Deep subwavelength integration of high-definition plasmonic nanostructures is of key importance in the development of future optical nanocircuitry for high-speed communication, quantum computation and lab-on-a-chip applications. To date, the experimental realization of proposed extended plasmonic networks consisting of multiple functional elements remains challenging, mainly because of the multi-crystallinity of commonly used thermally evaporated gold layers. This can produce structural imperfections in individual circuit elements that drastically reduce the yield of functional integrated nanocircuits. In this paper we demonstrate the use of large (>100 μm(2)) but thin (<80 nm) chemically grown single-crystalline gold flakes that, 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 micrometre-length scales. Our approach provides a possible solution to overcome the current fabrication bottleneck and realize high-definition plasmonic nanocircuitry.

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. 3 IntroductionMiniaturization and packaging density of integrated optics based on dielectrics is limited by the wavelength scale modal profiles of guided modes. 1 In contrast, plasmonic modes on noble metal nanostructures offer strong subwavelength confinement and therefore promise the realization of nanometer-scale integrated optical circuitry. 2,3 A truly subwavelength integrated photonic circuit based on plasmonic nano structures will generally consist of (i) a set of optical antennas 4,5,6,7 to efficiently excite specific local modes by far-field radiation, (ii) a very small footprint network of optical transmission lines 8 (OTLs) to distribute and manipulate plasmonic excitations, 9,10,11,12,13,14,15,16 and (iii) another set of optical antennas to efficiently convert local modes into propagating photons. The properties of metal nanoparticle chains, 17,18 metal nanowires, 19,20 line defects in plasmonic photonic crystals, 21 as well as gaps 22,23,24, and v-shaped grooves 9,25,26 in extended metal films have been explored as subwavelength waveguides for light.Efficient launching of specific guided modes on such structures is difficult since it requires matching of both, the small mode extension and the k-vector. It has been shown recently that efficient coupling between far-field photons and subwavelength spatial domains can be achieved using resonant optical antennas. 5,7,27,28,29,30,31 However, so far optical antennas have mostly been studied as isolated elements. Here we consider optical antennas as integral parts of an experimentally realizable integrated nanophotonic circuit where they act as efficient interfacing elements between propagating photons and guided modes of a plasmonic two-wire transmission line. We show by simulations that the principles of classical transmission line theory, e.g. impedance matching, 32 between the two-wire OTL and dipole antennas are fully applicable at optical frequencies. We further suggest tha...

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.PACS numbers: 84.40. Ba, 73.20.Mf, 78.67.Bf Light-matter interaction, i.e. absorption and emission of light as well as the control of its spectral and directional properties, can be optimized by means of antennalike plasmonic nano structures [1, 2]. This is of immediate importance in diverse fields of research ranging from solar energy conversion [3], photocatalytic [4] and sensing applications [5] to single-particle manipulation [6,7] and spectroscopy [8] as well as quantum optics and communication [9][10][11][12].RF-antenna designs are usually optimized for thin, infinitely good conducting wires that only support surface currents and are typically fed by transmission lines connected by infinitely narrow gaps [13]. For antennas at optical frequencies the general operation conditions deviate substantially from such ideal behaviour: (i) Antenna wire diameters are comparable to the electromagnetic penetration depth into the wire material leading to volume currents [14]. In the case of noble metals, such wires therefore exhibit plasmon resonances in the visible spectral range resulting in a reduced effective wavelength of wire waves [15]. (ii) Feeding (excitation) of optical antennas is often achieved by focused laser beams or quantum emitters. (iii) high-frequency-related effects such as the 'kinetic inductance' become significant [16]. It can therefore not be taken for granted that RF-inspired antenna designs, like dipole [17], bow tie [18,19] and Yagi-Uda antennas [20,21], represent 'optimal' geometries also at optical frequencies, although they provide a reasonable performance.Evolutionary algorithms (EAs) find optimized solutions to highly complex non-analytic problems by creating subsequent generations of individuals coded by their respective genomes that compete for the right to pass on their properties, according to a fitness parameter [22]. These optimized solutions can then be analyzed to foster the understanding of underlying physical principles. Evolutionary optimization has successfully been applied in various fields of research, including pulse shape optimization in coherent control of chemical reactions [23] and field localization in plasmonic structures [24,25]. Furthermore, evolutionary optimization has been used to aid the development of radio-wave antennas [26,27]. First attempts to employ such met...

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Chiral plasmonic nanostructures will be of increasing importance for future applications in the field of nano optics and metamaterials. Their sensitivity to incident circularly polarized light in combination with the ability of extreme electromagnetic field localization renders them ideal candidates for chiral sensing and for all-optical information processing. Here, the resonant modes of single plasmonic helices are investigated. We find that a single plasmonic helix can be efficiently excited with circularly polarized light of both equal and opposite handedness relative to that of the helix. An analytic model provides resonance conditions matching the results of full-field modeling. The underlying geometric considerations explain the mechanism of excitation and deliver quantitative design rules for plasmonic helices being resonant in a desired wavelength range. Based on the developed analytical design tool, single silver helices were fabricated and optically characterized. They show the expected strong chiroptical response to both handednesses in the targeted visible range. With a value of 0.45 the experimentally realized dissymmetry factor is the largest obtained for single plasmonic helices in the visible range up to now. arXiv:1811.04851v2 [physics.optics]