4 enables strong optical field confinement with much
enhanced nonlinear features as compared to simple metal or single (Ag/SiO2)1
structure. In the present work, the ultrafast nonlinear optical responses of
the above structures are investigated via femtosecond broadband optical
pump-probe technique. The enhanced nonlinear optical absorption is of reverse
saturation of absorption (RSA) nature, resulted due to free-carrier absorption
(FCA) and excited-state absorption (ESA) processes. The spectral nonlinearities
are closely related to the pump-induced modification of the metal's dielectric
functions, which are qualitatively visualized by transfer matrix and
two-temperature models. The ultrafast temporal evolution of nonlinear
absorption clearly demonstrated enhanced optical nonlinearity, disentangled by
the electron-electron and electron-phonon dynamic interactions at picosecond
time scales. A phenomenological pulse propagation model is employed that
incorporates the experimentally obtained nonlinear absorption coefficients and
different nonlinear effects exhibited by the system. Nonlinearity plays a
crucial role in controlling the ultrafast pulse propagation and could open a
new window for many nonlinear device applications. The findings of these new
optical materials could possibly pave the way for promising applications in
ultrafast photonics.
