Si-Jing Ding

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.

Integrating metallic nanoparticles of Au and Pt with MoS2–CdS hybrids for high-efficient photocatalytic hydrogen generation via plasmon-induced electron and energy transfer

Semiconductor-based photocatalytic H2 generation is a promising approach to convert solar energy, but single-component photocatalysts still suffer from low efficiency limited by the fast charge recombination. Here, we investigate the high-efficient photocatalytic hydrogen generation of (MoS2–CdS)/Au and (MoS2–CdS)/Pt hybrids, and demonstrate the plasmon-induced electron and energy transfer as well as the co-catalytic effect of metallic nanoparticles (NPs). In these hybrids, visible-light-harvester CdS NPs as well as plasmonic Au NPs or co-catalyst Pt NPs were grown on the monolayer MoS2 nanosheets. The photocatalytic H2 generation under visible light irradiation of (MoS2–CdS)/Au and (MoS2–CdS)/Pt is respectively 3.2 times and 2.4 times that of MoS2–CdS. Intriguingly, the co-effect of Au NPs and Pt NPs leads to the 17 times enhancement. The plasmonic Au NPs in the hybrids play multiply significant roles to increase efficiency of H2 generation: (1) enhance light-harvesting and charge separation in the MoS2–CdS subunit; (2) provide multiply plasmon-mediated hot electron injection channels; (3) amplify the co-catalyst effect of Pt. The present work offers a promising approach for the rational integration of multi-component photocatalyst to improve photocatalytic performance.

Plasmon-Enhanced Photoelectrochemical Current and Hydrogen Production of (MoS 2 -TiO 2 )/Au Hybrids

Three component hybrid (MoS2-TiO2)/Au substrate is fabricated by loading plasmonic Au nanorods on the MoS2 nanosheets coated TiO2 nanorod arrays. It is used for photoelectrochemical (PEC) cell and photocatalyst for hydrogen generation. Owing to the charge transfer between the MoS2-TiO2 hetero-structure, the PEC current density and hydrogen generation of TiO2 nanoarrays are enhanced 2.8 and 2.6 times. The broadband photochemical properties are further enhanced after Au nanorods loading. The plasmon resonance of Au nanorods provides more effective light-harvesting, induces hot-electron injection, and accelerates photo-excited charges separation. The results have suggested a route to construct nanohybrid by combining one-dimensional arrays and two-dimensional nanosheets, meanwhile have successfully utilized plasmonic nanorods as a sensitizer to improve the photochemical properties of the semiconductor nanocomposite.

Strongly Asymmetric Spectroscopy in Plasmon-Exciton Hybrid Systems due to Interference-Induced Energy Repartitioning

Recent intense effort has been devoted to exploring different manifestations of resonant excitations of strongly coupled plasmons and excitons, but so far such studies have been limited to situations where the Fano- or Rabi-type spectra are largely symmetric at zero detuning. Using a newly developed full quantum mechanical model, here we reveal the existence of a highly asymmetric spectroscopic regime for both the Rabi splitting and transparency dip. The asymmetric nature is inherently tied to the non-negligible exciton absorbance and is caused by substantial interference-induced energy repartitioning of the resonance peaks. This theoretical framework can be exploited to reveal the quantum behaviors of the two excitation entities with varying mutual coupling strengths in both linear and nonlinear regimes. We also use prototypical systems of rhodamine molecules strongly coupled with AuAg alloyed nanoparticles and well-devised control experiments to demonstrate the validity and tunability of the energy repartitioning and correlated electronic state occupations, as captured by the variations in the asymmetric spectroscopy and corresponding nonlinear absorption coefficient as a function of the Au:Ag ratio. The present study helps to substantially enrich our microscopic understanding of strongly coupled plasmon-exciton systems.

Plasmon-Modulated Excitation-Dependent Fluorescence from Activated CTAB Molecules Strongly Coupled to Gold Nanoparticles

Excitation-dependent fluorophores (EDFs) have been attracted increasing attention owing to their high tunability of emissions and prospective applications ranging from multicolor patterning to bio-imaging. Here, we report tunable fluorescence with quenching dip induced by strong coupling of exciton and plasmon in the hybrid nanostructure of CTAB* EDFs and gold nanoparticles (AuNPs). The quenching dip in the fluorescence spectrum is tuned by adjusting excitation wavelength as well as plasmon resonance and concentration of AuNPs. The observed excitation-dependent emission spectra with quenching dip are theoretically reproduced and revealed to be induced by resonant energy transfer from multilevel EDFs with wider width channels to plasmonic AuNPs. These findings provide a new approach to prepare EDF molecules and a strategy to modulate fluorescence spectrum via exciton-to-plasmon energy transfer.

Tunable plasmon resonance and enhanced second harmonic generation and upconverted fluorescence of hemispheric-like silver core/shell islands

We investigate tunable plasmon resonance and enhanced second harmonic generation (SHG) and up-converted fluorescence (UCF) of the hemispheric-like silver core/shell islands. The Ag, Ag/Ag2O, and Ag/Ag2O/Ag island films are prepared by using a sputtering technique. The SHG and UCF of the Ag/Ag2O/Ag core/shell islands near the percolating regime is enhanced 2.34 and 3.94 times compared to the sum of two individual counterparts of Ag/Ag2O core/shell and Ag shell islands. The ratio of SHG intensity induced by p- and s-polarization is 0.86 for the initial Ag islands and increase to 1.61 for the Ag/Ag2O/Ag core/shell samples. The tunable intensity ratio of SHG to UCF of the Ag islands treated by thermal and laser annealing processes is also observed. The physical mechanism of the enhanced SHG and UCF in the Ag/Ag2O/Ag core/shell islands is discussed. Our observations provide a new approach to fabricate plasmon-enhanced optical nonlinear nanodevices with tunable SHG and UCF.

Largely enhanced photocatalytic hydrogen production rate of CdS/(Au–ReS2) nanospheres by the dielectric–plasmon hybrid antenna effect

In this study, we synthesized CdS/(Au-ReS2) nanospheres that have highly efficient photocatalytic hydrogen production activity induced by dielectric-plasmon hybrid antenna resonance. As the diameter (D) of ReS2 nanospheres consisting of 2D nanosheets increases from 114 ± 11 to 218 ± 25 nm, the resonance wavelength of the ReS2 dielectric antenna is tuned from 380 to 620 nm and the hydrogen production rate for the CdS/(Au-ReS2) nanospheres increases by more than 1.85 times and reaches a value as high as 3060 μmol g-1 h-1, with a 9% weight percentage of Au. Due to the enhancements of the local electromagnetic field and excitation energy transfer by the ReS2-Au dielectric-plasmon hybrid antenna, the hydrogen production rate for the CdS/(Au-ReS2) nanospheres (D = 218 ± 25 nm) is 797, 319, 105 and 12 times larger than that for pure ReS2, Au-ReS2, CdS, and CdS-ReS2, respectively. Additionally, the persistence and reusability measurements indicate a favorable stability of CdS/(Au-ReS2). These results provide a strategy to prepare a new class of dielectric-plasmon hybrid antennas consisting of 2D materials and metal nanoparticles, which have promise in applications ranging from photocatalysis to nonlinear optics.

Plasmon-Enhanced Fluorescence of Rare Earth Nanocrystals

Rare earth (RE) nanocrystals (NCs) exhibit featured fluorescence properties and are widely used in many applications of biomedical labeling, display, sensing, light-emitting devices, etc. Plasmon-enhanced and -manipulated fluorescence of RE NCs usually shows different behaviors compared with organic dye molecules and semiconductor NCs. In this Chapter, we firstly present the enhanced upconversion fluorescence of RE fluoride NCs by the dopant-controlled synthesis. Then the preparation of plasmon/RE core/shell NCs by wet-chemical growth method is introduced. Finally, we demonstrate the plasmon-enhanced upconversion fluorescence of a single RE NC and by plasmonic array nanostructures.