Fan Nan

Unusual and Tunable One-Photon Nonlinearity in Gold-Dye Plexcitonic Fano Systems

Recent studies of the coupling between the plasmonic excitations of metallic nanostructures with the excitonic excitations of molecular species have revealed a rich variety of emergent phenomena known as plexcitonics. Here, we use a combined experimental and theoretical approach to demonstrate new and intriguing aspects in the ultrafast nonlinear responses of strongly coupled hybrid Fano systems consisting of gold nanorods decorated with near-infrared dye molecules. We show that the severely suppressed linear absorption around the Fano dip significantly enhances the unidirectional energy transfer from the plasmons to the excitons and further allows one-photon nonlinearity to be drastically and reversibly tuned. These striking observations are interpreted within a microscopic model stressing on two competing processes: saturated plasmonic absorption and weakened destructive Fano interference from the bleached excitonic absorption. The unusually strong one-photon nonlinearity revealed here provides a promising strategy in fabricating nanoplasmonic devices with both pronounced nonlinearities and good figures of merit.

Largely Enhanced Saturable Absorption of a Complex of Plasmonic and Molecular-Like Au Nanocrystals

A saturable absorber is a nonlinear functional material widely used in laser and photonic nanodevices. Metallic nanostructures have prominent saturable absorption (SA) at the plasmon resonance frequency owing to largely enhanced ground state absorption. However, the SA of plasmonic metal nanostructures is hampered by excited-state absorption processes at very high excitation power, which usually leads to a changeover from SA to reversed SA (SA→RSA). Here, we demonstrate tunable nonlinear absorption behaviours of a nanocomplex of plasmonic and molecular-like Au nanocrystals. The SA→RSA process is efficiently suppressed, and the stepwise SA→SA process is fulfilled owing to energy transfer in the nanocomplex. Our observations offer a strategy for preparation of the saturable absorber complex and have prospective applications in liquid lasers as well as one-photon nonlinear nanodevices.

Plasmon-Enhanced Light Harvesting of Chlorophylls on Near-Percolating Silver Films via One-Photon Anti-Stokes Upconversion

There exists a wealth of means of efficient utilization of solar energy in nature, with photosynthesis of chlorophylls as a prime example. Separately, artificially structured plasmonic materials are versatile in light harvesting and energy conversion. Using a simple and scalable design of near-percolating silver nanostructures, we demonstrate that the light-harvesting efficiency of chlorophylls can be drastically enhanced by tuning the plasmon frequency of the constituent silver nanoparticles to coincide with the maximal photon flux of sunlight. In particular, we show that the photon upconversion efficiency can be readily enhanced by over 20 folds, with the room-temperature fluorescence quantum yield increased by a factor of 2.63. The underlying mechanism for the upconversion enhancement is attributed to a one-electron-per-photon anti-Stokes process, involving absorption of a characteristic phonon mode of the chlorophylls. These findings suggest that chlorophylls can serve as molecular building blocks for high-efficiency light harvesting and solar energy conversion.

Strong tunability of cooperative energy transfer in Mn2＋-doped （Yb3＋, Er3＋）/NaYF4 nanocrystals by coupling with silver nanorod array

Fluorescent rare-earth ions are useful for efficient energy transfer via multichannels with different properties. Tuning these transfer processes in functional rare-earth materials has attracted considerable attention to satisfy the various demands of diverse practical applications. In this study, strong tunabilities of cooperative energy transfer and nonlinear upconversion emissions are realized using (Yb3+, Er3+)/NaYF4 nanocrystals with and without doped Mn2+ ions by adopting a plasmonic nanocavity composed of a silver nanorod array. The plasmon nanocavity can not only increase the energy transfer between Mn2+ and (Yb3+, Er3+) but also significantly enhance the radiative emission. This reveals a prominent nonlinear gain in the nanocavity nanosystems. These observations suggest the prospective applications in the design and preparation of rare-earth nanocrystals with excellent tunabilities of multiple functionalities.

Manipulating Nonlinear Emission and Cooperative Effect of CdSe/ZnS Quantum Dots by Coupling to a Silver Nanorod Complex Cavity

Colloidal semiconductor quantum dots have three-dimensional confined excitons with large optical oscillator strength and gain. The surface plasmons of metallic nanostructures offer an efficient tool to enhance exciton-exciton coupling and excitation energy transfer at appropriate geometric arrangement. Here, we report plasmon-mediated cooperative emissions of approximately one monolayer of ensemble CdSe/ZnS quantum dots coupled with silver nanorod complex cavities at room temperature. Power-dependent spectral shifting, narrowing, modulation, and amplification are demonstrated by adjusting longitudinal surface plasmon resonance of silver nanorods, reflectivity and phase shift of silver nanostructured film, and mode spacing of the complex cavity. The underlying physical mechanism of the nonlinear excitation energy transfer and nonlinear emissions are further investigated and discussed by using time-resolved photoluminescence and finite-difference time-domain numerical simulations. Our results suggest effective strategies to design active plasmonic complex cavities for cooperative emission nanodevices based on semiconductor quantum dots.

Sign-reversed and magnitude-enhanced nonlinear absorption of Au–CdS core–shell hetero-nanorods

We synthesis uniform Au–CdS core–shell hetero-nanorods and demonstrate the effective plasmon–exciton interaction induced optical nonlinear enhancement in metal–semiconductor hetero-nanostructures. After growing CdS semiconductor shell onto the Au nanorods, the longitudinal plasmon resonance exhibits considerable red-shift with enlarged absorption intensity. Nonlinear absorption responses transform from saturable absorption to reverse saturable absorption, and effective nonlinear absorption coefficient β is increased from −7.7 to +22.2 cm/GW. The observed behaviors indicate strong plasmon–exciton interaction and great local field enhancement.

Tunable nonlinear optical absorption in semiconductor nanocrystals doped with transition metal ions

Semiconductor nanocrystals (SNCs) doped with Mn2+ and Cu2+ ions (ZnSe:Mn, ZnSe:Mn/CdSe, and ZnSe:Cu) were synthesized and their nonlinear absorption properties were investigated using the typical open-aperture Z-scan technique. A reversal from reverse saturable absorption (RSA) to saturable absorption (SA) was observed in the ZnSe:Mn SNCs. In contrast, the SA signals were largely suppressed and only RSA signals were measured in the ZnSe:Mn/CdSe and ZnSe:Cu SNCs. These different nonlinear absorption properties implied strong interactions between the excited excitons and impurity ions, which agreed well with the analysis performed by the theory model of the exciton transition in the energy structures. The nonlinearities of the doped SNCs could be controllably tuned by the integration of different doped ions and host SNCs. This strategy shows promise for extensive potential applications in optical limiting and switching.

Upconversion luminescence properties of Mn2+-doped NaYF4:Yb/Er nanoparticles

NaYF4:Yb/Er upconversion nanoparticles doped with Mn2+ were synthesized by hydrothermal method. The upconversion photoluminescence measured by 975 nm continuous wave laser indicates that the as-synthesized samples generated green and red color emission with various intensity ratio ranging from 3.25 to 548.35, which is highly correlative to the dopant concentration of Mn2+. However, there is no red emission enhancement observed in Cu2+-doped NaYF4:Yb/Er nanoparticles.

Stepwise synthesis of cubic Au-AgCdS core-shell nanostructures with tunable plasmon resonances and fluorescence

Cubic Au-AgCdS core-shell nanostructures were synthesized through cation exchange method assisted by tributylphosphine (TBP) as a phase-transfer agent. Among intermediate products, Au-Ag core-shell nanocubes exhibited many high-order plasmon resonance modes related to the special cubic shape, and these plasmon bands red-shifted along with the increasing of particle size. The plasmon band of Au core first red-shifted and broadened at the step of Au-Ag₂S and then blue-shifted and narrowed at the step of Au-AgCdS. Since TBP was very crucial for the efficient conversion from Ag₂S to CdS, we found that both absorption and fluorescence of the final products could be controlled by TBP.

Size-dependent plasmon relaxation dynamics and saturable absorption in gold nanorods

The surface plasmon resonances of metallic nanoparticles are highly dependent on size and shape, which leads to high tunability of both linear and nonlinear optical responses. Here, we investigate the plasmon resonance and saturable absorption of gold nanorods (AuNRs) with different lengths and reveal the size dependence of electron relaxation dynamics. The strong plasmon absorption leads to ground-state bleaching and saturable absorption (SA) in AuNRs, Z-scan measurements demonstrate that the saturation threshold decreases as the length of AuNRs increases. The time-resolved differential transmittance reveals that the longer AuNRs have large relaxation times for hot electrons, which accounts for the decrease of the SA threshold.