Quantum tunneling between two plasmonic resonators links nonlinear quantum optics with terahertz nanoelectronics. We describe the direct observation of and control over quantum plasmon resonances at length scales in the range 0.4 to 1.3 nanometers across molecular tunnel junctions made of two plasmonic resonators bridged by self-assembled monolayers (SAMs). The tunnel barrier width and height are controlled by the properties of the molecules. Using electron energy-loss spectroscopy, we directly observe a plasmon mode, the tunneling charge transfer plasmon, whose frequency (ranging from 140 to 245 terahertz) is dependent on the molecules bridging the gaps.

A surface plasmon resonance (SPR) based graphene biosensor is presented. It consists of a graphene sheet coated above a gold thin film, which has been proposed and experimentally fabricated recently [ChemPhysChem 11, 585 (2010)]. The biosensor uses attenuated total reflection (ATR) method to detect the refractive index change near the sensor surface, which is due to the adsorption of biomolecules. Our calculations show that the proposed graphene-on-gold SPR biosensor (with L graphene layers) is (1 + 0.025 L) x gamma (where gamma > 1) times more sensitive than the conventional gold thin film SPR biosensor. The improved sensitivity is due to increased adsorption of biomolecules on graphene (represented by the factor gamma) and the optical property of graphene.

Optical sensors are widely used for refractive index measurement in chemical, biomedical and food processing industries. Due to specific field distribution of the resonances excited, optical sensors provide high sensitivity to ambient refractive index variations. The sensitivity of optical sensor is highly dependent on material and structure of the sensor. Here, we review six major categories of optical refractive index sensors using plasmonic and photonic structures: (i) metal-based propagating plasmonic eigenwave structures, (ii) metal-based localized plasmonic eigenmode structures, (iii) dielectric-based propagating photonic eigenwave structures, (iv) dielectric-based localized photonic eigenmode structures, (v) advanced hybrid structures, and (vi) 2D material integrated structures. Representative configurations working in the wavelength range of 400−2000 nm will be selected and compared in terms of bulk refractive index sensitivities, figure of merits and working wavelengths. A technology map is established in order to define the standard and development trend for optical refractive index sensors.

The crystal phase-based heterostructures of noble metal nanomaterials are of great research interest for various applications, such as plasmonics and catalysis. However, the synthesis of unusual crystal phases of noble metals still remains a great challenge, making the construction of heterophase noble metal nanostructures difficult. Here, we report a one-pot wetchemical synthesis of well-defined heterophase fcc-2H-fcc gold nanorods (fcc: face-centred cubic; 2H: hexagonal close-packed with stacking sequence of "AB") at mild conditions. Single particle-level experiments and theoretical investigations reveal that the heterophase gold nanorods demonstrate a distinct optical property compared to that of the conventional fcc gold nanorods. Moreover, the heterophase gold nanorods possess superior electrocatalytic activity for the carbon dioxide reduction reaction over their fcc counterparts under ambient conditions. First-principles calculations suggest that the boosted catalytic performance stems from the energetically favourable adsorption of reaction intermediates, endowed by the unique heterophase characteristic of gold nanorods.

Molecular electronic control over plasmons offers a promising route for on-chip integrated molecular plasmonic devices for information processing and computing. To move beyond the currently available technologies and to miniaturize plasmonic devices, molecular electronic plasmon sources are required. Here, we report on-chip molecular electronic plasmon sources consisting of tunnel junctions based on self-assembled monolayers sandwiched between two metallic electrodes that excite localized plasmons, and surface plasmon polaritons, with tunnelling electrons. The plasmons originate from single, diffraction-limited spots within the junctions, follow power-law distributed photon statistics, and have well-defined polarization orientations. The structure of the self-assembled monolayer and the applied bias influence the observed polarization. We also show molecular electronic control of the plasmon intensity by changing the chemical structure of the molecules and by bias-selective excitation of plasmons using molecular diodes.S urface plasmon polaritons (SPPs) confine and enhance local electromagnetic fields near surfaces of metallic nanostructures at optical frequencies and have the ability to propagate along subdiffractive metallic waveguides, thereby opening up new perspectives for integrated optoelectronic circuits at the nanoscale 1-4 . However, almost all these applications use large external light sources such as monochromatic lasers. To minimize the size of the light sources and ultimately the size of the plasmonic devices, plasmons have been excited on-chip using electrically driven light sources such as (organic) light-emitting diodes (LEDs) 5-8 , laser diodes 9 , silicon spheres 10 and single carbon nanotubes 11 instead of bulky lasers. Surface plasmons have also been directly excited by tunnelling electrons in metal-insulator-metal junctions based on metal oxides 12-14 or scanning tunnelling microscopes (STMs) using vacuum or molecular tunnelling barriers [15][16][17][18][19][20][21][22][23][24][25] . During the tunnelling process, most of the electrons tunnel elastically, but some tunnel inelastically and couple to a plasmon mode. Direct excitation of plasmons by tunnelling electrons is attractive because not only is no background light generated but potentially it is also fast 26 (on the timescale of quantum tunnelling) as no slow electron-hole recombination processes are required as is the case for electroluminescent (nano)light sources 5-11 . Here, we report a direct electronic plasmon source based on molecular tunnel junctions operating via throughmolecular-bond tunnelling, where the plasmonic properties can be electrically controlled at the molecular level (without the need for optical antennas 27,28 ).In molecular electronic devices, the tunnelling barrier height is defined by the electronic energy levels of the molecule(s) bridging two electrodes. The tunnelling barrier width is defined by the length of the bridging molecule. Hence, the tunnelling gaps in molecular electronic devices are always exact...

Reducing the gap between two metal nanoparticles down to atomic dimensions uncovers novel plasmon resonant modes. Of particular interest is a mode known as the charge transfer plasmon (CTP). This mode has been experimentally observed in touching nanoparticles, where charges can shuttle between the nanoparticles via a conductive path. However, the CTP mode for nearly touching nanoparticles has only been predicted theoretically to occur via direct tunneling when the gap is reduced to ~0.4 nm. Because of challenges in fabricating and characterizing gaps at these dimensions, experiments have been unable to provide evidence for this plasmon mode that is supported by tunneling. In this work, we consider an alternative tunneling process, that is, the well-known Fowler-Nordheim (FN) tunneling that occurs at high electric fields, and apply it for the first time in the theoretical investigation of plasmon resonances between nearly touching nanoparticles. This new approach relaxes the requirements on gap dimensions, and intuitively suggests that with a sufficiently high-intensity irradiation, the CTP can be excited via FN tunneling for a range of subnanometer gaps. The unique feature of FN tunneling induced CTP is the ability to turn on and off the charge transfer by varying the intensity of an external light source, and this could inspire the development of novel quantum devices.

We demonstrate strong chiral optical response in three-dimensional chiral nanoparticle oligomers in the wavelength regime between 700 and 3500 nm.We show in experiment and simulation that this broad-band response occurs at the onset of charge transfer between the individual nanoparticles. The ohmic contact causes a strong red shift of the fundamental mode, while the geometrical shape of the resulting fused particles still allows for an efficient excitation of higher order modes. Calculated spectra and field distributions confirm our interpretation and show a number of interacting plasmonic modes. Our results deepen the understanding of the chiral optical response in complex chiral plasmonic nanostructures and pave the road toward broad-band chiral optical devices with strong responses, for example, for chiral plasmon rulers or sensing applications.

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