Nonlinear Optical Properties Of CdSe-based Nanoplatelets

Colloidal semiconductor nanocrystals possess large optical nonlinearity due to the quantum confinement effect,and their nonlinear optical parameters can be conveniently tuned by their size,shape and composition.Therefore,colloidal semiconductor nanocrystals are believed to be the promising materials for the applications in nonlinear gain media,optical limiting,all optical switching,and biolabeling.Up to now,extensive work has been performed on the nonlinear optical properties of semiconductor nanocrystals,especially the different-shaped II-VI semiconductor nanocrystals,such as CdSe quantum dots(QDs),CdSe/Zn S core/shell QDs,Cd S nanorods(NRs),and CdSe/Cd S core/shell NRs.CdSe-based nanoplatelets(NPLs),one of the new members in II-VI semiconductor nanocrystals,possess large lateral size and small thickness easily controlled at atomic precision,and have attracted great attention for their superior optical and electronic properties.Recent experiment has demonstrated that CdSe NPLs show 10 times more efficient two-photon absorption(2PA)than their NRs and QDs counterparts,arising from the high two-dimensional density of states and the ultra-strong anisotropic confinement.However,the nonlinear optical absorption of CdSe NPLs strongly depends on the electronic transition properties of the material and the excitation parameters(energy,pulse duration and irradiance of the incident laser beam).It is highly necessary to comprehensively understand the nonlinear optical characteristics of CdSe-based NPLs working at nonresonance as well as at resonance so that we can better apply them to nonlinear optoelectronics.The main contents of this paper can be summarized as the following aspects:1.We synthesized CdSe-based nanoplatelets(NPLs)by solution-phase method and colloidal atomic layer deposition approach.The CdSe NPLs have a zinc blende structure with the average lateral sizes of 7 nm × 22 nm.The coated Cd S monolayer adopts the lattice parameters of CdSe core to form a dislocation-free interface.The addition of a single Cd S layer to both sides of CdSe NPLs makes the excitonic absorption peak redshift from 510 to 584 nm.2.We investigated the optical nonlinear absorption of CdSe NPLs with 532 nm ps pulses by open aperture Z-scan technique.The CdSe NPLs exhibit strong two-photon induced free carrier absorption(effective three photon absorption)upon the nonresonant excitation,resulting in a remarkable optical limiting behavior with the limiting threshold of approximately 75 GW/cm2.3.A nonlinear optical switching from saturable absorption(SA)to reverse saturable absorption(RSA)with increasing laser intensity has been observed when overcoating CdSe NPLs with a monolayer of Cd S shell to realize the resonant absorption.The SA behavior is originated from the ground state bleaching and the RSA behavior is attributed to the free carrier absorption.The calculated effective two-photon absorption cross section of per NPL is 2.3 × 108 GM,several orders of magnitude larger than that of CdSe quantum dots.