Xiao-Niu Peng

Tuning Gold Nanorod-Nanoparticle Hybrids into Plasmonic Fano Resonance for Dramatically Enhanced Light Emission and Transmission

We investigate the optical response of a gold nanorod array coupled with a semicontinuous nanoparticle film. We find that, as the gold nanoparticle film is adjusted to the percolating regime, the nanorod-film hybrids are tuned into plasmonic Fano resonance, characterized by the coherent coupling of discrete plasmonic modes of the nanorod array with the continuum band of the percolating film. Consequently, optical transmission of the percolating film is substantially enhanced. Even more strikingly, electromagnetic fields around the nanorod array become much stronger, as reflected by 2 orders of magnitude enhancement in the avalanche multiphoton luminescence. These findings may prove instrumental in the design of various plasmonic nanodevices.

Plasmon-mediated radiative energy transfer across a silver nanowire array via resonant transmission and subwavelength imaging.

Efficient plasmon-mediated excitation energy transfer between the CdSe/ZnS semiconductor quantum dots (QDs) across the silver nanowire array up to 560 nm in length is observed. The subwavelength imaging and spectral response of the silver nanowire arrays with near-field point-source excitations are revealed by theoretical simulations. Our studies demonstrate three advantages of the nanosystem: efficient exciton-plasmon conversion at the input side of the array through near-field strong coupling, directional waveguidance and resonant transmission via half-wave plasmon modes of the nanowire array, and subwavelength imaging at the output side of the array. These advantages allow a long-range radiative excitation energy transfer with a high efficiency and a good directionality.

Multipole-plasmon-enhanced förster energy transfer between semiconductor quantum dots via dual-resonance nanoantenna effects

Ag nanoparticles with dipole and quadrupole plasmon are employed as dual-resonant nanoantenna to enhance both “receiving” and “emitting” fields resonantly. Strong enhancement of photoluminescence (PL) and highly efficient plasmon-assisted Forster energy transfer between CdSe quantum dots are demonstrated by employing Ag dual-resonant nanoantenna and micro-PL spectroscopy. The collaboration effect of quadrupole and dipole plasmon of Ag dual-resonant nanoantenna could find applications in plasmonics such as biosensoring and optical information processing.

Plasmon-enhanced Förster energy transfer between semiconductor quantum dots: multipole effects

We experimentally demonstrated plasmon-assisted energy transfer (ET) between CdSe semiconductor quantum dots (QDs) self-assembled in a monolayer by using time-resolved micro-photoluminescence (PL) technique. The enhancements of PL intensity and ET efficiency were manipulated by adjusting thickness (Delta) of SiO(2) coating on large Ag nanoparticles. The PL enhancement factor of the acceptor QDs and the PL intensity ratio of acceptor-to-donor reached their maxima approximately 47 and approximately 14 when Delta = 7 nm, the corresponding ET efficiency reached 86%. We also presented theoretical analysis based on the rate equation. The theoretical calculations agreed with experimental data and revealed interesting physics of multipole effect, and metal nanoparticle induced quench effect and plasmon-enhanced Förster ET.

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.

Controllable energy transfer in fluorescence upconversion of NdF 3 and NaNdF 4 nanocrystals

The synthesized Nd fluoride nanocrystals exhibited different upconversion behaviors as dispersed and aggregated samples due to the different energy transfer mechanisms. When they were dispersed in water, the NaNdF(4) nanocrystals exhibited approximately 400 times stronger upconversion fluorescence than the NdF(3) nanocrystals. Remarkable upconversion behaviors were found when the nanocrystals were aggregated in the films. For the NdF(3) nanocrystals, the energy transfer processes (4)I(13/2)<--(4)F(3/2)-->(4)G(7/2) in the films generated avalanche upconversion emissions with a high slope of approximately 12.0, which could be due to the large avalanche cross relaxation rates and spectral broadening effect. In contrast, the spectral broadening effect in the NaNdF(4) NCs films increased the energy transfer (4)I(15/2)<--(4)F(3/2)-->(4)G(5/2) of the Nd(3+) ions, and induced a new upconversion emission at approximately 680 nm with the slope increased from 1.0 to 3.2.

High temperature sensitivity of manganese-assisted excitonic photoluminescence from inverted core/shell ZnSe:Mn/CdSe nanocrystals

We report efficient and long-lived exciton photoluminescence (PL) from inverted core/shell ZnSe:Mn/CdSe nanocrystals (NCs) at room temperature. Contrary to low temperature-sensitivity of Mn2+ PL from ZnSe:Mn/ZnSe NCs, the Mn2+-assisted excitonic PL from the ZnSe:4%Mn/CdSe NCs exhibits unusually high temperature-sensitivity (−2.4% per K) in the temperature range 298–334 K, which is even higher than that of the pure excitonic PL from the undoped ZnSe/CdSe NCs. Such unexpected temperature-dependence of the Mn2+-assisted excitonic PL is well explained by the deduced exciton rate equation involving the exciton-Mn2+ resonant energy transfer in the ZnSe:Mn/CdSe NCs.

Sublinear and superlinear photoluminescence from Nd doped anodic aluminum oxide templates loaded with Ag nanowires

We first time prepared Nd(3+) ions doped anodic aluminum oxide (Nd:AAO) templates, reported linear, sublinear and superlinear photoluminescence (PL) from Nd:AAO templates loaded with Ag nanowires in different excitation power regions, in which, the excitation laser with wavelength 805 nm resonantly pumped the population to (4)F5/2 states of Nd(3+), and the radiative transitions (4)F(3/2) -->(4)I(9/2) of Nd(3+) centered at 880 nm. The excitation power dependences of emission polarization ratio and the spectral width were also investigated. The observed nonlinear amplifications of the PL intensity implied strong interaction between randomly-dispersed Nd(3+) ions and ordered-arrayed Ag nanowires in AAO templates.

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.