Nonlinear Optical Characterization of InP@ZnS Core-Shell Colloidal Quantum Dots Using 532 nm, 10 ns Pulses

InP@ZnS core-shell colloidal quantum dots (CQDs) were synthesized and characterized using the z-scan technique. The nonlinear refraction and nonlinear absorption coefficients (γ = -2 × 10^-12 cm² W^-1, β = 4 × 10^-8 cm W^-1) of these CQDs were determined using 10 ns, 532 nm pulses. The saturable absorption (β = -1.4 × 10^-9 cm W^-1, I_sat = 3.7 × 10⁸ W cm^-2) in the 3.5 nm CQDs dominated at small intensities of the probe pulses (I ≤ 7 × 10⁷ W cm^-2) followed by reverse saturable absorption at higher laser intensities. We report the optical limiting studies using these CQDs showing the suppression of propagated nanosecond radiation in the intensity range of 8 × 10⁷-2 × 10⁹ W cm^-2. The role of nonlinear scattering is considered using off-axis z-scan scheme, which demonstrated the insignificant role of this process along the whole range of used intensities of 532 nm pulses. We discuss the thermal nature of the negative nonlinear refraction in the studied species.