Light-matter interaction at the atomic scale rules fundamental phenomena such as photoemission and lasing, while enabling basic everyday technologies, including photovoltaics and optical communications. In this context, plasmons -the collective electron oscillations in conducting materials-are important because they allow manipulating optical fields at the nanoscale. The advent of graphene and other two-dimensional crystals has pushed plasmons down to genuinely atomic dimensions, displaying appealing properties such as a large electrical tunability. However, plasmons in these materials are either too broad or lying at low frequencies, well below the technologically relevant nearinfrared regime. Here we demonstrate sharp near-infrared plasmons in lithographically-patterned wafer-scale atomically-thin silver crystalline films. Our measured optical spectra reveal narrow plasmons (quality factor ∼ 4), further supported by a low sheet resistance comparable to bulk metal in few-atomic-layer silver films down to seven Ag(111) monolayers. Good crystal quality and plasmon narrowness are obtained despite the addition of a thin passivating dielectric, which renders our samples resilient to ambient conditions. The observation of spectrally sharp and strongly confined plasmons in atomically thin silver holds great potential for electro-optical modulation and optical sensing applications. * These two authors contributed equally to the work. † Electronic address: enrique.ortega@ehu.es ‡ Electronic address: javier.garciadeabajo@icfo.es arXiv:1901.07739v2 [cond-mat.mes-hall]

Transparent conductors are essential in many optoelectronic devices, such as displays, smart windows, light-emitting diodes and solar cells. Here we demonstrate a transparent conductor with optical loss of ∼1.6%, that is, even lower than that of single-layer graphene (2.3%), and transmission higher than 98% over the visible wavelength range. This was possible by an optimized antireflection design consisting in applying Al-doped ZnO and TiO2 layers with precise thicknesses to a highly conductive Ag ultrathin film. The proposed multilayer structure also possesses a low electrical resistance (5.75 Ω sq−1), a figure of merit four times larger than that of indium tin oxide, the most widely used transparent conductor today, and, contrary to it, is mechanically flexible and room temperature deposited. To assess the application potentials, transparent shielding of radiofrequency and microwave interference signals with ∼30 dB attenuation up to 18 GHz was achieved.

The functionalities of a wide range of optical and opto-electronic devices are based on resonance effects and active tuning of the amplitude and wavelength response is often essential. Plasmonic nano-structures are an efficient way to create optical resonances, a prominent example is the extraordinary optical transmission (EOT) through arrays of nano-holes patterned in a metallic film. Tuning of resonances by heating, applying electrical or optical signals has proven to be more elusive, due to the lack of materials that can induce modulation over a broad spectral range and/or at high speeds. Here we show that nano-patterned metals combined with phase change materials (PCMs) can overcome this limitation due to the large change in optical constants which can be induced thermally or on an ultrafast timescale. We demonstrate resonance wavelength shifts as large as 385 nm --an order of magnitude higher than previously reported--by combining properly designed Au EOT nanostructures with Ge2Sb2Te5 (GST). Moreover, we show, through pump-probe measurements, repeatable and reversible, large-amplitude modulations in the resonances, especially at telecommunication wavelengths, over ps time scales and at powers far below those needed to produce a permanent phase transition. Our findings open a pathway to the design of hybrid metal-PCM nanostructures with ultrafast and widely tuneable resonance responses, which hold potential impact on active nanophotonic devices such as tuneable optical filters, smart windows, bio-sensors and reconfigurable memories.* These authors made equal contribution †valerio.pruneri@icfo.eu 2 Nanophotonic devices incorporating metallic elements can support plasmons, which are collective oscillations of conduction band electrons driven by an external electromagnetic field 1 . Plasmons can confine and guide light well below the diffraction limit, and when supported by suitably engineered nanostructures, they enable the design of disruptive devices for a wide range of applications, including perfect lenses . Plasmons also play an important role in the phenomenon of extraordinary optical transmission (EOT) of visible and infrared light through periodic arrays of subwavelength nanoholes drilled in metallic films. The observation of transmission resonances in these arrays is attributed to the resonant interaction between holes mediated by surface plasmons propagating on the film surfaces 7 . More precisely, transmission peaks emerge close to the Wood anomalies 8 and are well explained in terms of geometrical resonances in the periodic lattice 9,10,11 . An important challenge in the design of plasmonic nanostructures is the precise control of their optical responses in order to meet the requirements of specific device applications. This can be accomplished by casting nanostructures with appropriate materials and geometries. However, such an approach is static and limited by material inhomogeneity and fabrication tolerances. More critically, many applications (e.g., optical switching and modulation) ...

Polycrystalline graphene and metallic nanowires (NWs) have been proposed to replace indium tin oxide (ITO), the most widely used transparent electrode (TE) film on the market. However, the trade-off between optical transparency (Topt) and electrical sheet resistance (Rs) of these materials taken alone makes them difficult to compete with ITO. In this paper, we show that, by hot-press transfer of graphene monolayer on Ag NWs, the resulting combined structure benefits from the synergy of the two materials, giving a Topt-Rs trade-off better than that expected by simply adding the single material contributions Ag NWs bridge any interruption in transferred graphene, while graphene lowers the contact resistance among neighboring NWs and provides local conductivity in the uncovered regions in-between NWs. The hot-pressing not only allows graphene transfer but also compacts the NWs joints, thus reducing contact resistance. The dependence on the initial NW concentration of the effects produced by the hot press process on its own and the graphene transfer using hot press was investigated and indicates that a low concentration is more suitable for the proposed geometry. A TE film with Topt of 90% and Rs of 14 Ω/sq is demonstrated, also on a flexible glass substrate about 140 μm thick, a very attractive platform for efficient flexible electronic and photonic devices.

Transparent electrodes (TEs) having electrooptical trade‐offs better than state‐of‐the‐art indium tin oxide (ITO) are continuously sought as they are essential to enable flexible electronic and optoelectronic devices. In this work, a TiO2‐Ag‐ITO (TAI)‐based TE is introduced and its use is demonstrated in an inverted polymer solar cell (I‐PSCs). Thanks to the favorable nucleation and wetting conditions provided by the TiO2, the ultrathin silver film percolates and becomes continuous with high smoothness at very low thicknesses (3–4 nm), much lower than those required when it is directly deposited on a plastic or glass substrate. Compared to conventional ITO‐TE, the proposed TAI‐TE exhibits exceptionally lower electrical sheet resistance (6.2 Ω sq−1), higher optical transmittance, a figure‐of‐merit two times larger, and mechanical flexibility, the latter confirmed by the fact that the resistance increases only 6.6% after 103 tensile bending cycles. The I‐PSCs incorporating the TAI‐TE show record power conversion efficiency (8.34%), maintained at 96% even after 400 bending cycles.

Metallic nanowires are among the most promising transparent conductor (TC) alternatives to widely used indium tin oxide (ITO) because of their excellent trade-off between electrical and optical properties, together with their mechanical flexibility. However, they tend to suffer from relatively large surface roughness, instability against oxidation, and poor adhesion to the substrate. Embedding in a suitable material can overcome these shortcomings. Here we propose and demonstrate a new TC comprising silver nanowires (AgNWs) in an ultrathin polyimide foil that presents an optical transmission in the visible larger than ITO (>90%), while maintaining similar electrical sheet resistance (15 ohm/sq). The polyimide protects the Ag against environmental agents such as oxygen and water and, thanks to its deformability and very small thickness (5 μm), provides an ideal mechanical support to the NW's network, in this way ensuring extreme flexibility (bending radius as small as at least 1 mm) and straightforwardly removing any adhesion issue. The initial AgNWs' roughness is also reduced by a factor of about 15, reaching RMS values as low as 2.4 nm, suitable for the majority of applications. All these properties together with the simple fabrication technique based on all-solution processing put the developed TC in a competitive position as a lightweight, mechanically flexible and inexpensive substrate for consumer electronic and optoelectronic devices.

Silver, king among plasmonic materials, features low inelastic absorption in the visible-infrared (vis-IR) spectral region compared to other metals. In contrast, copper is commonly regarded as too lossy for actual applications. Here, we demonstrate vis-IR plasmons with quality factors >60 in long copper nanowires (NWs), as determined by electron energy-loss spectroscopy. We explain this result by noticing that most of the electromagnetic energy in these plasmons lies outside the metal, thus becoming less sensitive to inelastic absorption. Measurements for silver and copper NWs of different diameters allow us to elucidate the relative importance of radiative and nonradiative losses in plasmons spanning a wide spectral range down to <20 meV. Thermal population of such low-energy modes becomes significant and generates electron energy gains associated with plasmon absorption, rendering an experimental determination of the NW temperature. Copper is therefore emerging as an attractive, cheap, abundant material platform for high-quality plasmonics in elongated nanostructures.

Vibrational optical spectroscopies can be enhanced by surface plasmons to reach molecular-sized limits of detection and characterization. The level of enhancement strongly depends on microscopic details of the sample that are generally missed by macroscopic techniques. Here we investigate phonons in h-BN by coupling them to silver-nanowire plasmons, whose energy is tuned by modifying the nanowire length. Specifically, we use electron beam milling to accurately and iteratively change the nanowire length, followed by electron energy-loss spectroscopy to reveal the plasmon-enhanced vibrational features of h-BN. This allows us to investigate otherwise hidden bulk phonons and observe strong plasmon-phonon coupling. The new milling-and-spectroscopy technique holds great potential for resolving vibrational features in material nanostructures. arXiv:1905.12503v1 [cond-mat.mes-hall]