Great enhancement on two-photon photoluminescence imaging contrast of Au nanoparticles via double-pulse femtosecond laser excitation with controlled phase differences.

Au nanoparticles are attractive contrast agents for noninvasive living tissue imaging with deep penetration because of their strong two-photon photoluminescence (TPPL) intensity and excellent biocompatibility. However, the inevitable phototoxicity and huge auto-fluorescence are consistently associated with laser excitation. Therefore, enhancement of TPPL intensity and suppression of backgrounds are always highly desired under the demand of reducing excitation powers. In this work, we develop a double-pulse TPPL (DP-TPPL) scheme with controlled phase differences (Δφ) between the double pulses to significantly improve the signal-to-noise ratio (SNR) of TPPL imaging. Under the modulated phase (Δφ periodically varying between 0-2π), our results show that SNR can be improved from 4.3 to 1715, with an enhancement of up to 400 folds at the integration of 50 ms. More importantly, this enhancement can be unlimitedly lifted by increasing the number of photons or integration times in principle. Further boosting has been achieved by reducing the magnitude of background noises; subsequently, SNR is improved by more than 104 times. Our schemes offer great potential for reducing phototoxicity and extracting extremely weak signals from huge backgrounds and open up a new possibility for a rapid, flexible, and reliable medical diagnosis by TPPL imaging with diminished laser powers.

Coherent Interference Fringes of Two-Photon Photoluminescence in Individual Au Nanoparticles: The Critical Role of the Intermediate State.

The interaction between light and metal nanoparticles enables investigations of microscopic phenomena on nanometer length and ultrashort timescales, benefiting from strong confinement and enhancement of the optical field. However, the ultrafast dynamics of these nanoparticles are primarily investigated by multiphoton photoluminescence on picoseconds or photoemission on femtoseconds independently. Here, we presented two-photon photoluminescence (TPPL) measurements on individual Au nanobipyramids (AuNP) to reveal their ultrafast dynamics by double-pulse excitation on a global timescale ranging from subfemtosecond to tens of picoseconds. Two orders of magnitude photoluminescence enhancement, namely, coherent interference fringes, has been demonstrated. Power-dependent measurements uncovered the transform of the nonlinearity from 1 to 2 when the interpulse delay varied from tens of femtoseconds to tens of picoseconds. We proved that the real intermediate state plays a critical role in the observed phenomena, supported by numerical simulations with a three-state model. Our results provide insight into the role of intermediate states in the ultrafast dynamics of noble metal nanoparticles. The presence of the intermediate states in AuNP and the coherent control of state populations offer interesting perspectives for imaging, sensing, nanophotonics, and in particular, for preparing macroscopic superposition states at room temperature and low-power superresolution stimulated emission depletion microscopy.

Criteria for Assessing the Interlayer Coupling of van der Waals Heterostructures Using Ultrafast Pump-Probe Photoluminescence Spectroscopy.

van der Waals (vdW) heterostructures of transition metal dichalcogenides (TMDCs) provide an excellent paradigm for next-generation electronic and optoelectronic applications. However, the reproducible fabrications of vdW heterostructure devices and the boosting of practical applications are severely hindered by their unstable performance, due to the lack of criteria to assess the interlayer coupling in heterostructures. Here we propose a physical model involving ultrafast electron transfer in the heterostructures and provide two criteria, η (the ratio of the transferred electrons to the total excited electrons) and ζ (the relative photoluminescence variation), to evaluate the interlayer coupling by considering the electron transfer in TMDC heterostructures and numerically simulating the corresponding rate equations. We have proved the effectiveness and robustness of two criteria by measuring the pump-probe photoluminescence intensity of monolayer WS2 in the WS2/WSe2 heterostructures. During thermal annealing of WS2/WSe2, ζ varies from negative to positive values and η changes between 0 and 4.5 × 10-3 as the coupling strength enhanced; both of them can well characterize the tuning of interlayer coupling. We also design a scheme to image the interlayer coupling by performing PL imaging at two time delays. Our scheme offers powerful criteria to assess the interlayer coupling in TMDC heterostructures, offering opportunities for the implementation of vdW heterostructures for broadband and high-performance electronic and optoelectronic applications.

Photostable fluorescent molecules on layered hexagonal boron nitride: Ideal single-photon sources at room temperature.

Photoblinking and photobleaching are commonly encountered problems for single-photon sources. Numerous methods have been devised to suppress these two impediments; however, either the preparation procedures or the operating conditions are relatively harsh, making them difficult to apply to practical applications. Here, we reported giant suppression of both photoblinking and photobleaching of a single fluorescent molecule, terrylene, via the utilization of hexagonal boron nitride (h-BN) flakes as substrates. Experimentally, a much-prolonged survival time of terrylene has been determined, which can have a photostable emission over 2 h at room temperature under ambient atmospheres. Compared with single molecules on a SiO2/Si substrate or glass coverslip, a more than 100-fold increase in the total number of photons collected from each terrylene on h-BN flakes has been demonstrated. We also proved that the photostability of terrylene molecules can be well maintained for more than 6 months even under ambient conditions without any further protection. Our results demonstrate that the utilization of h-BN flakes to suppress photoblinking and photobleaching of fluorescent molecules has promising applications in the production of high-quality single-photon sources at room temperature.

Co-adsorption of an anionic dye in the presence of a cationic dye and a heavy metal ion by graphene oxide and photoreduced graphene oxide.

To investigate the adsorption behavior of contaminants with different adsorbents and co-adsorbates under identical conditions, the adsorption capacities of anionic orange II (OII) dye onto graphene oxide (GO) and photoreduced GO (PRGO) in a single-component system and in the presence of cationic methylene blue (MB) dye as well as heavy metal ion Pb2+ were explored. In this work, PRGO was prepared by solar light irradiation of a GO dispersion. GO and PRGO were characterized by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, Raman spectroscopy, atomic force microscopy, scanning electron microscopy, and transmission electron microscopy. The adsorption isotherms of OII, MB, and Pb2+ onto GO and PRGO in single and binary systems have been studied and analyzed by the Langmuir model. In the single system, the adsorption capacity of OII on GO can be promoted from 8.4 mg g-1 to 32.5 mg g-1 after solar light irradiation. While the adsorption capacities of MB and Pb2+ are not affected by the photoreduction process. In the binary system, a marked synergistic effect for the adsorption of OII has been determined in the presence of both MB and Pb2+, where the adsorption capacity of OII on PRGO has been improved from 8.4 mg g-1 to 295 mg g-1 and 105 mg g-1, enhancements of 35- and 12.5-fold, respectively. In contrast, the presence of OII leads to a mildly antagonistic effect on the adsorption of MB and Pb2+. These findings show that the adsorption of anionic dyes by graphene-based materials can be strongly improved in the presence of either cationic dyes or heavy metal ions, which will be of great value in practical applications.