Highly fluorescing biological labels with excitation in near-infrared window II have attracted the interest of scientific community as they are capable of increasing both penetration depth and imaging quality. However, studies on the utilization of quantum dots (QDs) in biological imaging appear to be rather limited to the near-infrared window I (NIR-I: 650−950 nm). We herein report on the observation of efficient photoluminescence (PL) in Mn 2+ -doped ZnS QDs excited by two-photon absorption (2PA) in near-infrared window II (NIR-II: 1000−1350 nm). Multiphoton-absorptioninduced PL measurements indicate that these biocompatible QDs exhibit a two-photon action cross-section of 265 GM at 1180 nm, the highest value reported to date among conventional fluorescent probes on excitation in NIR-II. This value is 1−2 orders of magnitude higher than that for organic dye molecules excited by NIR-I photons and 3−4 times greater than that of fluorescent proteins excited in the NIR-II. The underlying NIR-II excitation mechanism for the Mn 2+ emission at 586 nm on account of the 4 T 1 − 6 A 1 transition is attributed to the transitions from the valence subband of ZnS QDs (or ground states of Mn 2+ ions) to the excited states of Mn 2+ ions by direct two-photon absorption. Transient PL measurements reveal single exponential decay with a PL lifetime of 0.35 ± 0.03 ms irrespective of excitation wavelength, which are 4−5 orders longer than that of conventional fluorescent probes. With the excitation in NIR-II window and the unique combination of photophysical properties such as a greater two-photon action cross-section, a longer PL lifetime, and larger anti-Stokes shift (450 nm or more), Mn 2+ -doped ZnS QDs appear to be a promising candidate for deep tissue imaging applications.

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We report on the dynamical properties of photoexcited carriers, particularly the charge transfer, in CdS–CdSe–CdS segmented nanorods using femtosecond transient pump–probe spectroscopy. Design of this kind of heteronanostructures with the possibility of variation of the relative volumes of CdS and CdSe segments permits independent tuning of one-photon and two-photon absorption cross-sections over a wide range of wavelengths, with specific advantages in applications related to photovoltaics and multiphoton microscopy. Intensity-dependent charge transfer dynamics in CdS–CdSe–CdS segmented nanorods indicates that the rate of charge transfer from CdS to CdSe is influenced by the number of electron–hole pairs generated in the nanorod. We attribute this change in the rate constant to Auger recombination-assisted charge transfer, which becomes the predominant relaxation mechanism at high intensities. Charge transfer also results in a large two-photon absorption cross-section, on the order of 104 GM (1 GM = 10–50 cm4 s photon–1), at 1.55 eV in these heteronanostructures. Furthermore, two-photon absorption induced photoluminescence on near-infrared excitation (1.55–0.99 eV) suggests that the local field effects plays a role in determining the effective two-photon action cross-section of heteronanostructures, offering a platform for engineering optical nonlinearity.

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Poly(styrene‐co‐maleic anhydride) (SMA) is a synthetic copolymer with interesting thermal and membrane properties. Schiff bases are one of the most widely used organic compounds with chelating ligands having N, S, and O as donor atoms. A Schiff‐base‐modified SMA was synthesized by the reaction of the copolymer with salicylaldehyde thiosemicarbazone. Gold (Au) nanoparticles (NPs), synthesized by a citrate reduction method were used to prepare the polymer–Au nanocomposites. In this research, we explored and investigated the effects on the linear and nonlinear optical properties of the Schiff‐base‐modified SMA copolymer with the incorporation of Au NPs. Open‐aperture Z‐scan measurements were recorded for the polymer, modified polymer, and polymer–Au nanocomposites at 532 nm with an Nd:YAG laser with a repetition rate of 10 Hz and a pulse width of 5 ns. The results indicate that the addition of the Au NPs effectively enhanced the two‐photon absorption coefficients of the polymer and, thereby, provided a platform for the development of nonlinear optical devices with good optical‐limiting properties. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45377.

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