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 [001] 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
Photoluminescence (PL) features and nonlinear transmission of Cu-doped CdSe colloidal quantum dots (QDs) under nanosecond laser pulses excitation were investigated. Strong difference of the pump intensity dependent behavior of basic exciton transition and Cu dopants associated PL was revealed. The presence of exciton-phonon interaction and its strong influence on the PL and nonlinear properties of Cu-doped colloidal CdSe QDs are proved by the simultaneous linear growth of basic excitons PL, the growth of the Stokes shift and significant decrease of absorption at the basic exciton transition wavelength with the increase of pump intensity [1].
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.
The high-intensity nanosecond laser pulses scattering in strongly absorbing colloidal solutions of СdSe/ZnS quantum dots has been investigated. Different types of nonlinear Tyndall scattering mechanism was revealed as a function of excitation radiation intensity. At the low laser pulses intensity (up to 15 MW/cm2 ) saturation of the basic exciton transition in strongly absorbing colloidal solution of СdSe/ZnS quantum dots was observed. In this case of average laser pulses intensity (15-200 MW/cm2 ) the dominant scattering mechanism is scattering on dipoles induced by the electric field and scattering on density fluctuations of the dispersed medium around bleached quantum dots. At the higher intensity (200-4000 MW/cm2 ), the predominant scattering mechanism is the scattering on bubbles of gas formed around local heating centers – colloidal quantum dots.
We have investigated the nonlinear absorption of CdSe-based nanoplatelets (NPLs) with different thicknesses of shell in the case of resonant one-photon stationary excitation of exciton transitions by nanosecond pulses of mode-locked Nd:YAP laser. Decrease in absorption at the wavelength of whether light hole – electron and heavy hole – electron exciton transitions was revealed. Induced changing of both absorption doublet components was attributed to phase space filling effect and exciton energy conversion mechanism.
A simple way to create dynamic photonic crystals with different lattice symmetry by interference of non-coplanar laser beams in colloidal solution of quantum dots was demonstrated. With the proposed technique we have made micro-periodic dynamic semiconductor structure with strong nonlinear changing of refraction and absorption and analyzed the self-diffraction processes of two, three and four non-coplanar laser beams at the dynamic photonic crystal (diffraction grating) with hexagonal lattice structure. To reach the best uniform contrast of the structure and for better understanding of the problems, specially raised by the interference of multiple laser beams theoretical calculation of the periodic intensity field in the QDs solution were performed. It was demonstrated that dynamic photonic crystal structure and even it’s dimension can be easily tuned with a high speed by the laser beams polarization variation without changing the experimental setup geometry.
The goal of this work is the investigation of optical spectra features of zinc selenide (ZnSe), silver iodide (AgI) and its two-phase composite AgI-ZnSe nanostructures produced by laser ablation method, which can be used to design optical sensors and diffractive structures in integrated optics. Shifted to blue wavelengths relatively to the bulk semiconductor material band edge transmission spectra minima have been discovered for the ZnSe and AgI-ZnSe films. The observed minima of the transmission spectra are peculiar to the quantum energy spectra of semiconductor nanostructures. Discovered transmission spectra minima for the ZnSe and AgI-ZnSe films shifted to the short-wavelength region from the energy of the bulk material band gap can be the evidence of nanocrystals formation during the film growth by laser ablation, and which are characterized by the energy spectrum quantization and lower electron and upper hole quantum confinement levels shifts from the bottom of the conduction and the top valence bands, respectively.
One-dimensional dynamic photonic crystal was formed by a periodic spatial modulation of dielectric permittivity induced by the two ultrashort laser pulses interference in semiconductor quantum dots CdSe/ZnS (QDs) colloidal solution intersecting at angle θ. The fundamental differences of dynamic photonic crystals from static ones which determine the properties of these transient structures are the following. I. Dynamic photonic crystals lifetimes are determined by the nature of nonlinear changes of dielectric permittivity. II. The refractive index changing is determined by the intensity of the induced standing wave maxima and nonlinear susceptibility of the sample. We use the pump and probe method to create the dynamic one-dimensional photonic crystal and to analyze its features. Two focused laser beams are the pump beams, that form in the colloidal solution of quantum dots dynamic one-dimensional photonic crystal. The picosecond continuum, generated by the first harmonic of laser (1064 nm) passing through a heavy water is used as the probe beam. The self-diffraction of pumping beams on self induced dynamic one-dimensional photonic crystal provides information about spatial combining of laser beams.
Two optical systems modeling of laser and broadband radiation focusing, that is necessary for realization of the pump and probe method, was carried out in this work. Modeling was utilized to construct experimental setup for transmission spectra measuring of studied sample by probe nanosecond broadband radiation (coumarin photoluminescence) depending on the intensity of the nanosecond laser pump pulses. The saturation effect of absorption and the induced charge Stark-effect coexistence and predominate issue of these effects are determined by power of optical excitation. In dependence of tuning of excitation radiation frequency from basic exciton transition frequency nonlinear effects in colloidal CdSe/ZnS quantum dots has been investigated.
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