Atomic-Scale Confinement of Resonant Optical Fields

Abstract

2The interaction of light and matter, i.e. absorption and emission of photons, can be considerably enhanced in the presence of strongly localized and therefore highly intense optical near fields 1 .Plasmonic antennas consisting of pairs of closely spaced metal nano particles have gained much attention in this context since they provide the possibility to strongly concentrate optical fields into the gap between the two metal particles [2][3][4] . Pairs of closely spaced metal nanoparticles supporting plasmonic gap resonances consequently find broad applications, e.g. in single-emitter surfaceenhanced spectroscopy 5,6 , quantum optics 7 , extreme nonlinear optics 8-10 , optical trapping 11 , metamaterials 12, 13 and molecular opto-electronics 14 .The success of metal-insulator-metal structures is based on two fundamental properties of their anti-symmetric electromagnetic gap modes. (i) As a direct consequence of the boundary conditions, the dominating field components normal to the metal-dielectric interfaces are sizable only inside the dielectric gap. This means that for anti-symmetric gap modes the achievable field confinement is not limited by the skin depth of the metal, but is solely determined by the actual size of the gap. (ii) Since the free electrons of the metal respond resonantly to an external optical frequency field, enormous surface charge accumulations, accompanied by ultra-intense optical near fields, will occur. In addition, with decreasing gap width, stronger attractive coulomb forces 4 across the gap lead to further surface-charge accumulation and a concomitantly increased near-field intensity enhancement. We therefore conclude that an experimental realization of atomic-scale concentration of electromagnetic fields at visible frequencies is possible but it requires atomic-scale shape control of the field-confining structure, i.e. the gap, as well as a careful assignment and selection of suitable optical modes.Here we achieve atomic-scale confinement of electromagnetic fields at visible frequencies by combining for the first time both atomic-scale shape control of the field confining structure 15 as well as a careful selection and assignment of suitable optical modes [16][17][18] . We study single-crystalline nanorods which self-assemble into side-by-side aligned dimers with gap widths below 0.5 nm. Sideby-side aligned nanorod dimers possess various distinguishable symmetric and anti-symmetric 3 modes in the visible range 19 . In contrast to previous work 20-22 we demonstrate full control over symmetric and anti-symmetric optical modes by means of white-light scattering experiments. We experimentally demonstrate the presence of atomic-scale light confinement in these structures by observing an extreme > 800 meV hybridization splitting of corresponding symmetric and antisymmetric dimer modes. Our results open new perspectives for atomically-resolved spectroscopic imaging, deeply nonlinear optics and attosecond physics, cavity optomechanics and ultra-sensing as well as quantum optics.To obtain nano...

The Morphology of Narrow Gaps Modifies the Plasmonic Response

“…The behavior predicted for the latter is similar to that for spherical particles, with a set of bonding modes that strongly red-shift with narrowing gaps and that determine both the near-and the far-field response. 17 In contrast, for the corresponding flat-gap antennas, we found two different sets of modes, 20,21,84 longitudinal antenna plasmons and transverse cavity plasmons, which behave in fundamentally different ways. The LAPs dominate the far-field response and saturate spectrally for narrow gaps.…”

Nonlinear Photon-Assisted Tunneling Transport in Optical Gap Antennas

“…When the separation distance between the individual constituents of the optical antenna reduces, large Coulomb splitting of the underlying plasmonic modes takes place [9][10][11]. For an atomic-scale gap, charge transfer plasmons are driving the antenna response in the so-called quantum tunneling regime [12][13][14][15].…”

Nanooptics of Plasmonic Nanomatryoshkas: Shrinking the Size of a Core–Shell Junction to Subnanometer

“…22 More importantly, advances in the synthesis of NMs with precise and uniform subnanometer gaps provide a better platform for experimental investigation of quantum plasmonic effects, 17,25,36,37 compared to typical nanoparticle dimer structures fabricated by various state-of-the-art techniques including high-resolution electron-beam lithography, 30 electron-beam induced manipulation, 26 dual AFM tip approaching, 28 and dropcasting. 27,31 In this Letter, we present a comprehensive investigation of the optical properties of NMs with different nanogap widths ranging from 100 nm down to the subnanometer regime. The larger gaps are realized by controlling the thickness of a SiO 2 layer inside the gap, while the subnanometer gap is created using a selfassembled monolayer (SAM) of 1,4-benzenedithiol (1,4-BDT) molecules as well as of 4-methylbenzenethiol (4-MBT) molecules.…”