Modification Of Quantum Dots' Optical Properties With Metal Surfaces,Optical Multilayers And Resonant Cavities

Benefiting from quantum confinement effect,semiconductor quantum dots?QDs?with high photoluminescence quantum yield?QY?and larger third-order nonlinear susceptibility,can be widely used in light emitting diodes,optical switching,QD lasers and other fields.If the QDs are combined with particular device geometries including metal surfaces,optical multilayers and resonant cavities,their QY and nonlinear optical response could be boosted due to enhancement of the photonic mode density or local field intensity surrounding them.This paper focuses on the modulation of these devices on optical properties of colloidal semiconductor QDs and its application.Ag/SiO₂/CdS-ZnS plasmon couplers with different SiO₂ shell thickness were synthesized to modify emission from trap-rich CdS-ZnS core-shell QDs by adjusting the distance between the QDs and Ag nanoparticles.When the thickness of the SiO₂ shell is 10 nm,optimum enhancements of the surface state emission?SSE??17 times?and band-edge emission?BEE??4 times?are simultaneously realized and the SSE to BEE intensity ratio is increased to 55%.As a result,a bright white-light emission from single-size QDs is achieved.The experimental results show that the fluorescence enhancement is mainly due to suppression of the non-radiative decay rate(k_nr).CdSe-ZnS core-shell QDs-embedded SiO₂?SiO₂:QDs?layer and TiO2 layer were used to fabricate a 5-period all-dielectric optical multilayer.The third-order nonlinear refractive index?n²?of SiO₂:QDs layers in the multilayer is nearly triple of that of the SiO₂:QDs film due to the local field enhancement in the multilayer.The photoinduced change in refractive index can shift the band edge,so transmission change of the multilayer is?17 folds of that of the SiO₂:QDs film.Under excitation of a 30 GW/cm² femtosecond laser beam,a transmission decrease of 80%is realized.Pump-probe experiments show that the response time of multilayer optical switching is-400 fs.We synthesized "giant" CdSe-CdS core-shell QDs with a 15 monolayer?ML?CdS shell and focused on removing interfacial potential barrier through femtosecond laser annealing and,consequently,transferred amplified spontaneous emission?ASE?from bulk-like CdS shell to quantum-confined CdSe core.Besides the ASE at 643 nm,two shorter-wavelength ASE peaks corresponding to optical transitions of the second?1P?and the third?1D?electron quantization shells of CdSe core appear also,indicating that Auger recombination?AR?is very effectively suppressed.Moreover,phase-pure wurtzite CdSe-CdS core-shell QDs with controlled shell thickness were also synthesized.Benefiting from increased volume,electron-hole partial spatial separation and nearly defect-free alloyed interface,this series of QDs exhibit more than 3 orders of magnitude decrease in AR rates with increasing thickness.As a result,the ASE threshold of the QDs with a 11 ML CdS shell is found to reach a minimum of 16 ?J/cm².A record long lifetime?>1000 ps?and extraordinarily large bandwidth?>170 nm?of optical gain are observed by employing ultrafast transient absorption spectroscopy.Finally,we made the QDs to form coffee-ring microcavities,and then achieved ultralow threshold??2 ?J/cm²?single-mode lasing.