Two-photon absorption in ZnSe and ZnSe∕ZnS core/shell quantum structures

Abstract

The third order nonlinear optical properties of two different sized ZnSe and ZnSe∕ZnS quantum dots (QDs) are investigated. The nonlinear absorption is measured at 806nm using Ti:sapphire 100fs laser pulses in an open aperture Z-scan setup. Two-photon absorption (2PA) is found to be dominant in core and core shell QDs. 2PA cross section is enhanced by three orders of magnitude compared bulk ZnSe. 2PA cross section is observed to increase with reduction in QD diameter, due to strong confinement effect. ZnSe∕ZnS QDs exhibit higher 2PA cross section compared with corresponding ZnSe QDs, indicating better passivation of the QD surface.

Near resonant and nonresonant third-order optical nonlinearities of colloidal InP/ZnS quantum dots

“…1,8,23 Interestingly, the TPA cross-sections of CdS and CdSe QDs were found to increase with size, 1,23 while those for ZnS and ZnSe QDs to decrease. 24,25 The opposite trend of TPA cross-sections of QDs with size arises from the competition between decreased quantum confinement effect and increased density of state (DOS) when the sizes of QDs increase. 1,25 The larger TPA cross-section of QD606 than that of QD548 indicates the increased DOS prevails over the decreased quantum confinement effect for InP/ZnS QDs.…”

“…24,25 The opposite trend of TPA cross-sections of QDs with size arises from the competition between decreased quantum confinement effect and increased density of state (DOS) when the sizes of QDs increase. 1,25 The larger TPA cross-section of QD606 than that of QD548 indicates the increased DOS prevails over the decreased quantum confinement effect for InP/ZnS QDs. By taking into account the volume fraction and local field effect, the intrinsic TPA coefficient of InP/ZnS QDs b QD can be deduced by:…”

Observation of enhanced two-photon absorption in ZnS quantum dots at an ultraviolet wavelength

“…, where f is the volume fraction of the QDs solutions, and £ is the local field correction, which depends on the dielectric constant of the solvent and QDs [12] . The refractive index n QDs 0 of the QDs of radius R can be obtained using the relation 9 n QDs 0…”

“…The observed variation of the NLA coefficient with the intensity demonstrates the presence of two-photon-generated free-carrier nonlinearities resulting in the effective fifthorder nonlinearity under strong excitation [7,17−19] . The value of the free-carrier absorption (FCA) cross section induced by the 2PA in the R II sample is calculated from the measured intensity-dependent OA zscan traces using the fast analytical procedure of Gu [11] , b ZnS QDs at 532 nm [7] , c ZnS QDs at 795 nm [12] , and d Bulk ZnS at 532 nm [13] Fig . 3.…”

“…The wavelength-dependent refractive index (n) of the bulk ZnS is calculated using the Sellmeier dispersion relation [16] : 7 times larger than that of bulk ZnS, and they are at least one order of magnitude larger than those of ZnSe/ZnS core/shell and ZnSe QDs [12] . In Table 1, the calculated values of the NLR coefficient (n 2 ) of all samples are almost the same and of the same order of magnitude as that of bulk ZnS.…”