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The features of the nonlinear absorption of CdSe/CdS core-shell nanocrystals based on 5 monolayer (ML) CdSe nanopletelets (NPLs) in the case of one-photon excitation of the exciton transitions by means of ultra-short laser pulses (non-stationary regime) were investigated. CdSe NPLs were synthesized by colloidal method at the temperature of 210 oC. Optical absorption spectrum of as prepared CdSe NPLs showed narrow excitonic absorption bands at 463 and 436nm corresponding to hh-e and lh-e, respectively, which indicates that the CdSe NPLs have 5ML thickness. The CdSe/CdS core-shell NPLs were obtained using method of colloidal atomic layer deposition (c-ALD). The c-ALD method allows obtaining core-shell NPLs with thickness control at the atomic monolayer level. The obtained CdSe/CdS core-shell NPLs showed narrow and pronounced hh-e and lh-e transitions characteristic for cadmium chalcogonide NPLs, which indicates their high uniformity in terms of thickness. Resonant excitation of heavy hole and light hole excitons was realized for 5CdSe/CdS, 5CdSe/2CdS, correspondingly, and non-resonant excitation both heavy hole and light hole excitons was carried out for 5CdSe/3CdS NPLs.
Excitation of colloidal solution of NPLs was carried out by the second harmonic of passively Q-switched Nd3+:YAG laser (2w, λ=532 nm, the pulse duration is 30 ps). Nonlinear transmission spectra evolution was measured while changing pumping intensity. The variation of excitation intensity was realized by neutral optical filters. The increase in transmission of the exciton transitions at the excitation wavelength was observed for all three samples. This feature of nonlinear change in transmission is attributed to phase space filling effect. The greater induced bleaching was discovered for resonantly excited sample. The saturation intensity of all samples were measured about 50 MW/cm2 for non-stationary excitation regime. The transmission increases in absolute value ΔT=T-T0≈30%, with relative change in transmission ΔT/T0≈50% in the case of resonant excitation of excitons, and ΔT≈15%, ΔT/T0≈35% in the case of resonant excitation. The role of up-conversion and down-conversion processes were defined.
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