Plasmonics is a rapidly developing field at the boundary of physical optics and condensed matter physics. It studies phenomena induced by and associated with surface plasmons-elementary polar excitations bound to surfaces and interfaces of nanostructured good metals. This Roadmap is written collectively by prominent researchers in the field of plasmonics. It encompasses selected aspects of nanoplasmonics. Among them are fundamental aspects such as quantum plasmonics based on quantum-mechanical properties of both underlying materials and plasmons themselves (such as their quantum generator, spaser), plasmonics in novel materials, ultrafast (attosecond) nanoplasmonics, etc. Selected applications of nanoplasmonics are also reflected in this Roadmap, in particular, plasmonic waveguiding, practical applications of plasmonics enabled by novel materials, thermo-plasmonics, plasmonic-induced photochemistry and photo-catalysis. This Roadmap is a concise but authoritative overview of modern plasmonics. It will be of interest to a wide audience of both fundamental physicists and chemists and applied scientists and engineers.

ABSTRACT:Understanding the temperature dependence of the optical properties of thin metal films is critical for designing practical devices for high temperature applications in a variety of research areas, including plasmonics and near-field radiative heat transfer. Even though the optical properties of bulk metals at elevated temperatures have been studied, the temperature-dependent data for thin metal films, with thicknesses ranging from few tens to few hundreds of nanometers, is largely missing. In this work we report on the optical constants of single-and polycrystalline gold thin films at elevated temperatures in the wavelength range from 370 to 2000 nm. Our results show that while the real part of the dielectric function changes marginally with increasing temperature, the imaginary part changes drastically. For 200-nm-thick single-and polycrystalline gold films the imaginary part of the dielectric function at 500 0 C becomes nearly twice larger than that at room temperature. In contrast, in thinner films (50-nm and 30-nm) the imaginary part can show either increasing or decreasing behavior within the same temperature range and eventually at 500 0 C it becomes nearly 3-4 times larger than that at room temperature. The increase in the imaginary part at elevated temperatures significantly reduces the surface plasmon polariton propagation length and the quality factor of the localized surface plasmon resonance for a spherical particle. We provide experiment-fitted models to describe the temperature-dependent gold dielectric function as a sum of one Drude and two critical point oscillators. These causal analytical models could enable accurate multiphysics modelling of gold-based nanophotonic and plasmonic elements in both frequency and time domains. KEYWORDS: nanophotonics, plasmonics, metamaterials, metal optics, high temperatures, gold thin films, analytical models2 Nanometer-scale field localization is at the heart of metal-based nanophotonics, namely plasmonics [1][2][3][4] . In plasmonic nanostructures, strong field confinement -or so-called 'hot spots' -arise due to the excitation of subwavelength oscillations of free electrons coupled to the incident electromagnetic field at the metal-dielectric interface, known as surface plasmons 5 . Such nanoscale hot spots lead to high energy densities that inevitably increase the local temperature of the plasmonic material under study. Recently, there has been a growing interest in plasmonicsbased local heating applications such as heat-assisted magnetic recording, thermophotovoltaics, and photothermal therapy 6,7 .However, theoretical modeling of plasmonic structures in such local-heating based systems has so far been performed using room-temperature optical constants, i.e. with thermal analysis and optical material properties being decoupled. Therefore, probing the temperature dependence of the optical properties of thin metal films is critical for both gaining an insight into the physical process associated with elevated temperatures and for accurate modeling of ...

Due to their exceptional plasmonic properties, noble metals such as gold and silver have been the materials of choice for the demonstration of various plasmonic and nanophotonic phenomena. However, noble metals' softness, lack of tailorability and low melting point along with challenges in thin film fabrication and device integration have prevented the realization of real-life plasmonic devices. In the recent years, titanium nitride (TiN) has emerged as a promising plasmonic material with good metallic and refractory (high temperature stable) properties. The refractory nature of TiN could enable practical plasmonic devices operating at elevated temperatures for energy conversion and harsh-environment industries such as gas and oil. Here we report on the temperature dependent dielectric functions of TiN thin films of varying thicknesses in the technologically relevant visible and near-infrared wavelength range from 330 nm to 2000 nm for temperatures up to 900 0 C using in-situ high temperature ellipsometry. Our findings show that the complex dielectric function of TiN at elevated temperatures deviates from the optical parameters at room temperature, indicating degradation in plasmonic properties both in the real and imaginary parts of the dielectric constant. However, quite strikingly, the relative changes of the optical properties of TiN are significantly smaller compared to its noble metal counterparts. In fact, at temperatures over 400 0 C the quality factors of localized surface plasmon resonances and propagating surface plasmons in thin TiN films become nearly the same as those in polycrystalline noble metals. Furthermore, no structural degradation was observed in any of TiN films upon heat treatment. Solids, 1969Solids, , 30, 2765Solids, -2769. Figures:

Hot-carriers in plasmonic nanostructures, generated via plasmon decay, play key roles in applications like photocatalysis and in photodetectors that circumvent band-gap limitations. However, direct experimental quantification of steady-state energy distributions of hot-carriers in nanostructures has so far been lacking. We present transport measurements from single-molecule junctions, created by trapping suitably chosen single molecules between an ultra-thin gold film supporting surface plasmon polaritons and a scanning probe tip, that can provide quantification of plasmonic hot-carrier distributions. Our results show that Landau damping is the dominant physical mechanism of hot-carrier generation in nanoscale systems with strong confinement. The technique developed in this work will enable quantification of plasmonic hot-carrier distributions in nanophotonic and plasmonic devices.

Theoretical studies have been conducted to comprehend these characteristics in ultrathin gold films. [6,7] However, the fabrication of such thin films is generally quite difficult. For conventional plasmonic metals, such as silver and gold, continuous, smooth films with thicknesses lower than 10 nm are challenging to produce because of island formation and large defect concentrations which result from their high surface energy. [8][9][10] On the other hand, transition metal nitrides (TiN, ZrN, etc.) can be grown epitaxially on substrates such as c-sapphire and MgO, [11] enabling the formation of continuous ultrathin films down to 2 nm while maintaining high quality. [12] These materials have gained much interest for plasmonic applications for the visible and near-infrared region due to their tailorable optical properties and refractory quality. [13] TiN has already been shown to have potential applications in photovoltaics, [14,15] waveguiding, [16] modulators, [17,18] and nonlinear optical devices. [3,19] Although some of these devices use films of ≈10 nm, there is additional interest in the formation of ultrathin metallic films (<10 nm) which may facilitate additional applications. To support this, there have been efforts to grow ultrathin TiN films using atomic layer deposition (ALD). However, these films become dielectric below 5 nm and may not be useful for nanophotonic applications. [20] This is because TiN thin films grown using ALD with a TiCl 4 precursor are more prone to Cl impurities, which could result in a degradation of the metallic properties in thinner films. [21][22][23] In the present study, we have grown continuous, epitaxial, ultrathin TiN films that remain plasmonic in the optical range via DC reactive magnetron sputtering. Using sputtering to grow the film decreases the chance of contamination since only Ti, N, and Ar are introduced into the system, which helps maintain the metallic properties in ultrathin films. The dielectric permittivity of these films was extracted using spectroscopic ellipsometry, while Hall measurements give further insight into the optical behavior of the films. Results and DiscussionUltrathin TiN films with thicknesses of 2, 4, 6, 8, and 10 nm were grown on MgO using DC reactive magnetron sputtering. Due to the lattice matching between TiN (4.24 Å) and substrates such as MgO (4.21 Å) and sapphire (4.76 Å), high quality,Overcoming the challenge of growing ultrathin metallic films is of great importance for practical applications of nanoplasmonic structures. In the present work, epitaxial, ultrathin (<10 nm) films of plasmonic TiN are grown on MgO using DC reactive magnetron sputtering. The optical properties of the films are studied through variable angle spectroscopic ellipsometry and Hall measurements. As the film thickness decreases, they become less metallic and exhibit higher loss while still remaining plasmonic in the optical range. These trends are related to the decreasing carrier concentration in the thinner films and increased scattering, respectively. Howev...

Metasurfaces are thin two-dimensional metamaterial layers that allow or inhibit the propagation of electromagnetic waves in desired directions. For example, metasurfaces have been demonstrated to produce unusual scattering properties of incident plane waves or to guide and modulate surface waves to obtain desired radiation properties. These properties have been employed, for example, to create innovative wireless receivers and transmitters. In addition, metasurfaces have recently been proposed to confine electromagnetic waves, thereby avoiding undesired leakage of energy and increasing the overall efficiency of electromagnetic instruments and devices. The main advantages of metasurfaces with respect to the existing conventional technology include their low cost, low level of absorption in comparison with bulky metamaterials, and easy integration due to their thin profile. Due to these advantages, they are promising candidates for real-world solutions to overcome the challenges posed by the next generation of transmitters and receivers of future high-rate communication systems that require highly precise and efficient antennas, sensors, active components, filters, and integrated technologies. This Roadmap is aimed at binding together the experiences of prominent researchers in the field of metasurfaces, from which explanations for the physics behind the extraordinary properties of these structures shall be provided from viewpoints of diverse theoretical backgrounds. Other goals of this endeavour are to underline the advantages and limitations of metasurfaces, as well as to lay out guidelines for their use in present and future electromagnetic devices. This Roadmap is divided into five sections: 1. Metasurface based antennas. In the last few years, metasurfaces have shown possibilities for advanced manipulations of electromagnetic waves, opening new frontiers in the design of antennas. In this section, the authors explain how metasurfaces can be employed to tailor the radiation properties of antennas, their remarkable advantages in comparison with conventional antennas, and the future challenges to be solved. 2. Optical metasurfaces. Although many of the present demonstrators operate in the microwave regime, due either to the reduced cost of manufacturing and testing or to satisfy the interest of the communications or aerospace industries, part of the potential use of metasurfaces is found in the optical regime. In this section, the authors summarize the classical applications and explain new possibilities for optical metasurfaces, such as the generation of superoscillatory fields and energy harvesters. 3. Reconfigurable and active metasurfaces. Dynamic metasurfaces are promising new platforms for 5G communications, remote sensing and radar applications. By the insertion of active elements, metasurfaces can break the fundamental limitations of passive and static systems. In this section, we have contributions that describe the challenges and potential uses of active components in metasurfaces, including new studies on non-Foster, parity-time symmetric, and non-reciprocal metasurfaces. 4. Metasurfaces with higher symmetries. Recent studies have demonstrated that the properties of metasurfaces are influenced by the symmetries of their constituent elements. Therefore, by controlling the properties of these constitutive elements and their arrangement, one can control the way in which the waves interact with the metasurface. In this section, the authors analyze the possibilities of combining more than one layer of metasurface, creating a higher symmetry, increasing the operational bandwidth of flat lenses, or producing cost-effective electromagnetic bandgaps. 5. Numerical and analytical modelling of metasurfaces. In most occasions, metasurfaces are electrically large objects, which cannot be simulated with conventional software. Modelling tools that allow the engineering of the metasurface properties to get the desired response are essential in the design of practical electromagnetic devices. This section includes the recent advances and future challenges in three groups of techniques that are broadly used to analyze and synthesize metasurfaces: circuit models, analytical solutions and computational methods.

By combining first principles theoretical calculations and experimental optical and structural characterization such as spectroscopic ellipsometry, X-ray spectroscopy, and electron microscopy, we study the dielectric permittivity and plasmonic properties of ultrathin TiN films at an atomistic level. Our results indicate a remarkably persistent metallic character of ultrathin TiN films and a progressive red shift of the plasmon energy as the thickness of the film is reduced. The microscopic origin of this trend is interpreted in terms of the characteristic two-band electronic structure of the system. Surface oxidation and substrate strain are also investigated to explain the deviation of the optical properties from the ideal case. This paves the way to the realization of ultrathin TiN films with tailorable and tunable plasmonic properties in the visible range for applications in ultrathin metasurfaces and flexible optoelectronic devices.

Silver holds a unique place in plasmonics compared to other noble metals owing to its low losses in the visible and near-IR wavelength ranges. With a growing interest in local heating and high temperature applications of plasmonics, it is becoming critical to characterize the dielectric function of nanometer-scale thin silver films at higher temperatures, especially near the breakdown temperature, which depends on the film thickness and crystallinity. So far, such a comprehensive study has been missing. Here we report the in situ high temperature ellipsometry measurements of ultrasmooth and epitaxial quality crystalline silver films, along with electron beam evaporated polycrystalline silver films at temperatures up to 700 °C, in the wavelength range of 330−2000 nm. Our findings show that the dielectric function of all the films changes remarkably at elevated temperatures with larger relative changes observed in polycrystalline films. In addition, low-loss epitaxial films were found to be thermally more stable at elevated temperatures. We demonstrate the importance of our findings for high temperature applications with a numerical simulation of field enhancement in a bow-tie nanoantenna, a near field transducer commonly used for heat-assisted magnetic recording. The simulated field profiles at elevated temperatures showed significant deviations compared to those at room temperature, clearly suggesting that the use of room temperature optical properties in modeling elevated temperature applications can be misleading due to the thermal deviations in the Ag dielectric function. We also provide causal analytical models describing the elevated temperature Ag dielectric functions.