  We employed open-aperture Z-scan technique to unveil the third-order optical
nonlinearity in ZnCo2O4 (ZCO) microflowers. Our results indicate that
intersystem crossing (ISC) lifetime can be used as simple tool to demonstrate
remarkably contrasting optical nonlinearity in ZCO. Ultrafast transient
absorption measurements reveal that ISC from singlet to triplet state takes
place in 5 ps. For femtosecond laser pulses, when the pulse duration is shorter
than ISC lifetime, saturable absorption (SA) takes place for all intensities.
On the contrary, when the pulse duration is longer than ISC for nanosecond
excitation, we observe transition from SA to reverse SA (RSA) at higher
intensities via excited-state absorption. We envisage that benefiting from SA
and RSA, ZCO emerges as potential candidate for mode locking and optical
limiting devices.
