Observation of pulse-width dependent saturable and reverse saturable absorption in spinel ZnCo2O4 microflowers

We exploited intersystem crossing to demonstrate remarkably contrasting optical nonlinearity in ZnCo2O4 (ZCO) microflowers. Ultrafast transient absorption measurements reveal that intersystem crossing (ISC) from singlet to triplet state takes place in 5 ps. When the pulse-width is shorter than ISC lifetime for femtosecond excitation, saturable absorption (SA) takes place for all intensities. On the contrary, when the pulse-width 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.