Modification of ground-state chemical reactivity via light-matter coherence in infrared cavities.

Reaction-rate modifications for chemical processes due to strong coupling between reactant molecular vibrations and the cavity vacuum have been reported; however, no currently accepted mechanisms explain these observations. In this work, reaction-rate constants were extracted from evolving cavity transmission spectra, revealing resonant suppression of the intracavity reaction rate for alcoholysis of phenyl isocyanate with cyclohexanol. We observed up to an 80% suppression of the rate by tuning cavity modes to be resonant with the reactant isocyanate (NCO) stretch, the product carbonyl (CO) stretch, and cooperative reactant-solvent modes (CH). These results were interpreted using an open quantum system model that predicted resonant modifications of the vibrational distribution of reactants from canonical statistics as a result of light-matter quantum coherences, suggesting links to explore between chemistry and quantum science.

Ultrafast Transmission Modulation and Recovery via Vibrational Strong Coupling.

Strong coupling between vibrational modes and cavity optical modes leads to the formation of vibration-cavity polaritons, separated by the vacuum Rabi splitting. The splitting depends on the square root of the concentration of absorbers confined in the cavity, which has important implications on the response of the coupled system after ultrafast infrared excitation. In this work, we report on solutions of W(CO)6 in hexane with a concentration chosen to access a regime that borders on weak coupling. Under these conditions, large fractions of the W(CO)6 oscillators can be excited, and the anharmonicity of the molecules leads to a commensurate reduction in the Rabi splitting. We report excitation fractions > 0.4, depending on excitation pulse intensity, and show drastic increases in transmission that can be modulated on the picosecond time scale. In comparison to previous experiments, the transient spectra that we observe are much simpler because excited-state transitions lie outside of the transmission spectrum of the cavity, thereby contributing only weakly to the spectra. We find that the Rabi splitting recovers with the characteristic vibrational relaxation lifetime and anisotropy decay of uncoupled W(CO)6, implying that polaritons are not directly involved in the relaxation we observe after the first few ps. The results help corroborate the model that we proposed to describe the results at higher concentrations and show that the ground-state bleach of cavity-coupled molecules has a broad, multisigned spectral response.

Surface plasmon polariton-induced hot carrier generation for photocatalysis.

Non-radiative plasmon decay in noble metals generates highly energetic carriers under visible light irradiation, which opens new prospects in the fields of photocatalysis, photovoltaics, and photodetection. While localized surface plasmon-induced hot carrier generation occurs in diverse metal nanostructures, inhomogeneities typical of many metal-semiconductor plasmonic nanostructures hinder predictable control of photocarrier generation and therefore reproducible carrier-mediated photochemistry. Here, we generate traveling surface plasmon polaritons (SPPs) at the interface between a noble metal/titanium dioxide (TiO2) heterostructure film and aqueous solution, enabling simultaneous optical and electrochemical interrogation of plasmon-mediated chemistry in a system whose resonance may be continuously tuned via the incident optical excitation angle. To the best of our knowledge, this is the first experimental demonstration of SPP-induced hot carrier generation for photocatalysis. We found electrochemical photovoltage and photocurrent responses as SPP-induced hot carriers drive both solution-based oxidation of methanol and the anodic half-reaction of photoelectrochemical water-splitting in sodium hydroxide solution. A strong excitation angle dependence and linear power dependence in the electrochemical photocurrent confirm that the photoelectrochemical reactions are SPP-driven. SPP-generated hot carrier chemistry was recorded on gold and silver and with two different excitation wavelengths, demonstrating potential for mapping resonant charge transfer processes with this technique. These results will provide the design criteria for a metal-semiconductor hybrid system with enhanced hot carrier generation and transport, which is important for the understanding and application of plasmon-induced photocatalysis.

Low-Power Light Guiding and Localization in Optoplasmonic Chains Obtained by Directed Self-Assembly.

Optoplasmonic structures contain plasmonic components embedded in a defined photonic environment to create synergistic interactions between photonic and plasmonic components. Here, we show that chains of optical microspheres containing gold nanoparticles in their evanescent field combine the light guiding properties of a microsphere chain with the light localizing properties of a plasmonic nanoantenna. We implement these materials through template guided self-assembly and investigate their fundamental electromagnetic working principles through combination of electromagnetic simulations and experimental characterization. We demonstrate that optoplasmonic chains implemented by directed self-assembly achieve a significant reduction in guiding losses when compared with conventional plasmonic waveguides and, at the same time, retain the light localizing properties of plasmonic antennas at pre-defined locations. The results reinforce the potential of optoplasmonic structures for realizing low-loss optical interconnects with high bandwidth.

Directed Assembly of Optoplasmonic Hybrid Materials with Tunable Photonic-Plasmonic Properties.

Optoplasmonic materials are metallo-dielectric hybrid structures that combine metallic and dielectric components in defined geometries in which plasmonic and photonic modes synergistically interact. These beneficial interactions can be harnessed by integrating plasmonic nanoantennas into defined photonic environments generated, for instance, by discrete optical resonators or extended systems of diffractively coupled nanoparticles. Optoplasmonic structures facilitate photonic-plasmonic mode coupling and offer degrees of freedom for creating optical fields with predefined amplitude and phase in space and time that are absent in conventional photonic or plasmonic structures. This Perspective reviews the fundamental electromagnetic mechanisms underlying selected optoplasmonic approaches with an emphasis on materials available through template-guided self-assembly strategies.

Demonstration of efficient on-chip photon transfer in self-assembled optoplasmonic networks.

Plasmonic nanoantennas facilitate the manipulation of light fields on deeply sub-diffraction-limited length scales, but high dissipative losses in metals make new approaches for an efficient energy transfer in extended on-chip integrated plasmonic circuits mandatory. We demonstrate in this article efficient photon transfer in discrete optoplasmonic molecules comprising gold nanoparticle (NP) dimer antennas located in the evanescent field of a 2 µm diameter polystyrene bead, which served as an optical microcavity (OM). The optoplasmonic molecules were generated through a guided self-assembly strategy in which the OMs were immobilized in binding sites generated by quartz (SiO2) or silicon posts that contained plasmonic nanoantennas on their tips. Control of the post height facilitated an accurate positioning of the plasmonic antennas into the evanescent field of the whispering gallery modes located in the equatorial plane of the OM. Cy3 and Cy5.5 dyes were tethered to the plasmonic antennas through oligonucleotide spacers to act as on-chip light sources. The intensity of Cy3 was found to be increased relative to that of Cy5.5 in the vicinity of the plasmonic antennas where strongly enhanced electric field intensity and optical density of states selectively increase the excitation and emission rates of Cy3 due to spectral overlap with the plasmon. The fluorescent dyes preferentially emitted into the OM, which efficiently trapped and recirculated the photons. We experimentally determined a relative photon transfer efficiency of 44% in non-optimized self-assembled optoplasmonic molecules in this proof-of-principle study.

Electromagnetic field enhancement and spectrum shaping through plasmonically integrated optical vortices.

We introduce a new design approach for surface-enhanced Raman spectroscopy (SERS) substrates that is based on molding the optical powerflow through a sequence of coupled nanoscale optical vortices "pinned" to rationally designed plasmonic nanostructures, referred to as Vortex Nanogear Transmissions (VNTs). We fabricated VNTs composed of Au nanodiscs by electron beam lithography on quartz substrates and characterized their near- and far-field responses through combination of computational electromagnetism, and elastic and inelastic scattering spectroscopy. Pronounced dips in the far-field scattering spectra of VNTs provide experimental evidence for an efficient light trapping and circulation within the nanostructures. Furthermore, we demonstrate that VNT integration into periodic arrays of Au nanoparticles facilitates the generation of high E-field enhancements in the VNTs at multiple defined wavelengths. We show that spectrum shaping in nested VNT structures is achieved through an electromagnetic feed-mechanism driven by the coherent multiple scattering in the plasmonic arrays and that this process can be rationally controlled by tuning the array period. The ability to generate high E-field enhancements at predefined locations and frequencies makes nested VNTs interesting substrates for challenging SERS applications.

Photonic-plasmonic mode coupling in on-chip integrated optoplasmonic molecules.

We investigate photonic-plasmonic mode coupling in a new class of optoplasmonic materials that comprise dielectric microspheres and noble metal nanostructures in a morphologically well-defined on-chip platform. Discrete networks of optoplasmonic elements, referred to as optoplasmonic molecules, were generated through a combination of top-down fabrication and template-guided self-assembly. This approach facilitated a precise and controllable vertical and horizontal positioning of the plasmonic elements relative to the whispering gallery mode (WGM) microspheres. The plasmonic nanostructures were positioned in or close to the equatorial plane of the dielectric microspheres where the fields associated with the plasmonic modes can synergistically interact with the evanescent fields of the WGMs. We characterized the far-field scattering spectra of discrete optoplasmonic molecules that comprised two coupled 2.048 µm diameter polystyrene microspheres each encircled by four 148 nm diameter Au nanoparticles (NPs), through far-field scattering spectroscopy. We observed a broadening of the TE and TM modes in the scattering spectra of the optoplasmonic dimers indicative of an efficient photonic-plasmonic mode coupling between the coupled photonic modes of the WGM resonators and the localized surface plasmon modes of the NPs. Our experimental findings are supported by generalized multiple particle Mie theory simulations, which provide additional information about the spatial distributions of the near fields associated with the photonic-plasmonic hybrid modes in the investigated optoplasmonic molecules. The simulations reveal partial localization of the spectrally sharp hybrid modes outside of the WGM microspheres on the Au NPs where the local E-field intensity is enhanced by approximately 2 orders of magnitude over that of an individual Au NP.

Periodic nanotemplating by selective deposition of electroless gold island films on particle-lithographed dimethyldichlorosilane layers.

Uniform hexagonal arrays of diverse nanotemplated metal structures were formed via selective electroless gold plating on particle-lithographed dimethyldichlorosilane layers. Surface-associated water at silica bead interstices was shown to correlate with the formation of silane rings with outer ring diameters ranging from 522.5+/-29.7 to 1116.9+/-52.6 nm and/or spherical gold nanoparticles with diameters from 145.5+/-20.2 to 389.1+/-51.1 nm in the array. Reproducibility and millimeter-size scalability of the array were achieved without the need for expensive and sophisticated lithography or metal deposition equipment. The formation of each structure was explained on the basis of the silanization mechanism and microscopic characterization, as well as dimensional analysis of the nanostructures. This new, facile, and versatile method enables fine fabrication of regular metal nanoparticle array platforms to improve optical and plasmonic features in nanoelectronics and nanophotonic devices.

Enhanced uniformity in arrays of electroless plated spherical gold nanoparticles using tin presensitization.

Gold nanoparticle arrays created with electroless gold plating provide a unique means of transforming nanocylinders usually formed in electron beam lithography to spherical nanoparticles. Alone, electroless gold plating is not selective to the substrate and results in the formation of a gold film on all exposed surfaces of an electron beam patterned sample, including the electron resist. Undesired gold plating occurred near patterned features on the substrate surface, which was reduced by increasing post-spin-coat cure time. When the electron resist is removed, some nanocylinders break off with the gold film, leaving partial cylinders or holes in the patterned elements. By presensitizing the substrate surface with tin, gold cylinders may be selectively deposited to the substrate surface without forming a film on the electron resist. Tin presensitized arrays were produced with 47.1 +/- 7.4 nm radius gold nanoparticles with an interparticle distance of 646.0 +/- 12.4 nm. Defects from sheared, missing, and redeposited Au particles associated with the resist removal were minimized, resulting in enhanced size and shape uniformity of pillars and arrays. Hollow particles were eliminated, and relative standard deviation in particle size was reduced by 7.4% on average, while elongation was reduced 12.3% when astigmatism was eliminated.

Electroless gold island thin films: photoluminescence and thermal transformation to nanoparticle ensembles.

Electroless gold island thin films are formed by galvanic replacement of silver reduced onto a tin-sensitized silica surface. A novel approach to create nanoparticle ensembles with tunable particle dimensions, densities, and distributions by thermal transformation of these electroless gold island thin films is presented. Deposition time is adjusted to produce monomodal ensembles of nanoparticles from 9.5 +/- 4.0 to 266 +/- 22 nm at densities from 2.6 x 1011 to 4.3 x 108 particles cm-2. Scanning electron microscopy and atomic force microscopy reveal electroless gold island film structures as well as nanoparticle dimensions, densities, and distributions obtained by watershed analysis. Transmission UV-vis spectroscopy reveals photoluminescent features that suggest ultrathin EL films may be smoother than sputtered Au films. X-ray diffraction shows Au films have predominantly (111) orientation.