Surface plasmon-mediated photocatalysis has attracted great interest. One model system is that p-aminothiophenol (PATP) could be dimerized into a new molecule of p,p′-dimercaptoazobenzene on noble metal surfaces under laser illumination. The mechanism of this catalytic reaction has caused wide discussion, but it is not fully understood yet. We performed a series of experiments for PATP and p-nitrothiophenol by tip-enhanced Raman spectroscopy and surface-enhanced Raman spectroscopy in a well-controlled gas environment (high vacuum, air, N2, and O2), and on different substrates (Au, Ag, Cu, Al, and corresponding oxides). The experimental results indicate that the electron acceptor or donor plays a decisive role in whether the reactions can occur or not.
Plasmonic nanostructures can be used as nanoscale heat sources under light illumination. The heat generation leads to the increase of local temperature and thus the change of permittivities of both the plasmonic metals and surrounding dielectric media due to the thermo-optic effect. In this talk, I will present our research on using the photothermal properties of silver nanowires to modulate propagating surface plasmons and using gold nanorods with tunable surface plasmon resonances to demonstrate resonant scattering enhanced photothermal imaging.
The hybrid systems of multiple quantum emitters coupled with plasmonic waveguides provide promising building blocks for the future integrated quantum nanophotonic circuits. The techniques that can selectively excite adjacent quantum emitters in a diffraction-limited area are of great importance for studying the plasmon-mediated interaction between quantum emitters and manipulating the generation and propagation of single plasmons in nanophotonic circuits. We show that by modulating the electric field on the nanowire using the interference of surface plasmons, multiple quantum dots coupled with a silver nanowire can be controllably excited. We experimentally demonstrate the selective excitation of two quantum dots with a separation distance of about 100 nm. Our work provides a new kind of optical excitation and characterization method overcoming the diffraction limit, and adds a new tool for studying and manipulating single quantum emitters and single plasmons for quantum plasmonic circuitry applications.
The research of the fabrication of plasmonic nanowire waveguides and circuits for nanophotonic circuitry applications by lithographic fabrication method has attracted much attention. Here we report an approach for fabricating metal nanowire networks by electron beam lithography and metal film deposition techniques. The gold nanowire structures without adhesion layer are fabricated on quartz substrates and a thin layer of Al2O3 film is deposited using atom layer deposition for immobilizing the nanostructures. During the Al2O3 deposition process, the thermal annealing effect can decrease the surface plasmon loss on the nanowires. Y-shaped gold nanowire networks are fabricated and the surface plasmons can be routed to different branches by controlling the length of the main nanowire and the polarization of the excitation light. The simulation results of the electric field distributions show that the zigzag distribution of electric field on the main wire determines the surface plasmon routing. The interference of surface plasmons in the nanowire network can modulate the output intensity to realize Boolean logic operations. AND, OR, XOR and NOT gates are realized in three-terminal nanowire networks, and NAND, NOR and XNOR gates are realized in four-terminal nanowire networks. This work provides a new way for on-chip integrated plasmonic circuits.
The interactions between surface plasmons in metal nanostructures and excitons in quantum emitters lead to many
interesting phenomena that are strongly dependent on the quantum yield of surface plasmons. The experimental
measurement of this quantum yield is hindered due to the difficulty in distinguishing all the possible exciton
recombination channels. By utilizing the propagation of surface plasmons, we experimentally measured the decay rates
of all exciton recombination channels, and thus obtained the quantum yield of single surface plasmons generated by a
quantum dot coupled with a silver nanowire.
Plasmonic devices can be used to construct nanophotonic circuits and are very promising candidates for next-generation
information technology. The functions of plasmonic circuits rely on the rigorous control of plasmon modes. Two
different methods were proposed to control the propagation of surface plasmons (SPs) supported by Ag nanowires
(NWs). The first one is modulating the beat period of the near-field distribution pattern, which can be realized by
depositing Al2O3 layer or changing the refractive index of surrounding medium. The beat period increasing by 90 nm per
nanometer of Al2O3 coating or by 16 μm per refractive index unit was obtained in experiments. The second one is
introducing local structural symmetry breaking to realize mode conversion of SPs. Three typical structures including
NW-nanoparticle (NP) structure, branched NW and bent NW were used to investigate the mode conversion. It’s revealed
that the mode conversion is a scattering induced process. The lossy characteristic of SPs at optical frequencies typically
limits the propagation length and hinders the further development of integrated plasmonic circuits. CdSe
nanobelt/Al2O3/Ag film hybrid plasmonic waveguide was proposed to compensate the loss of SPs by using an optical
pump-probe technique. Compared to the measured internal gain, the propagation loss was almost fully compensated for
the TM mode. These results for mode control and loss compensation of propagating SPs are important for constructing
functional nanophotonic circuits.
We report the first experimental demonstration of plasmon-exciton coupling between silver nanowire (NW) and a pair of
quantum dots (QDs). The resolving of single surface plasmons (SPs) generated in the NW-QD pair system is achieved.
The accurate positions of the two QDs and NW ends are obtained by using a maximum likelihood single molecule
localization method, and the separation distances between the two QDs range from microns to 200 nm within the
diffraction limit. Parameters including the SP propagation length and the wire terminal reflectivity are experimentally
determined and taken into account. The efficiency of plasmon generation due to the exciton-plasmon coupling is
obtained for each QD.
KEYWORDS: Surface plasmons, Polarization, Silver, Near field optics, Dielectric polarization, Near field, Near field scanning optical microscopy, Optical fibers, Radio propagation, Nanowires
The comprehensive understanding of the excitation and propagation of surface plasmons (SPs) on metallic nanowires
(NWs) is essential for potential applications of these materials as nanoscale optical waveguides. Combining theory and
different experimental methods, we did intensive study on the excitation and propagation of SP modes in crystal Ag
nanowires. We found the excitation of NW SP modes is strongly affected by the excitation configuration. When an
optically “thick” NW is radiated at the end of the NW, several SP modes could be excited simultaneously with
appropriate incident polarization. If the NW is in the medium of uniform refractive index, the coherent superposition of
these SP modes generates chiral SPs in single NW, and the handedness of the chiral SPs can be controlled by the input
polarization angle. When we use a near field scanning optical fiber tip to excite the SPs on metallic nanowires from the
middle of the NW, we also found multiple SP modes in the NWs can be excited through polarization selective near field
interaction. The excitation mechanism of the tip-induced SP propagation is quite different from the previous wire-endlaunching
scheme. We found the input coupling efficiency is modulated by Fabry-Pérot interferences in the near field
coupling case.
The local electric field distribution of propagating surface plasmons along Ag nanowires can be imaged by coating the
nanowires with a layer of quantum dots, which provide a useful tool to study the plasmon propagation. In simple
photonic networks composed of Ag nanowires, plasmons can be controllably routed to a specific nanowire output. The
underlying physical mechanism is that the plasmon interferences modulate the near field distribution and thus control the
output intensity. The plasmon interference can result in combinations of optical signals that execute specific
interferometric Boolean logic operations. And a complete family of Boolean logic gates is realized in the simple
nanowire networks. The primary nanowire in the network can be viewed as the plasmonic equivalent of a bus in a central
processing unit. Furthermore, a plasmonic NOR gate is demonstrated by cascading OR gate and NOT gate. To realize
the cascaded NOR gate, the plasmon wave packet should overlap with the junction between the main wire and the
branch wire for the control signal.
Surface plasmon resonances in metal nanostructures can lead to novel optical properties. The greatly enhanced
electromagnetic field makes surface-enhanced Raman scattering (SERS) a highly sensitive spectroscopic technique. We
employed Ag nanowires as plasmonic waveguide and achieved
remote-excitation SERS at a few molecules level. The
junctions between metal nanowires and nanoparticles offer hot spots for SERS, while the enhancement strongly depends
on the laser polarization. We studied the polarization dependence in Au nanowire-nanoparticle systems of different
geometry. The polarization of Raman-scattered light in SERS is a rarely studied topic. We found nanoantennas
composed of a few nanoparticles can manipulate the polarization of emission light. A nanoparticle trimer is the simplest
nanoantenna to realize the polarization control. By tuning the position and size of the third particle, emission polarization
can be modified in a controllable way. In addition, the refractivity of the surrounding media also plays a crucial role for
the emission polarization.
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