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Plasmonic double helical antennas are a means to funnel circularly polarized states of light down to the nanoscale. Here, an existing design tool for single helices is extended to the case of double helices and used to design antennas that combine large chiroptical interaction strength with highly directional light emission. Full-field numerical modeling underpins the design and provides additional insight into surface charge distributions and resonance widths. The experimentally realized double helical antennas were studied regarding their polarization-dependent transmission behavior resulting in a large and broadband dissymmetry factor in the visible range. Since the polarization of light is an important tool for implementing logic functionality in photonic and quantum photonic devices, these helices are potential building blocks for future nanophotonic circuits, but also for chiral metamaterials or phase plates., Comment: 11 pages with Supporting Information, 10 figures

Plasmonic antennas with helical geometry are capable transducers between linearly polarized dipole emission and purely circular polarized far-fields. Besides large Purcell enhancements they possess a wide tunability due to the geometry dependence of their resonant modes. Here, the coupling of a dipole emitter embedded in a thin film to plasmonic single and double helices is numerically studied. Using a higher-order finite element method (FEM) the wavelength dependent Purcell enhancement of a dipole with different positions and orientations is calculated and the far-fields with respect to their chirality and radiation patterns are analyzed. Both single and double helices demonstrate highly directional and circularly polarized far-fields for resonant excitation but with significantly improved directional radiation for the case of double helices.

Achieving reliable and quantifiable performance in large-area surface-enhanced Raman spectroscopy (SERS) substrates has long been a formidable challenge. It requires substantial signal enhancement while maintaining a reproducible and uniform response. Conventional SERS substrates are typically made of inhomogeneous materials with random resonator geometries and distributions. As a result, they exhibit several or broadened plasmonic resonances, undesired absorptive losses, and inhomogeneous field enhancement. These limitations diminish the signal strength and hamper reproducibility, making it difficult to conduct comparative studies with high sensitivity. In this study, we propose an approach that utilizes monocrystalline gold flakes to fabricate well-defined gratings composed of plasmonic double-wire resonators, which are fabricated through focused ion-beam lithography. The geometry of the double wire grating substrate (DWGS) was evolutionary optimized to achieve efficient enhancement for both excitation and emission processes. The use of monocrystalline material minimizes absorption losses while enhancing the shape fidelity during the nanofabrication process. The DWGS shows notable reproducibility (RSD=6.6%), repeatability (RSD=5.6%), and large-area homogeneity over areas $>10^4\,\mu$m$^2$. Moreover, it provides a SERS enhancement factor of $\approx 10^6$ for 4-Aminothiophenol (4-ATP) analyte and detection capability for sub-monolayer coverage. The DWGS demonstrates reusability, as well as long-term stability on the shelf. Experimental validation with various analytes, in different states of matter, including biological macromolecules, confirms the sensitive and reproducible nature of DWGSs, thereby establishing them as a promising SERS substrate design for future sensing applications., Comment: 22 pages, 6 Figures, 12 pages Supplementary

Classical electrodynamics describes the optical response of systems using bulk electronic properties and infinitesimally thin boundaries. However, due to the quantum nature of electrons, interfaces have a finite thickness. Non-classical surface effects become increasingly important as ever smaller nanoscale systems are realized and eventually dominate over volume-related phenomena. Investigating the response of surface electrons in such systems, therefore, becomes imperative. One way to gain control over non-classical interface effects and study them is through electrical gating, as the static screening charges reside exclusively at the surface. Here, we investigate the modulation of the surface response upon direct electric charging of a single plasmonic nanoresonator by measuring the resulting changes in resonance. We analyze the observed effects within the general framework of surface-response functions and provide a basic model derived from electron spill-out within the local-response approximation (LRA). Our observed change in resonance frequency is well accounted for by assuming a modulation of the in-plane surface current. Surprisingly, we also measure a change in the resonance width, where adding electrons to the surface leads to a narrowing of the plasmonic resonance, i.e., reduced losses. The description of such effects requires considering nonlocal effects and the inclusion of a possible anisotropy of the perturbed surface permittivity. Our experiment, therefore, opens a vast field of investigations on how to gain control over the surface response in plasmonic resonators and to develop ultrafast and extremely small electrically driven plasmonic modulators and metasurfaces by leveraging electrical control over non-classical surface effects., Comment: 10 pages, 4 figures, 15 pages Supplementary

Plasmonic dimer antennas create strong field enhancement by squeezing light into a nanoscale gap. These optical hotspots are highly attractive for boosting nonlinear processes, such as harmonic generation, photoelectron emission, and ultrafast electron transport. Alongside large field enhancement, such phenomena often require control over the field asymmetry in the hotspot, which is challenging considering the nanometer length scales. Here, by means of strongly enhanced second harmonic generation, we demonstrate unprecedented control over the field distribution in a hotspot by systematically introducing geometrical asymmetry to the antenna gap. We use focused helium ion beam milling of mono-crystalline gold to realize asymmetric-gap dimer antennas in which an ultra-sharp tip with 3 nm apex radius faces a flat counterpart, conserving the bonding antenna mode and the concomitant field enhancement at the fundamental frequency. By decreasing the tip opening angle, we are able to systematically increase both field enhancement and asymmetry, thus enhancing second harmonic radiation to the far-field, which is nearly completely suppressed for equivalent symmetric dimer antennas. Combining these findings with second harmonic radiation patterns as well as quantitative nonlinear simulations, we further obtain remarkably detailed insights into the mechanism of second harmonic generation at the nanoscale. Our results open new opportunities for the realization of novel nonlinear nanoscale systems, where the control over local field asymmetry in combination with large field enhancement is essential to create nonreciprocal functionalities.

We employ electric circuit networks to study topological states of matter in non-Hermitian systems enriched by parity-time symmetry $\mathcal{PT}$ and chiral symmetry anti-$\mathcal{PT}$ ($\mathcal{APT}$). The topological structure manifests itself in the complex admittance bands which yields excellent measurability and signal to noise ratio. We analyze the impact of $\mathcal{PT}$ symmetric gain and loss on localized edge and defect states in a non-Hermitian Su--Schrieffer--Heeger (SSH) circuit. We realize all three symmetry phases of the system, including the $\mathcal{APT}$ symmetric regime that occurs at large gain and loss. We measure the admittance spectrum and eigenstates for arbitrary boundary conditions, which allows us to resolve not only topological edge states, but also a novel $\mathcal{PT}$ symmetric $\mathbb{Z}_2$ invariant of the bulk. We discover the distinct properties of topological edge states and defect states in the phase diagram. In the regime that is not $\mathcal{PT}$ symmetric, the topological defect state disappears and only reemerges when $\mathcal{APT}$ symmetry is reached, while the topological edge states always prevail and only experience a shift in eigenvalue. Our findings unveil a future route for topological defect engineering and tuning in non-Hermitian systems of arbitrary dimension., Comment: 4 pages, 3 figures, 6 pages supplement (separate file)

The emission rate of a point dipole can be strongly increased in presence of a well-designed optical antenna. Yet, optical antenna design is largely based on radio-frequency rules, ignoring e.g.~ohmic losses and non-negligible field penetration in metals at optical frequencies. Here we combine reciprocity and Poynting's theorem to derive a set of optical-frequency antenna design rules for benchmarking and optimizing the performance of optical antennas driven by single quantum emitters. Based on these findings a novel plasmonic cavity antenna design is presented exhibiting a considerably improved performance compared to a reference two-wire antenna. Our work will be useful for the design of high-performance optical antennas and nanoresonators for diverse applications ranging from quantum optics to antenna-enhanced single-emitter spectroscopy and sensing., Comment: 4 pages + appendices; 2 + 4 figures, 1 Table

Coupling mechanical degrees of freedom with plasmonic resonances has potential applications in optomechanics, sensing, and active plasmonics. Here we demonstrate a suspended two-wire plasmonic nano-antenna acting like a nano-electrometer. The antenna wires are supported and electrically connected via thin leads without disturbing the antenna resonance. As a voltage is applied, equal charges are induced on both antenna wires. The resulting equilibrium between the repulsive Coulomb force and the restoring elastic bending force enables us to precisely control the gap size. As a result the resonance wavelength and the field enhancement of the suspended optical nano-antenna (SONA) can be reversibly tuned. Our experiments highlight the potential to realize large bandwidth optical nanoelectromechanical systems (NEMS).

Nanoantennas for light enhance light-matter interaction at the nanoscale making them useful in optical communication, sensing, and spectroscopy. So far nanoantenna engineering has been largely based on rules derived from the radio frequency domain which neglect the inertia of free metal electrons at optical frequencies causing phenomena such as complete field penetration, ohmic losses and plasmon resonances. Here we introduce a general and scalable evolutionary algorithm that accounts for topological constrains of the fabrication method and therefore yields unexpected nanoantenna designs exhibiting strong light localization and enhancement which can directly be "printed" by focused-ion beam milling. The fitness ranking in a hierarchy of such antennas is validated experimentally by two-photon photoluminescence. Analysis of the best antennas' operation principle shows that it deviates fundamentally from that of classical radio wave-inspired designs. Our work sets the stage for a widespread application of evolutionary optimization to a wide range of problems in nano photonics., Comment: 3 figures; please see the supplementary information for additional data

The design of nano-antennas is so far mainly inspired by radio-frequency technology. However, material properties and experimental settings need to be reconsidered at optical frequencies, which entails the need for alternative optimal antenna designs. Here a checkerboard-type, initially random array of gold cubes is subjected to evolutionary optimization. To illustrate the power of the approach we demonstrate that by optimizing the near-field intensity enhancement the evolutionary algorithm finds a new antenna geometry, essentially a split-ring/two-wire antenna hybrid which surpasses by far the performance of a conventional gap antenna by shifting the n=1 split-ring resonance into the optical regime., Comment: Also see Supplementary material, as attached to the main paper

Deep subwavelength integration of high-definition plasmonic nanostructures is of key importance for the development of future optical nanocircuitry for high-speed communication, quantum computation and lab-on-a-chip applications. So far the experimental realization of proposed extended plasmonic networks consisting of multiple functional elements remains challenging, mainly due to the multi-crystallinity of commonly used thermally evaporated gold layers. Resulting structural imperfections in individual circuit elements will drastically reduce the yield of functional integrated nanocircuits. Here we demonstrate the use of very large (>100 micron^2) but thin (<80 nm) chemically grown single-crystalline gold flakes, which, after immobilization, serve as an ideal basis for focused-ion beam milling and other top-down nanofabrication techniques on any desired substrate. Using this methodology we obtain high-definition ultrasmooth gold nanostructures with superior optical properties and reproducible nano-sized features over micrometer length scales. Our approach provides a possible solution to overcome the current fabrication bottleneck and to realize high-definition plasmonic nanocircuitry., Comment: 28 pages, 14 figures, including Supporting Information (11 pages, 10 figures)

An experimentally realizable prototype nanophotonic circuit consisting of a receiving and an emitting nano antenna connected by a two-wire optical transmission line is studied using finite-difference time- and frequency-domain simulations. To optimize the coupling between nanophotonic circuit elements we apply impedance matching concepts in analogy to radio frequency technology. We show that the degree of impedance matching, and in particular the impedance of the transmitting nano antenna, can be inferred from the experimentally accessible standing wave pattern on the transmission line. We demonstrate the possibility of matching the nano antenna impedance to the transmission line characteristic impedance by variations of the antenna length and width realizable by modern microfabrication techniques. The radiation efficiency of the transmitting antenna also depends on its geometry but is independent of the degree of impedance matching. Our systems approach to nanophotonics provides the basis for realizing general nanophotonic circuits and a large variety of derived novel devices.