Optimizing Substrate-Mediated Plasmon Coupling toward High-Performance Plasmonic Nanowire Waveguides

Seeking better plasmonic waveguides is of critical importance for minimizing photonic circuits into the nanometer scale. We have made a theoretical study of the properties of surface plasmon polaritons in a metallic nanowire over substrate (NWOS) configuration. The dielectric substrate breaks the symmetry of the system and mediates the coupling of different primary wire plasmons. The lowest order hybridized mode can be used for subwavelength plasmonic waveguiding for NWOS with thin wire, for a low-permittivity substrate, and in the shorter wavelength region. For NWOS with a high-permittivity substrate, leaky radiation into the substrate raises the propagation losses so that the propagation distance is shorter in the longer wavelength region. By simply adding a high-permittivity layer onto the low-permittivity substrate, we show that leaky radiation can be blocked and high-performance plasmonic waveguiding can be extended to the near-infrared region. Importantly, the NWOS configuration is compatible with current silicon technologies and can be designed into various deep subwavelength active devices such as electro-optical or all-optical modulators

Quantum Yield of Single Surface Plasmons Generated by a Quantum Dot Coupled with a Silver Nanowire

The interactions between surface plasmons (SPs) in metal nanostructures and excitons in quantum emitters (QEs) lead to many interesting phenomena and potential applications that are strongly dependent on the quantum yield of SPs. The difficulty in distinguishing all the possible exciton recombination channels hinders the experimental determination of SP quantum yield. Here, we experimentally measured for the first time the quantum yield of single SPs generated by the exciton–plasmon coupling in a system composed of a single quantum dot and a silver nanowire (NW). By utilizing the SP guiding property of the NW, the decay rates of all the exciton recombination channels, i.e., direct free space radiation channel, SP generation channel, and nonradiative damping channel, are quantitatively obtained. It is determined that the optimum emitter-NW coupling distance for the largest SP quantum yield is about 10 nm, resulting from the different distance-dependent decay rates of the three channels. These results are important for manipulating the coupling between plasmonic nanostructures and QEs and developing on-chip quantum plasmonic devices for potential nanophotonic and quantum information applications

Low Frequency Vibration Assisted Catalytic Aquathermolysis of Heavy Crude Oil

Low frequency vibration was applied to assist the catalytic aquathermolysis reaction of heavy oil for the first time. The optimum vibration parameters were first optimized by orthogonal experiments: vibration acceleration is 3 m·s^–2, vibration time is 90 min, and vibration frequency is 20 Hz, and the efficient consequences of the parameters are as follows: vibration acceleration > vibration time > vibration frequency. Under the optimum vibration parameters, heavy oil viscosity could be reduced by 88.2% after reaction, and the viscosity bounce rate of treated oil is 4.9%. To evaluate the vibration’s performance, the structure and group compositions of the oil before and after reaction were characterized by modern chemical analysis techniques, such as column chromatography, elemental analysis, gas chromatography, and Fourier transform infrared spectrometery. It is found that vibration cannot initiate new reactions in the process of catalytic aquathermolysis, but it can promote the original reactions and deepen the reaction degree such as dealcoholization reaction, hydrogenation reaction, ring-opening reaction, and alkyl side chain removal reaction, etc. Compared to catalytic aquathermolysis reaction, vibration assisted catalytic aquathermolysis can further decrease the average molecular weight of heavy oil, increase the saturate and aromatic contents, decrease the resin and asphaltene contents, improve the ratio of <i>N</i>_H/<i>N</i>_C, and decrease the heteroatoms content of heavy oil. Vibration plays more important role in the in situ catalytic aquathermolysis reactions due to the fact that vibration could aid to reduce the adsorption of catalyst and help the catalysts contact with heavy oil sufficiently in the porous media. The preliminary results proved that the vibration assisted in situ catalytic aquathermolysis technique is feasible and it has some practical value

Resolving Single Plasmons Generated by Multiquantum-Emitters on a Silver Nanowire

Surface plasmons, the collective oscillations of electrons at metal surface, provide the ability to enhance the weak interaction between individual quantum emitters and photons for quantum information applications. The generation of single plasmons by coupling silver nanowire with single quantum emitters opens the prospects of using quantum optical techniques to control single surface plasmons and designing novel quantum plasmonic devices. However, the real applications will deal with multiple plasmons generated from multiple quantum emitters. Here we report the first experimental demonstration of resolving single plasmons generated by a pair of quantum dots (QDs) on a silver nanowire waveguide. The accurate positions of the two QDs with separation ranging from micrometers to 200 nm within the diffraction limit are determined by using super-resolution imaging method. The efficiency of plasmon generation due to the exciton–plasmon coupling is obtained for each QD. Our research takes a crucial step toward the experimental study of coupled systems of multiple quantum emitters and plasmonic waveguides and would shed new light on the study of light-matter interactions for potential quantum optics and quantum information applications

Direct Visual Evidence for Quinoidal Charge Delocalization in Poly-<i>p</i>-phenylene Cation Radical

Recently, X-ray crystallographic evidence of quinoidal charge delocalization in poly-p-phenylene cation radicals was reported [Banerjee, M. et al., J. Am. Chem. Soc. 2007, 129, 8070]. In this paper, direct visual evidence for quinoidal charge delocalization in quaterphylene (QP) is shown with three-dimensional (3D) charge difference densities. It was revealed that the extra positive charge mainly localized on the two center units at the ground state, while the extra positive charge will delocalize to the two outer units upon electronic state transitions by photoexcitation. The 2D plots together with the corresponding charge difference densities were interpreted as large distance−charge oscillations, implying that in the positive species upon excitation a nearly free oscillating motion of a hole occurs. For the QP cation radical, the transition dipole moment of S1 represents mesoscopic dipole antennae

Multiple-Particle Nanoantennas for Enormous Enhancement and Polarization Control of Light Emission

We investigate the light emission from dipolar emitters located within nanoparticle antennas. It is found that the enormous emission enhancement can reach nearly a million fold. For multinanoparticle antennas, the polarization of the emissions strongly depends on the geometry of the antennas, the emitted wavelengths, and the dielectric functions of surrounding media. It is shown that a polarization nanorotator, which modulates the emission polarization on the nanometer scale, can be readily realized by varying either the geometry or surrounding media of nanoparticle antennas

Emergence of material momentum in optical media

Understanding the momentum of light when propagating through optical media is not only fundamental for studies as varied as classical electrodynamics and polaritonics in condensed matter physics, but also for important applications such as optical-force manipulations and photovoltaics. From a microscopic perspective, an optical medium is in fact a complex system that can split the light momentum into the electromagnetic field, as well as the material electrons and the ionic lattice. Here, we disentangle the partition of momentum associated with light propagation in optical media, and develop a quantum theory to explicitly calculate its distribution. The material momentum here revealed, which is distributed among electrons and ionic lattice, leads to the prediction of unexpected phenomena. In particular, the electron momentum manifests through an intrinsic DC current, and strikingly, we find that under certain conditions this current can be along the photonic wave vector, implying an optical pulling effect on the electrons. Likewise, an optical pulling effect on the lattice can also be observed, such as in graphene during plasmon propagation. We also predict the emergence of boundary electric dipoles associated with light transmission through finite media, offering a microscopic explanation of optical pressure on material boundaries

Rotational Doppler cooling and heating

Doppler cooling is a widely used technique to laser cool atoms and nanoparticles exploiting the Doppler shift involved in translational transformations. The rotational Doppler effect arising from rotational coordinate transformations should similarly enable optical manipulations of the rotational degrees of freedom in rotating nanosystems. Here, we show that rotational Doppler cooling and heating (RDC and RDH) effects embody rich and unexplored physics, such as a strong dependence on particle morphology. For geometrically confined particles, such as a nanorod that can represent diatomic molecules, RDC and RDH follow similar rules as their translational Doppler counterpart, where cooling and heating are always observed at red- or blue-detuned laser frequencies, respectively. Surprisingly, nanosystems that can be modeled as a solid particle shows a strikingly different response, where RDH appears in a frequency regime close to their resonances, while a detuned frequency produces cooling of rotation. We also predict that the RDH effect can lead to unprecedented spontaneous chiral symmetry breaking, whereby an achiral particle under linearly polarized illumination starts spontaneously rotating, rendering it nontrivial compared to the translational Doppler effect. Our results open up new exciting possibilities to control the rotational motion of molecules and nanoparticles

Creating Hot Nanoparticle Pairs for Surface-Enhanced Raman Spectroscopy through Optical Manipulation

We use optical tweezers to move single silver nanoparticles into near-field contact with immobilized particles, forming isolated surface-enhanced Raman spectroscopy (SERS) active Ag particle dimers. The surface-averaged SERS intensity increases by a factor ∼20 upon dimerization. Electrodynamics calculations indicate that the final approach between the particles is due to “optical binding”. The described methodology may facilitate controlled single molecule SERS analysis

Surface-Enhanced Raman Spectroscopy and Nanogeometry: The Plasmonic Origin of SERS

Using a series of highly regular nanostructures consisting of periodic Ag nanowires fabricated in porous aluminum oxide, we validate the overwhelmingly plasmonic origin of the most intense SERS signals such as those responsible for single-molecule SERS, demonstrating its sensitive dependence on the system's nanogeometry. By varying the interwire gap distance from 35 to 10 nm, the SERS intensity excited with 785 nm laser light, increased over 200-fold. These observations were shown to agree quantitatively with electromagnetic field calculations carried out using the free space green's tensor method