Nonlinear Optical Properties of 2D CdSe Nanoplatelets in a Nonresonant Regime

The nonlinear optical response of CdSe and CdSe/CdS core/shell nanoplatelets (NPLs) in toluene suspension was determined by using Z-scan and optical Kerr gate (OKG) techniques at 800 nm in the femtosecond regime. First, an analysis of the nonlinearities based on the Z-scan resulted in n(2) = 1.4 x 10(-16) cm(2)/W and n(2) = 1.3 x 10(-16) cm(2)/W for CdSe and CdSe/CdS core/shell NPLs, respectively. Fifth-order nonlinear absorption behavior was predominant, resulting in alpha(4) = 4.0 x 10(-2) cm/GW for CdSe NPLs and alpha(4) = 9.0 x 10(-3) cm/GW for CdSe/CdS core/shell NPLs for an optical intensity of 60.97 and 122 GW/cm(2), respectively. The nonlinear refractive index obtained from Z-scan is in good agreement with those obtained from the OKG, and the response time obtained from OKG showed a decay of less than 1 ps for all studied materials, about 5 times slower than the pulse duration. For optical intensities in the range between 30.45 and 182.8 GW/cm(2), NLA was not observed for toluene but was observed for CdSe and CdSe/CdS core/shell NPLs, revealing that the presence of the NPLs decreases the threshold of the absorptive nonlinearity. The results are discussed and explained based on the well-known materials' electronic structure.