Da-Jie Yang

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.

Tunable Fano Resonance in Rod-Ring Plasmonic Nanocavities

Tunable Fano resonances in the gold rod-ring plasmonic nanocavities with strong coherent coupling are demonstrated by finite-difference time-domain (FDTD) method. For one rod-one ring nanocavity, symmetry breaking activates the high-order plasmon modes in the ring and causes a Fano resonance with a dip in the extinction spectrum of the cavity by the coherent coupling between the bright rod mode and dark ring mode. The addition of a non-resonant rod introduces a second dark mode, which could be successively excited by the first dark ring mode, and produces double Fano resonances. By adjusting the rod number and configuration of the nanocavity, the extinction line shape can be controlled and the near-field distribution can be dramatically modified. An intriguing phenomenon of superposition beats in the plasmon damping process is revealed, which induces an energy swap between the rod and ring. The antenna effect of the resonant rod as well as the energy transfer and distribution in the nanocavities are also discussed.

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.

Plasmonic phase modulator based on novel loss-overcompensated coupling between nanoresonator and waveguide

We present that surface plasmon polariton, side-coupled to a gain-assisted nanoresonator where the absorption is overcompensated, exhibits a prominent phase shift up to π maintaining the flat unity transmission across the whole broad spectra. Bandwidth of this plasmonic phase shift can be controlled by adjusting the distance between the plasmonic waveguide and the nanoresonator. For a moderate distance, within bandwidth of 100 GHz, the phase shift and transmission are constantly maintained. The plasmonic phase can be shift-keying-modulated by a pumping signal in the gain-assisted nanoresonator. A needed length in our approach is of nanoscale while already suggested types of plasmonic phase modulator are of micrometer scale in length. The energy consumption per bit, which benefits from the nano size of this device, is ideally low on the order of 10 fJ/bit. The controllable plasmonic phase shift can find applications in nanoscale Mach–Zehnder interferometers and other phase-sensitive devices as well as directly in plasmonic phase shift keying modulators.

Enhanced Second Harmonic Generation by Mode Matching in Gain-assisted Double-plasmonic Resonance Nanostructure

We theoretically study the gain-assisted double plasmonic resonances to enhance second harmonic generation (SHG) in a centrosymmetric multilayered silver-dielectric-gold-dielectric (SDGD) nanostructure. Introducing gain media into the dielectric layers can not only compensate the dissipation and lead to giant amplification of surface plasmons (SPs), but also excite local quadrupolar plasmon which can boost SHG by mode matching. Specifically, as the quadrupolar mode dominates SHG in our nanostructure, under the mode matching condition, the intensity of second harmonic near-field can be enhanced by 4.43 × 102 and 1.21 × 105 times when the super-resonance is matched only at the second harmonic (SH) frequency or fundamental frequency, respectively. Moreover, the intensity of SHG near-field is enhanced by as high as 6.55 × 107 times when the nanostructure is tuned to double super-resonances at both fundamental and SH frequencies. The findings in this work have potential applications in the design of nanosensors and nanolasers.

Asymmetric growth of Au-core/Ag-shell nanorods with a strong octupolar plasmon resonance and an efficient second-harmonic generation

Colloidal Au-core/Ag-shell nanorods with an asymmetric transverse cross-section and a strong octupolar plasmon resonance are synthesized by the controlled growth of Ag shells on one side of the Au cores. A largely enhanced second harmonic generation (SHG) from these asymmetric core–shell nanorods is demonstrated for the first time by tuning the dipolar and the octupolar plasmon modes to make them resonant with the fundamental and harmonic frequencies, respectively. The SHG intensity of the Au–Ag nanorods with dual-frequency resonances is enhanced by 21 times compared to that of the bare Au nanorods. The co-existence of the dipolar, quadrupolar, and octupolar radiations in the SHG is revealed. Additionally, the cellular uptake of the Au–Ag nanorods is monitored and the evolution of the membrane bleb is successfully observed by the SHG imaging. Our observations provide a strategy for enhancing the SHG of the colloidal metal nanoparticles and can have applications in chemical process monitoring and biological sensing.

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.

Plasmon-assisted site-selective growth of Ag nanotriangles and Ag-Cu 2 O hybrids

We report a plasmon-assisted growth of metal and semiconductor onto the tips of Ag nanotriangles (AgNTs) under light irradiation. The site-selective growth of Ag onto AgNTs are firstly demonstrated on the copper grids and amine-coated glass slides. As the irradiation time increases, microscopic images indicate that AgNTs gradually touch with each other and finally “weld” tip-to-tip together into the branched chains. Meanwhile, the redshift of plasmon band is observed in the extinction spectra, which agrees well the growth at the tips of AgNTs and the decrease of the gaps between the adjacent nanotriangles. We also synthesize AgNT-Cu2O nanocomposites by using a photochemical method and find that the Cu2O nanoparticles preferably grow on the tips of AgNTs. The site-selective growth of Ag and Cu2O is interpreted by the local field concentration at the tips of AgNTs induced by surface plasmon resonance under light excitation.