In this work, the second harmonic generation was investigated in colloidal solution of CdSe nanoplatelets with two and three monolayers of CdS shell in the case of the two-photon laser excitation of the light-hole – electron transition (1lh-e). In addition to the two-photon photoluminescence spectrum a narrow spectral peak was observed at the wavelength of the laser’s second harmonic radiation, which intensity increases quadratically with incident laser intensity growth taking into account linear absorption of colloidal solution. Also the cause of the second harmonic generation in CdSe/CdS nanoplatelets was considered and explained. It was suggested that the second harmonic in CdSe/CdS colloidal solution is mostly generated by surface atoms for the part of the nanoplatelets satisfying to phase matching condition of the fundamental and second harmonic waves. The concentration of CdSe/CdS nanoplatelets in colloidal solution at which the second harmonic generation can be observed was determined.
Semiconductor nanocrystals have been actively studied due to their unique physical and chemical properties and are actively being implemented in nanophotonics devices. Nanostructures created by the colloidal synthesis with design shape, size and crystal structure are widely used. Recently, colloidal semiconductor quantum wells (nanoplatelets) have been created. Colloidal semiconductor nanoplatelets (NPLs) are atomically flat nanocrystals which demonstrate a zinc blend crystal structure with a  axis. Strong quantum confinement of NPLs and high exciton binding energy are provided by anisotropic of nanocrystals with several nanometers thick and tens of nanometers in lateral dimensions which can be used to tune the optical absorption and photoluminescence spectra.
In this paper we present the peculiarities of excitation, interaction and relaxation of excitons in colloidal CdSe nanoplates depend on type and thickness of shell in the case of one-photon exciton excitation by laser pulses (λ=540 nm, τ=10 ns). The linear and nonlinear absorption spectra of colloidal CdSe NPLs were studied. The linear absorption spectrum of the NPLs demonstrate three well-resolved absorption bands that correspond to heavy hole, light hole and spin-orbital exciton transitions at room temperatures due to the almost complete absence size dispersion of nanocrystals. The differential transmission spectra allowed us to reveal experimentally the lowest four band structure of CdSe/CdS nanoplatelets at the Γ point of Brillouin zone and its modification with CdS shell thickness changing for the first time. In addition, the features of exciton-exciton interaction, exciton-phonon interaction, as well as the process of energy transfer between light and heavy excitons to exciton relaxation were investigated. The rate equations describing the exciton-exciton and exciton-electron interactions was applied for analyzing the recombination and interaction of excitons in the colloidal NPLs under high excitation densities.
This work was partially supported by the Russian Foundation for Basic Research №20-32-70001
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.