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Plasmon polaritons (plasmons, for simplicity) have become paramount for tailored nanoscale light-matter interaction, and extensive research has been conducted to monitor(1,2) and manipulate their spatial(3) and spatio-temporal dynamics(4). These dynamics result from the superposition of various plasmon modes, which are classical wave packets. Beyond this classical picture, plasmon modes are treated as quasiparticles(5,6) and they are considered essential for the realization of future nanoscale quantum functionality(7-10). Implementing and demonstrating such functionality requires access to the quasiparticle's quantum state to monitor and manipulate its corresponding quantum wave packet dynamics in Hilbert space. Here we report the local detection of nanoscale plasmon quantum wave packets using plasmon-assisted electron emission as a signal in coherent two-dimensional nanoscopy(11). The observation of a quantum coherence oscillating at the third harmonic of the plasmon frequency is traced back to the superposition of energetically non-adjacent plasmon occupation number states and is therefore a direct fingerprint of the quantum wave packet. Beyond demonstrating the existence of a plasmon quantum wave packet via the coherence between certain occupation number states and providing an improved model for plasmon-assisted electron emission processes, the results may enable time-dependent probing and manipulation of coupled quantum states and dynamics on the nanoscale.

When photons interact with matter, forces and torques occur due to the transfer of linear and angular momentum, respectively. The resulting accelerations are small for macroscopic objects but become substantial for microscopic objects with small masses and moments of inertia, rendering photon recoil very attractive to propel micro- and nano-objects. However, until now, using light to control object motion in two or three dimensions in all three or six degrees of freedom has remained an unsolved challenge. Here we demonstrate light-driven microdrones (size roughly 2 μm and mass roughly 2 pg) in an aqueous environment that can be manoeuvred in two dimensions in all three independent degrees of freedom (two translational and one rotational) using two overlapping unfocused light fields of 830 and 980 nm wavelength. To actuate the microdrones independent of their orientation, we use up to four individually addressable chiral plasmonic nanoantennas acting as nanomotors that resonantly scatter the circular polarization components of the driving light into well-defined directions. The microdrones are manoeuvred by only adjusting the optical power for each motor (the power of each circular polarization component of each wavelength). The actuation concept is therefore similar to that of macroscopic multirotor drones. As a result, we demonstrate manual steering of the microdrones along complex paths. Since all degrees of freedom can be addressed independently and directly, feedback control loops may be used to counteract Brownian motion. We posit that the microdrones can find applications in transport and release of cargos, nanomanipulation, and local probing and sensing of nano and mesoscale objects.

Visible and infrared photons can be detected with a broadband response via the internal photoeffect. By using plasmonic nanostructures, i.e. nanoantennas, wavelength selectivity can be introduced to such detectors through geometry-dependent resonances. Also, additional functionality, like electronic responsivity switching and polarization detection have been realized. However, previous devices consisted of large arrays of nanostructures to achieve detectable photocurrents. Here we show that this concept can be scaled down to a single antenna level, resulting in detector dimensions well below the resonance wavelength of the device. Our design consists of a single electrically-connected plasmonic nanoantenna covered with a wide-bandgap semiconductor allowing broadband photodetection in the VIS/NIR via injection of hot carriers. We demonstrate electrical switching of the color sensitivity as well as polarization detection. Our results hold promise for the realization of ultra small, highly integratable photodetectors with advanced functionality., Comment: 8 pages, 4 figures

Coherent perfect absorption (CPA) describes the absence of all outgoing modes from a lossy resonator, driven by lossless incoming modes. Here, we show that for nanoresonators that also exhibit radiative losses, e.g., plasmonic nanoantennas, a generalized version of CPA (gCPA) can be applied. In gCPA outgoing modes are suppressed only for a subset of (guided plasmonic) modes while other (radiative) modes are treated as additional loss channels - a situation typically referred to as perfect impedance matching. Here we make use of gCPA to show how to achieve perfect impedance matching between a single nanowire plasmonic waveguide and a plasmonic nanoantenna. Antennas with both radiant and subradiant characteristics are considered. We further demonstrate potential applications in background-free sensing.

We use mono-crystalline gold platelets with ultra-smooth surfaces and superior plasmonic properties to investigate the formation of interference patterns caused by surface plasmon polaritons (SPPs) with scattering-type scanning near-field microscopy (s-SNOM) at 521~nm and 633~nm. By applying a Fourier analysis approach, we can identify and separate several signal channels related to SPPs launched and scattered by the AFM tip and the edges of the platelet. Especially at the excitation wavelength of 633~nm, we can isolate a region in the center of the platelets where we find only contributions of SPPs which are launched by the tip and reflected at the edges. These signatures are used to determine the SPP wavelength of $\lambda_{SPP}=606$ nm in good agreement with theoretical predictions. Furthermore, we were still able to measure SPP signals after 20~$\upmu$m propagation, which demonstrates impressively the superior plasmonic quality of these mono-crystalline gold platelets., Comment: Main 20 pages, 8 figures, Supplemental Information 5 pages, 3 figures

The electrical excitation of guided plasmonic modes at the nanoscale enables integration of optical nanocircuitry into nanoelectronics. In this context, exciting plasmons with a distinct modal field profile constitutes a key advantage over conventional single-mode integrated photonics. Here, we demonstrate the selective electrical excitation of the lowest-order symmetric and antisymmetric plasmonic modes in a two-wire transmission line. We achieve mode selectivity by precisely positioning nanoscale excitation sources, i.e., junctions for inelastic electron tunneling, within the respective modal field distribution. By using advanced fabrication that combines focused He-ion beam milling and dielectrophoresis, we control the location of tunnel junctions with sub-10 nm accuracy. At the far end of the two-wire transmission line, the guided plasmonic modes are converted into far-field radiation at separate spatial positions showing two distinct orthogonal polarizations. Hence, the resulting device represents the smallest electrically driven light source with directly switchable polarization states with possible applications in display technology.

The photon spin is an important resource for quantum information processing as is the electron spin in spintronics. However, for subwavelength confined optical excitations, polarization as a global property of a mode cannot be defined. Here, we show that any polarization state of a plane-wave photon can reversibly be mapped to a pseudospin embodied by the two fundamental modes of a subwavelength plasmonic two-wire transmission line. We design a device in which this pseudospin evolves in a well-defined fashion throughout the device reminiscent of the evolution of photon polarization in a birefringent medium and the behavior of electron spins in the channel of a spin field-effect transistor. The significance of this pseudospin is enriched by the fact that it is subject to spin-orbit locking. Combined with optically active materials to exert external control over the pseudospin precession, our findings could enable spin-optical transistors, that is, the routing and processing of quantum information with light on a subwavelength scale.

Focused beams of helium ions are a powerful tool for high-fidelity machining with spatial precision below 5 nm. Achieving such a high patterning precision over large areas and for different materials in a reproducible manner, however, is not trivial. Here, we introduce the Python toolbox FIB-o-mat for automated pattern creation and optimization, providing full flexibility to accomplish demanding patterning tasks. FIB-o-mat offers high-level pattern creation, enabling high-fidelity large-area patterning and systematic variations in geometry and raster settings. It also offers low-level beam path creation, providing full control over the beam movement and including sophisticated optimization tools. Three applications showcasing the potential of He ion beam nanofabrication for two-dimensional material systems and devices using FIB-o-mat are presented.

To investigate the enhanced light-matter interaction mediated by optical nanoantennas, there is a need for reliable quantum emitters (QE). Nanodiamonds (ND) with N-V-centers and single-walled carbon nanotubes (SWCNT) are a good choice, since they are photostable and presumably do not blink.

Most currently studied optical antenna geometries are based on radio wave antenna designs. However, material properties at optical frequency largely differ from the perfect metal case at radio frequencies. Recently, evolutionary algorithms (EA) [1] were used in the field of Plasmonics to find novel geometries however with strongly limited configuration space [2]. We developed an EA capable of handling complex multiparticle geometries. In a first application we optimized near-field intensity enhancement produced by antenna structures in a single point [3]. We found a novel geometry, i.e. a nano antenna/split-ring hybrid antenna for which the near-field intensity enhancement is strongly increased. Yet these structures are not suitable for experimental realization. Here we show the results of an adapted EA which uses primitive elements that are compatible with an experimental fabrication step using FIB. The performance of the realized structures is characterized by means of confocal two-photon-photoluminescence-(2PPL)-microscopy. We find that the hierarchy of performances found in individuals taken from sequential generations of the EA can be reproduced in fabricated structures.

Nanoscale optical integration is nowadays a strategic technological challenge and the ability of generating and manipulating nonlinear optical processes in sub-wavelength volumes is pivotal to realize efficient sensing probes and photonic sources for the next-generation communication technologies. Yet, confining nonlinear processes below the diffraction limit remains a challenging task because phase-matching is not a viable approach at the nanoscale. The localized fields associated to the resonant modes of plasmonic and dielectric nanoantennas offer a route to enhance and control nonlinear processes in highly confined volumes. In my talk I will discuss two nonlinear platforms based on plasmonic and dielectric nanostructures. The first relies on a broken symmetry antenna design, which brings about an efficient second harmonic generation (SHG). We recently applied this concept to an extended array of non-centrosymmetric nanoantennas for sensing applications. I will also show the evidence of a cascaded second-order process in Third Harmonic Generation (THG) in these nanoantennas. Recently, dielectric nanoantennas emerged as an alternative to plasmonic nanostructures for nanophotonics applications, thanks to their sharp magnetic and electric Mie resonances along with the low ohmic losses in the visible/near-infrared region of the spectrum. I will present our most recent studies on the nonlinear properties of AlGaAs dielectric nanopillars. The strong localized modes along with the broken symmetry in the crystal structure of AlGaAs allow obtaining more than two orders of magnitude higher SHG efficiency with respect to plasmonic nanoantennas with similar spatial footprint and using the same pump power. I will also discuss a few key strategies we recently adopted to optically switch the SHG in these antennas even on the ultrafast time scale. Finally, I will show how to effectively engineer the sum frequency generation via the Mie resonances in these nanoantennas. These results draw a viable blueprint towards room-temperature all optical logic operation at the nanoscale.

We describe a setup for time-resolved photoemission electron microscopy with aberration correction enabling 3 nm spatial resolution and sub-20 fs temporal resolution. The latter is realized by our development of a widely tunable (215–970 nm) noncollinear optical parametric amplifier (NOPA) at 1 MHz repetition rate. We discuss several exemplary applications. Efficient photoemission from plasmonic Au nanoresonators is investigated with phase-coherent pulse pairs from an actively stabilized interferometer. More complex excitation fields are created with a liquid-crystal-based pulse shaper enabling amplitude and phase shaping of NOPA pulses with spectral components from 600 to 800 nm. With this system we demonstrate spectroscopy within a single plasmonic nanoslit resonator by spectral amplitude shaping and investigate the local field dynamics with coherent two-dimensional (2D) spectroscopy at the nanometer length scale (“2D nanoscopy”). We show that the local response varies across a distance as small as 33 nm in our sample. Further, we report two-color pump–probe experiments using two independent NOPA beamlines. We extract local variations of the excited-state dynamics of a monolayered 2D material (WSe2) that we correlate with low-energy electron microscopy (LEEM) and reflectivity measurements. Finally, we demonstrate the in situ sample preparation capabilities for organic thin films and their characterization via spatially resolved electron diffraction and dark-field LEEM.We describe a setup for time-resolved photoemission electron microscopy with aberration correction enabling 3 nm spatial resolution and sub-20 fs temporal resolution. The latter is realized by our development of a widely tunable (215–970 nm) noncollinear optical parametric amplifier (NOPA) at 1 MHz repetition rate. We discuss several exemplary applications. Efficient photoemission from plasmonic Au nanoresonators is investigated with phase-coherent pulse pairs from an actively stabilized interferometer. More complex excitation fields are created with a liquid-crystal-based pulse shaper enabling amplitude and phase shaping of NOPA pulses with spectral components from 600 to 800 nm. With this system we demonstrate spectroscopy within a single plasmonic nanoslit resonator by spectral amplitude shaping and investigate the local field dynamics with coherent two-dimensional (2D) spectroscopy at the nanometer length scale (“2D nanoscopy”). We show that the local response varies across a distance as small as 33 n...

In the vicinity of metallic nanostructures, absorption and emission rates of optical emitters can be modulated by several orders of magnitude1,2. Control of such near-field light–matter interaction is essential for applications in biosensing3, light harvesting4 and quantum communication5,6 and requires precise mapping of optical near-field interactions, for which single-emitter probes are promising candidates7–11. However, currently available techniques are limited in terms of throughput, resolution and/or non-invasiveness. Here, we present an approach for the parallel mapping of optical near-field interactions with a resolution of

We describe the wet-chemical synthesis of high-aspect-ratio single-crystalline gold platelets with thicknesses down to 20 nm and edge lengths up to 0.2 mm. By employing statistical analysis of a large number of platelets, we investigate the effect of temperature on the growth velocities of the top and side facets for constant concentrations of the three common ingredients: ethylene glycol, chloroauric acid, and water. We further show that by varying the chemical environment during growth, the ratio between the growth velocities can be adjusted, and thus thickness and lateral size can be tuned independently. Very large but ultrathin single-crystalline gold platelets represent an important starting material for top-down nanofabrication and may also find applications as transparent conducting substrates as well as substrates for high-end scanning probe and electron microscopy.

Unlocking the true potential of optical spectroscopy on the nanoscale requires development of stable and low-noise laser sources. Here, we have developed a low-noise supercontinuum (SC) source based on an all-normal dispersion fiber pumped by a femtosecond fiber laser and demonstrate high resolution, spectrally resolved near-field measurements in the near-infrared (NIR) region. Specifically, we explore the reduced-noise requirements for aperture-less scattering-type scanning near-field optical microscopy (s-SNOM), including inherent pulse-to-pulse fluctuation of the SC. We use our SC light source to demonstrate the first NIR, spectrally resolved s-SNOM measurement, a situation where state-of-the-art commercial SC sources are too noisy to be useful. We map the propagation of surface plasmon polariton (SPP) waves on monocrystalline gold platelets in the wavelength region of 1.34-1.75 μm in a single measurement, thereby characterizing experimentally the dispersion curve of the SPP in the NIR. Our results represent a technological breakthrough that has the potential to enable a wide range of new applications of low-noise SC sources in near-field studies.

Yagi-Uda antennas are a key technology for efficiently transmitting information from point to point using radio waves. Since higher frequencies allow higher bandwidths and smaller footprints, a strong incentive exists to shrink Yagi-Uda antennas down to the optical regime. Here we demonstrate electrically-driven Yagi-Uda antennas for light with wavelength-scale footprints that exhibit large directionalities with forward-to-backward ratios of up to 9.1 dB. Light generation is achieved via antenna-enhanced inelastic tunneling of electrons over the antenna feed gap. We obtain reproducible tunnel gaps by means of feedback-controlled dielectrophoresis, which precisely places single surface-passivated gold nanoparticles in the antenna gap. The resulting antennas perform equivalent to radio-frequency antennas and combined with waveguiding layers even outperform RF designs. This work paves the way for optical on-chip data communication that is not restricted by Joule heating but also for advanced light management in nanoscale sensing and metrology as well as light emitting devices., Nanoantennas have been developed to direct light, but most still rely on laboratory scale light sources. Here, the authors demonstrate electrically-driven directional emission in the optical frequency range using a nanogap in conjunction with a Yagi-Uda antenna nanostructure.

Gold nanostructures have important applications in nanoelectronics, nano-optics as well as in precision metrology due to their intriguing opto-electronic properties. These properties are governed by the bulk band structure but to some extend are tunable via geometrical resonances. Here we show that the band structure of gold itself exhibits significant size-dependent changes already for mesoscopic critical dimensions below 30 nm. To suppress the effects of geometrical resonances and grain boundaries, we prepared atomically flat ultrathin films of various thicknesses by utilizing large chemically grown single-crystalline gold platelets. We experimentally probe thickness-dependent changes of the band structure by means of two-photon photoluminescence and observe a surprising 100-fold increase of the nonlinear signal when the gold film thickness is reduced below 30 nm allowing us to optically resolve single-unit-cell steps. The effect is well explained by density functional calculations of the thickness-dependent 2D band structure of gold.

Plasmonic resonators can be designed to support spectrally well-separated discrete modes. The associated characteristic spatial patterns of intense electromagnetic hot-spots can be exploited to enhance light-matter interaction. Here, we study the local field dynamics of individual hot-spots within a nanoslit resonator by detecting characteristic changes of the photoelectron emission signal on a scale of ∼12 nm using time-resolved photoemission electron microscopy (TR-PEEM) and by excitation with the output from a 20 fs, 1 MHz noncollinear optical parametric amplifier (NOPA). Surprisingly, we detect apparent spatial variations of the

The optimization of nonlinear optical processes at the nanoscale is a crucial step for the development of nanoscale photon sources for quantum-optical networks. The development of innovative plasmonic nanoantenna designs and hybrid nanostructures to enhance optical nonlinearities in very small volumes represents one of the most promising routes. In such systems, the upconversion of photons can be achieved with high efficiencies via third-order processes, such as third harmonic generation (THG), thanks to the resonantly-enhanced volume currents. Conversely, second-order processes, such as second harmonic generation (SHG), are often inhibited by the symmetry of metal lattices and of common nanoantenna geometries. SHG and THG processes in plasmonic nanostructures are generally treated independently, since they both represent a small perturbation in the light-matter interaction mechanisms. In this work, we demonstrate that this paradigm does not hold in general, by providing evidence of a cascaded process in THG, which is fueled by SHG and sizably contributes to the overall yield. We address this mechanism by unveiling an anomalous fingerprint in the polarization state of the nonlinear emission from non-centrosymmetric gold nanoantennas and point out that such cascaded processes may also appear for structures that exhibit only moderate SHG yields - signifying its general relevance in plasmon-enhanced nonlinear optics. The presence of this peculiar mechanism in THG from plasmonic nanoantennas at telecommunication wavelengths allows gaining further insight on the physics of plasmon-enhanced nonlinear optical processes. This could be crucial in the realization of nanoscale elements for photon conversion and manipulation operating at room-temperature., 25 pages, 4 figures

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).

Helium ion milling of chemically-synthesized micron-sized gold flakes is performed to fabricate ultra-narrow nanoslit cavities with a varying length and width down to 5 nm. Their plasmon resonances are characterized by one-photon photoluminescence spectroscopy. The combination of fabrication based on single-crystalline gold and resonant modes with low radiative losses leads to remarkably high quality factors of up to 24. Multiple Fabry-Perot-type resonances in the visible/near infrared spectral range are observed due to the achieved narrow slit widths and the resulting short effective wavelengths of nanoslit plasmons. These features make nanoslit cavities attractive for a range of applications such as surface-enhanced spectroscopy, ultrafast nano-optics and strong light-matter coupling.

The optimization of nonlinear optical processes at the nanoscale is a key challenge in nanoscience. In this framework, plasmon-enhanced nonlinear effects together with the development of innovative nanoantenna designs and hybrid nanostructures are receiving a lot of attention [1-2]. We recently devised a plasmonic nanoantenna working in the near-infrared region of the electromagnetic spectrum, which allows boosting the SHG efficiency. This is achieved by optimizing the nanoantenna geometry to feature (i) a double resonant response at both the excitation and emission wavelengths, (ii) a spatial overlap between the modes involved in the process and (iii) a broken symmetry, to enable dipole-allowed SHG. We found that this nanoantenna behaves like a strongly coherent nanoscale light source, featuring a marked THG along with an intense SHG [3-4]. We find evidence of a SHG-mediated cascaded effect in THG [5]. We have identified a THG polarization behavior that strongly deviates from that of a bulk (3)-mediated effect and unveils a significant contribution coming from the cascaded coherent sum of a SHG photon and a pump photon. References [1] Kauranen, M.; Zayats, A. V., Nature Photon. 2012, 6, 737-748. [2] Butet, J.; Brevet, P. F.; Martin, O. J. F., ACS Nano 2015, 9, 10545–10562. [3] Celebrano, M.; et al., Nat. Nanotechnol. 2015, 10, 412−417. [4] Sartorello, G.; et al., ACS Photon. 2016, 3, 1517−1522. [5] Mu, X.; et al., Y., Opt. Lett. 2000, 25, 117-119.

Scanning probe technology is used to examine near-field strong coupling of a plasmonic resonator and a single colloidal quantum dot., Strong coupling and the resultant mixing of light and matter states is an important asset for future quantum technologies. We demonstrate deterministic room temperature strong coupling of a mesoscopic colloidal quantum dot to a plasmonic nanoresonator at the apex of a scanning probe. Enormous Rabi splittings of up to 110 meV are accomplished by nanometer-precise positioning of the quantum dot with respect to the nanoresonator probe. We find that, in addition to a small mode volume of the nanoresonator, collective coherent coupling of quantum dot band-edge states and near-field proximity interaction are vital ingredients for the realization of near-field strong coupling of mesoscopic quantum dots. The broadband nature of the interaction paves the road toward ultrafast coherent manipulation of the coupled quantum dot-plasmon system under ambient conditions.

Antennas play a key role in today’s wireless communication networks and it would be hugely beneficial to extent their use into the optical regime. However, classical signal generators do not work at those frequencies and therefore new concepts are needed. Here, we demonstrate how to electrically drive an optical nanoantenna using an atomic-scale feed gap provided by a gold-particle pushed into a precisely tailored interstice between two antenna arms. Upon applying a voltage, inelastic electron tunneling leads to current fluctuations in the optical regime and, hence, light emission. We show how the antennas spectrally shape the emission, how the exact particle position influences these properties and how to increase the directivity via Yagi-Uda arrangements or plasmonic waveguides structures in order to make electricallydriven optical nanoantennas more suitable for on-chip data communication.

We fabricated asymmetric nanoslit cavities by milling with helium ion microscope on the edge of single-crystal gold stripes with one end open to the air. The cavities exhibit high-Q multiple resonances in the visible range.

Two-photon polymerization using femtosecond laser pulses at a wavelength of 515 nm is used for three-dimensional patterning of photosensitive, biocompatible inorganic-organic hybrid polymers (ORMOCER(A (R))s). In order to fabricate millimeter-sized biomedical scaffold structures with interconnected pores, medium numerical aperture air objectives with long working distances are applied which allow voxel lengths of several micrometers and thus the solidification of large scaffolds in an adequate time. It is demonstrated that during processing the refraction of the focused laser beam at the air/material interface leads to strong spherical aberration which decreases the peak intensity of the focal point spread function along with shifting and severely extending the focal region in the direction of the beam propagation. These effects clearly decrease the structure integrity, homogeneity and the structure details and therefore are minimized by applying a positioning and laser power adaptation throughout the fabrication process. The results will be discussed with respect to the resulting structural homogeneity and its application as biomedical scaffold.

Researchers demonstrate an electrically driven nanoscale transmitter based on the broadband quantum shot noise of electrons tunnelling across a feed gap. Unlike radiowave antennas, so far optical nanoantennas cannot be fed by electrical generators. Instead, they are driven by light1 or indirectly via excited discrete states in active materials2,3 in their vicinity. Here we demonstrate the direct electrical driving of an in-plane optical antenna by the broadband quantum-shot noise of electrons tunnelling across its feed gap. The spectrum of the emitted photons is determined by the antenna geometry and can be tuned via the applied voltage. Moreover, the direction and polarization of the light emission are controlled by the antenna resonance, which also improves the external quantum efficiency by up to two orders of magnitude. The one-material planar design offers facile integration of electrical and optical circuits and thus represents a new paradigm for interfacing electrons and photons at the nanometre scale, for example for on-chip wireless communication and highly configurable electrically driven subwavelength photon sources.

Chemically synthesized single-crystalline gold microplates have been attracting increasing interest because of their potential as high-quality gold films for nanotechnology. We present the growth of tens of nanometers thick and tens of micrometers large single-crystalline gold plates directly on solid substrates by solution-phase synthesis. Compared to microplates deposited on substrates from dispersion phase, substrate-grown plates exhibit significantly higher quality by avoiding severe small-particle contamination and aggregation. Substrate-grown gold plates also open new perspectives to study the growth mechanism via step-growth and observation cycles of a large number of individual plates. Growth models are proposed to interpret the evolution of thickness, area and shape of the plates. It is found that the plate surface remains smooth after regrowth, implying the applicability of regrowth for producing giant plates as well as unique single-crystalline nano-structures.

The demand for using nanostructures fabricated by focused ion beam (FIB) on delicate substrates or as building blocks for complex devices motivates the development of protocols that allow FIB-fabricated nanostructures to be transferred from the original substrate to the desired target. However, transfer of FIB-fabricated nanostructures is severely hindered by FIB-induced welding of structure and substrate. Here we present two (ex and in situ) transfer methods for FIB-fabricated nanostructures based on a silica-gold bilayer evaporated onto a bulk substrate. Utilizing the poor adhesion between silica and gold, the nanostructures can be mechanically separated from the bulk substrate. For the ex situ transfer, a spin-coated poly(methyl methacrylate) film is used to carry the nanostructures so that the bilayer can be etched away after being peeled off. For the in situ transfer, using a micro-manipulator inside the FIB machine, a cut-out piece of silica on which a nanostructure has been fabricated is peeled off from the bulk substrate and thus carries the nanostructure to a target substrate. We demonstrate the performance of both methods by transferring plasmonic nano-antennas fabricated from single-crystalline gold flakes by FIB milling to a silicon wafer and to a scanning probe tip.

We experimentally investigate the smallest germanium waveguide cavity resonators on silicon that can be designed to work around 1.55 mu m wavelength and observe an almost 30-fold enhancement in the collected spontaneous emission per unit volume when compared to a continuous germanium film of the same thickness. The enhancement is due to an effective combination of (i) excitation enhancement at the pump wavelength, (ii) emission enhancement (Purcell effect) at the emission wavelength, and (iii) effective beaming by the nanoresonators, which act as optical antennas to enhance the radiation efficiency. Our results set a basis for the understanding and engineering of light emission based on subwavelength, CMOS-compatible nanostructures operating at telecommunication wavelengths.

Quantum photonics holds great promise for future technologies such as secure communication, quantum computation, quantum simulation, and quantum metrology. An outstanding challenge for quantum photonics is to develop scalable miniature circuits that integrate single-photon sources, linear optical components, and detectors on a chip. Plasmonic nanocircuits will play essential roles in such developments. However, for quantum plasmonic circuits, integration of stable, bright, and narrow-band single photon sources in the structure has so far not been reported. Here we present a plasmonic nanocircuit driven by a self-assembled GaAs quantum dot. Through a planar dielectric-plasmonic hybrid waveguide, the quantum dot efficiently excites narrow-band single plasmons that are guided in a two-wire transmission line until they are converted into single photons by an optical antenna. Our work demonstrates the feasibility of fully on-chip plasmonic nanocircuits for quantum optical applications.

Quantum photonics holds great promise for future technologies such as secure communication, quantum computation, quantum simulation, and quantum metrology. An outstanding challenge for quantum photonics is to develop scalable miniature circuits that integrate single-photon sources, linear optical components, and detectors on a chip. Plasmonic nanocircuits will play essential roles in such developments. Plasmonic components feature ultracompact geometries and can be controlled more flexibly and more energy-efficiently compared to conventional dielectric components due to strong field confinement and enhancement. Moreover, plasmonic components are compatible with electronic circuits, thanks to their deep subwavelength sizes as well as their electrically conducting materials. However, for quantum plasmonic circuits, integration of stable, bright, and narrow-band single photon sources in the structure has so far not been reported. Here we present a quantum plasmonic nanocircuit driven by a self-assembled GaAs quantum dot. The quantum dot efficiently excites narrow-band single plasmons that are guided in a two-wire transmission line until they are converted into single photons by an optical antenna. Our work demonstrates the feasibility of fully on-chip plasmonic nanocircuits for quantum optical applications.

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

Exact quantitative understanding of plasmon propagation along nanowires is mandatory for designing and creating functional devices. Here we investigate plasmon transmission through top-down fabricated monocrystalline gold nanowires on a glass substrate. We show that the transmission through finite-length nanowires can be described by Fabry-P\'{e}rot oscillations that beat with free-space propagating light launched at the incoupling end. Using an extended Fabry-P\'{e}rot model, experimental and simulated length dependent transmission signals agree quantitatively with a fully analytical model., Comment: 5 pages, 4 figures

In the vicinity of metallic nanostructures, absorption and emission rates of optical emitters can be modulated by several orders of magnitude

The design of noble-metal plasmonic devices and nanocircuitry requires a fundamental understanding and control of the interference of plasmonic modes. Here we report the first visualization of the propagation and interference of guided modes in a showcase plasmonic nanocircuit using normal-incidence nonlinear two-photon photoemission electron microscopy (PEEM). We demonstrate that in contrast to the commonly used grazing-incidence illumination scheme, normal-incidence PEEM provides a direct image of the structure’s near-field intensity distribution due to the absence of beating patterns and despite the transverse character of the plasmonic modes. Based on a simple heuristic numerical model for the photoemission yield, we are able to model all experimental findings if global plane wave illumination and coupling to multiple input/output ports, and the resulting interference effects are accounted for.

We discriminate different field dynamics across distances as small as 33 nm within a plasmonic slit resonator using aberration-corrected photoemission electron microscopy and a tunable broadband optical parametric amplifier at 1 MHz repetition rate.