  Nonlinear optical (NLO) response in terms of reverse saturable absorption
(RSA) has been exploited extensively for optical limiting. Here, we
experimentally demonstrate that flower-like hybrid nanostructures of NiCo2O4
and Multiwall Carbon Nanotubes (NCO@MWCNT) exhibit a strong non-linearity in
their absorption, specifically an excited state absorption (ESA) induced RSA,
when exposed to nanosecond laser pulses. We obtain strong nonlinear absorption
coefficient (\b{eta}) and nonlinear refractive index (n2) in hybrid NCO@MWCNT,
the values that are 2-times and 2-orders of magnitude higher, respectively
compared to NCO or MWCNT alone. This offers straightforward application in
optical limiting with optical limiting (FOL) and optical onset (FON) threshold
ca. 2-10 times lower than benchmark NLO materials, e.g., graphene, family of
2-D transition metal dichalcogenides (TMDC) materials and recently established
NCO. Notably, for femtosecond pumping, NLO response of NCO@MWCNT is dominated
by saturable absorption (SA) with a week contribution from two photon
absorption (TPA), arising respectively from MWCNT and NCO. A remarkable sign
reversal along with a larger amplitude is obtained for n2 thanks to the charge
transfer from MWCNT to NCO.
