Eliminating irreproducibility in SERS substrates

Irreproducibility in surface-enhanced Raman spectroscopy (SERS) due to variability among substrates is a source of recurrent debate within the field. It is regarded as a major hurdle towards the widespread adoption of SERS as a sensing platform. Most of the literature focused on developing substrates for various applications considers reproducibility of lower importance. Here, we address and analyse the sources of this irreproducibility in order to show how these can be minimised. We apply our findings to a simple substrate demonstrating reproducible SERS measurements with relative standard deviations well below 1% between different batches and days. Identifying the sources of irreproducibility and understanding how to reduce these can aid in the transition of SERS from the lab to real world applications.Isaac Newton Trust Leverhulme Trust Winton Programme for the Physics of Sustainability Trinity College, University of Cambridg

Suppressed quenching and strong-coupling of Purcell-enhanced single-molecule emission in plasmonic nanocavities

An emitter in the vicinity of a metal nanostructure is quenched by its decay through nonradiative channels, leading to the belief in a zone of inactivity for emitters placed within <10 nm of a plasmonic nanostructure. Here we demonstrate and explain why in tightly coupled plasmonic resonators forming nanocavities “quenching is quenched” due to plasmon mixing. Unlike isolated nanoparticles, such plasmonic nanocavities show mode hybridization, which can massively enhance emitter excitation and decay via radiative channels, here experimentally confirmed by laterally dependent emitter placement through DNA-origami. We explain why this enhancement of excitation and radiative decay can be strong enough to facilitate single-molecule strong coupling, as evident in dynamic Rabi-oscillations

Near-Field Optical Drilling of Sub-λ Pits in Thin Polymer Films

Under UV illumination, polymer films can undergo chain scission and contract. Using this effect, tightly focused laser light is shown to develop runaway near-field concentration that drills sub-100 nm pits through a thin film. This subwavelength photolithography can be controlled in real time by monitoring laser scatter from the evolving holes, allowing systematic control of the void diameter. Our model shows how interference between the substrate and film together with near-field focusing by the evolving crevice directs this formation and predicts minimum pit sizes in films of 100 nm thickness on gold substrates. The smallest features so far are 60 nm diameter pits using 447 nm light focused onto polystyrene through a ×100 objective (NA = 0.8). Such arrays of pits can be easily used as masks for fabricating more complex nanostructures, such as plasmonic nanostructures and biomicrofluidic devices. This demonstration shows the potential for harnessing near-field feedback in optical direct-writing for nanofabrication.This research is supported by UK Engineering and Physical Sciences Research Council Grants EP/G060649/1, EP/N016920/1, and EP/L027151/1, ERC Grant LINASS 320503, and Leverhulme Trust (ECF-2016-606). R.C. acknowledges support from the Dr. Manmohan Singh scholarship from St John’s College, University of Cambridge

Single-molecule optomechanics in "picocavities"

Trapping light with noble metal nanostructures overcomes the diffraction limit and can confine light to volumes typically on the order of 30 cubic nanometers. We found that individual atomic features inside the gap of a plasmonic nanoassembly can localize light to volumes well below 1 cubic nanometer ("picocavities"), enabling optical experiments on the atomic scale. These atomic features are dynamically formed and disassembled by laser irradiation. Although unstable at room temperature, picocavities can be stabilized at cryogenic temperatures, allowing single atomic cavities to be probed for many minutes. Unlike traditional optomechanical resonators, such extreme optical confinement yields a factor of 10$^{6}$ enhancement of optomechanical coupling between the picocavity field and vibrations of individual molecular bonds. This work sets the basis for developing nanoscale nonlinear quantum optics on the single-molecule level.Supported by Project FIS2013-41184-P from MINECO (Ministerio de Economía y Competitividad) and IT756-13 from the Basque government consolidated groups (M.K.S., Y.Z., A. Demetriadou, R.E., and J.A.); the Winton Programme for the Physics of Sustainability (F.B.); the Dr. Manmohan Singh scholarship from St. John’s College (R.C.); the UK National Physical Laboratory (C.C.); the Fellows Gipuzkoa Program of the Gipuzkoako Foru Aldundia via FEDER funds of the European Union “Una manera de hacer Europa” (R.E.); UK Engineering and Physical Sciences Research Council grants EP/G060649/1 and EP/L027151/1; and European Research Council grant LINASS 320503

Interrogating Nanojunctions Using Ultraconfined Acoustoplasmonic Coupling

Single nanoparticles are shown to develop a localized acoustic resonance, the bouncing mode, when placed on a substrate. If both substrate and nanoparticle are noble metals, plasmonic coupling of the nanoparticle to its image charges in the film induces tight light confinement in the nanogap. This yields ultrastrong “acoustoplasmonic” coupling with a figure of merit 7 orders of magnitude higher than conventional acousto-optic modulators. The plasmons thus act as a local vibrational probe of the contact geometry. A simple analytical mechanical model is found to describe the bouncing mode in terms of the nanoscale structure, allowing transient pump-probe spectroscopy to directly measure the contact area for individual nanoparticles.This work is supported by UK EPSRC grants EP/G060649/1, EP/L027151/1 and ERC grant LINASS 320503, as well as the Winton Programme for the Physics of Sustainability (FB, YdV-IR, JM), the Dr Manmohan Singh Scholarship from St John’s College (RC)

Mapping SERS in CB:Au Plasmonic Nanoaggregates

In order to optimize surface-enhanced Raman scattering (SERS) of noble metal nanostructures for enabling chemical identification of analyte molecules, careful design of nanoparticle structures must be considered. We spatially map the local SERS enhancements across individual micro-aggregates comprised of monodisperse nanoparticles separated by rigid monodisperse 0.9 nm gaps and show the influence of depositing these onto different underlying substrates. Experiments and simulations show that the gaps between neighbouring nanoparticles dominate the SERS enhancement far more than the gaps between nanoparticles and substrate

Suppressed Quenching and Strong-Coupling of Purcell-Enhanced Single-Molecule Emission in Plasmonic Nanocavities

An emitter in the vicinity of a metal nanostructure is quenched by its decay through nonradiative channels, leading to the belief in a zone of inactivity for emitters placed within <10 nm of a plasmonic nanostructure. Here we demonstrate and explain why in tightly coupled plasmonic resonators forming nanocavities “quenching is quenched” due to plasmon mixing. Unlike isolated nanoparticles, such plasmonic nanocavities show mode hybridization, which can massively enhance emitter excitation and decay via radiative channels, here experimentally confirmed by laterally dependent emitter placement through DNA-origami. We explain why this enhancement of excitation and radiative decay can be strong enough to facilitate single-molecule strong coupling, as evident in dynamic Rabi-oscillations.We acknowledge support from EPSRC Grants EP/G060649/1 and EP/L027151/1 and European Research Council Grant LINASS 320503. N.K. and A.D. contributed equally to this work. R.C. acknowledges support from the Dr. Manmohan Singh scholarship from St. John’s College

Unrelenting plasmons

Following a brief historic introduction to plasmons, their useful properties and early applications, we highlight some of the key advances in the field over the past decade. We then discuss new directions for the future, such as the use of 2D materials and strong coupling phenomena, which are likely to shape the field over the next ten years. For centuries, metals were employed in optical applications only as mirrors and gratings. New vistas opened up in the late 1970s and early 1980s with the discovery of surface-enhanced Raman scattering and the use of surface plasmon (SP) resonances for sensing. However, it was not until the 1990s, with the appearance of accurate and reliable nanofabrication techniques, that plasmonics blossomed1. Initially, the attention focused on the exploitation of SPs (collective electronic oscillations at the surface of metals) for sensing, subwavelength waveguiding and extraordinary optical transmission2. Since then, the scientific and technological interest in SPs has expanded. Correspondingly, as illustrated in Fig. 1, the number of publications in the field has increased in a steady exponential fashion for more than two decades, and the momentum driving plasmonics research looks set to continue (...

Plasmonic tunnel junctions for single-molecule redox chemistry

Nanoparticles attached just above a flat metallic surface can trap optical fields in the nanoscale gap. This enables local spectroscopy of a few molecules within each coupled plasmonic hotspot, with near thousand-fold enhancement of the incident fields. As a result of non-radiative relaxation pathways, the plasmons in such sub-nanometre cavities generate hot charge carriers, which can catalyse chemical reactions or induce redox processes in molecules located within the plasmonic hotspots. Here, surface-enhanced Raman spectroscopy allows us to track these hot-electron-induced chemical reduction processes in a series of different aromatic molecules. We demonstrate that by increasing the tunnelling barrier height and the dephasing strength, a transition from coherent to hopping electron transport occurs, enabling observation of redox processes in real time at the single-molecule level.We acknowledge financial support from EPSRC grants EP/G060649/1, EP/I012060/1, EP/L027151/1, ERC grant LINASS 320503. F.B. acknowledges support from the Winton Programme for the Physics of Sustainability. S.J.B. thanks the European Commission for a Marie Curie Fellowship (NANOSPHERE, 658360). M.K. thanks the European Commission for a Marie Curie Fellowship (SPARCLEs, 7020005). P.N. acknowledges support from the Harvard University Center for the Environment (HUCE). R.C. acknowledges support from the Dr Manmohan Singh scholarship from St John’s College. C.C. acknowledges support from the UK National Physical Laboratories. R.S. acknowledges computational resources provided by the Center for Computational Innovations (CCI) at Rensselaer Polytechnic Institute