Optical Properties and Plasmonic Performance of Titanium Nitride

Titanium nitride (TiN) is one of the most well-established engineering materials nowadays. TiN can overcome most of the drawbacks of plasmonic metals due to its high electron conductivity and mobility, high melting point and due to the compatibility of its growth with Complementary Metal Oxide Semiconductor (CMOS) technology. In this work, we review the dielectric function spectra of TiN and we evaluate the plasmonic performance of TiN by calculating (i) the Surface Plasmon Polariton (SPP) dispersion relations and (ii) the Localized Surface Plasmon Resonance (LSPR) band of TiN nanoparticles, and we demonstrate a significant plasmonic performance of TiN

Laser Annealing as a Platform for Plasmonic Nanostructuring

Nanoconstruction of metals is a significant challenge for the future manufacturing of plasmonic devices. Such a technology requires the development of ultra‐fast, high‐throughput and low cost fabrication schemes. Laser processing can be considered as such and can potentially represent an unrivalled tool towards the anticipated arrival of modules based in metallic nanostructures, with an extra advantage: the ease of scalability. Specifically, laser nanostructuring of either thin metal films or ceramic/metal multilayers and composites can result on surface or subsurface plasmonic patterns, respectively, with many potential applications. In this chapter, the photo‐thermal processes involved in surface and subsurface nanostructuring are discussed and processes to develop functional plasmonic nanostructures with pre‐determined morphology are demonstrated. For the subsurface plasmonic conformations, the temperature gradients that are developed spatially across the metal/dielectric structure during the laser processing can be utilized. For the surface plasmonic nanoassembling, the ability to tune the laser\u27s wavelength to either match the absorption spectral profile of the metal or to be resonant with the plasma oscillation frequency can be utilised, i.e. different optical absorption mechanisms that are size‐selective can be probed. Both processes can serve as a platform for stimulating further progress towards the engineering of large‐scale plasmonic devices

Conductive nitrides: growth principles, optical and electronic properties, and their perspectives in photonics and plasmonics

The nitrides of most of the group IVb-Vb-VIb transition metals (TiN, ZrN, HfN, VN, NbN, TaN, MoN, WN) constitute the unique category of conductive ceramics. Having substantial electronic conductivity, exceptionally high melting points and covering a wide range of work function values, they were considered for a variety of electronic applications, which include diffusion barriers in metallizations of integrated circuits, Ohmic contacts on compound semiconductors, and thin film resistors, since early eighties. Among them, TiN and ZrN are recently emerging as significant candidates for plasmonic applications. So the possible plasmonic activity of the rest of transition metal nitrides (TMN) emerges as an important open question. In this work, we exhaustively review the experimental and computational (mostly ab initio) works in the literature dealing with the optical properties and electronic structure of TMN spanning over three decades of time and employing all the available growth techniques. We critically evaluate the optical properties of all TMN and we model their predicted plasmonic response. Hence, we provide a solid understanding of the intrinsic (e.g. the valence electron configuration of the constituent metal) and extrinsic (e.g. point defects and microstructure) factors that dictate the plasmonic performance. Based on the reported optical spectra, we evaluate the quality factors for surface plasmon polariton and localized surface plasmon for various TMN and critically compare them to each other. We demonstrate that, indeed TiN and ZrN along with HfN are the most well-performing plasmonic materials in the visible range, while VN and NbN may be viable alternatives for plasmonic devices in the blue, violet and near UV ranges, albeit in expense of increased electronic loss. Furthermore, we consider the alloyed ternary TMN and by critical evaluation and comparison of the reported experimental and computational works, we identify the emerging optimal tunable plasmonic conductors among the immense number of alloying combinations

Photoluminescence enhancement of ZnO via coupling with surface plasmons on Al thin films

We present that the ultra-violet emission of ZnO can be enhanced, as much as six-times its integral intensity, using an Al thin interlayer film between the Si substrate and ZnO thin film and a postfabrication laser annealing process. The laser annealing is a cold process that preserves the chemical state and integrity of the underlying aluminum layer, while it is essential for the improvement of the ZnO performance as a light emitter and leads to enhanced emission in the visible and in the ultraviolet spectral ranges. In all cases, the metal interlayer enhances the intensity of the emitted light, either through coupling of the surface plasmon that is excited at the Al/ZnO interface, in the case of light-emitting ZnO in the ultraviolet region, or by the increased back reflection from the Al layer, in the case of the visible emission. In order to evaluate the process and develop a solid understanding of the relevant physical phenomena, we investigated the effects of various metals as interlayers (Al, Ag, and Au), the metal interlayer thickness, and the incorporation of a dielectric spacer layer between Al and ZnO. Based on these experiments, Al emerged as the undisputable best choice of metal interlayers because of its compatibility with the laser annealing process, as well as due to its high optical reflectivity at 380 and 248 nm, which leads to the effective coupling with surface plasmons at the Al/ZnO interfaces at 380 nm and the secondary annealing of ZnO by the back-reflected 248 nm laser beam

Embedded metal oxide plasmonics using local plasma oxidation of AZO for planar metasurfaces

New methods for achieving high‐quality conducting oxide metasurfaces are of great importance for a range of emerging applications from infrared thermal control coatings to epsilon‐near‐zero nonlinear optics. This work demonstrates the viability of plasma patterning as a technique to selectively and locally modulate the carrier density in planar Al‐doped ZnO (AZO) metasurfaces without any associated topographical surface profile. This technique stands in strong contrast to conventional physical patterning which results in nonplanar textured surfaces. The approach can open up a new route to form novel photonic devices with planar metasurfaces, for example, antireflective coatings and multi‐layer devices. To demonstrate the performance of the carrier‐modulated AZO metasurfaces, two types of devices are realized using the demonstrated plasma patterning. A metasurface optical solar reflector is shown to produce infrared emissivity equivalent to a conventional etched design. Second, a multiband metasurface is achieved by integrating a Au visible‐range metasurface on top of the planar AZO infrared metasurface. Independent control of spectral bands without significant cross‐talk between infrared and visible functionalities is achieved. Local carrier tuning of conducting oxide films offers a conceptually new approach for oxide‐based photonics and nanoelectronics and opens up new routes for integrated planar metasurfaces in optical technology

Simulating the opto-thermal processes involved in laser induced self-assembly of surface and sub-surface plasmonic nano-structuring

Nano-structuring of metals is one of the greatest challenges for the future of plasmonic and photonic devices. Such a technology calls for the development of ultra-fast, high-throughput and low cost fabrication techniques. Laser processing accounts for the aforementioned properties, representing an unrivalled tool towards the anticipated arrival of modules based in metallic nano-structures, with an extra advantage: the ease of scalability. Specifically, laser nano-structuring of an ultra-thin metal film or an alternating metal film on a substrate/metal film on a substrate results respectively on surface (metallic nanoparticles on the surface of the substrate) or subsurface (metallic nanoparticles embedded in a dielectric matrix) plasmonic patterns with many applications. In this work we investigate theoretically the photo-thermal processes involved in surface and sub-surface plasmonic nano-structuring and compare to experiments. To this end, we present a design process and develop functional plasmonic nano-structures with pre-determined morphology by tuning the annealing parameters like the laser fluence and wavelength and/or the structure parameters like the thickness of the metallic film and the volume ratio of the metal film on a substrate-metal composite. For the surface plasmonic nano-structuring we utilize the ability to tune the laser's wavelength to either match the absorption spectral profile of the metal or to be resonant with the plasma oscillation frequency, i.e. we utilize different optical absorption mechanisms that are size-selective. Thus, we overcome a great challenge of laser induced self assembly by combining simultaneously large-scale character with nanometer scale precision. For subsurface plasmonic nano-structuring, on the other hand, we utilize the temperature gradients that are developed spatially across the metal/dielectric nano-composite structure during the laser treatment. We find that the developed temperature gradients are strongly depended on the nanocrystalline character of the dielectric host which determines its thermal conductivity, the composition of the ceramic/metal and the total thickness of the nano-composite film. The aforementioned material parameters combined with the laser annealing parameters can be used to pre-design the final morphology of the sub-surface plasmonic structure. The proposed processes can serve as a platform that will stimulate further progress towards the engineering of plasmonic devices

Self-assembled plasmonic templates produced by microwave annealing: applications to surface-enhanced Raman scattering

Perhaps the simplest method for creating metal nanoparticles on a substrate is by driving their self-assembly with the thermal annealing of a thin metal film. By properly tuning the annealing parameters one hopes to discover a recipe that allows the pre-determined design of the NP arrangement. However, thermal treatment is known for detrimental effects and is not really the manufacturer's route of choice when it comes to large-scale applications. An alternative method is the use of microwave annealing, a method that has never been applied for metal processing, due to the high reflectance of microwave radiation at the surface of a metal. However, in this work we challenge the widely used nanostructuring methods by proving the microwave's annealing ability to produce plasmonic templates, out of extremely thin metal films, by simply using a domestic microwave oven apparatus. We show that this process is generic and independent of the deposition method used for the metal and we further quantify the suitability of these plasmonic templates for use in surface-enhanced Raman scattering applications

Laser-driven structural modifications and diffusion phenomena of plasmonic AlN/Ag stratified films

Laser annealing (LA) of AlN/Ag multilayers was proven to be an effective process to control the structure and dispersion of Ag into the AlN resulting in intense coloration via the localized surface plasmon resonance, which is of particular importance for decorative applications. In this work we present a study of the structural changes occurring in various AlN/Ag multilayers after LA, in an effort to establish firm knowledge of the diffusion and re-nucleation mechanisms that occur during the laser process. We investigate the effect of the basic LA parameters, such as the laser wavelength (193 and 248 nm), fluence (400–700 mJ/cm2), pressure (1 and 10 Bar) and number of pulses (1 and 2) and we show that the main processes is the Ag particle enhancement close to the film surface as a result of additive outidiffusion Ag and the formation of nanoparticles of varying size

Enhanced electrical and optical properties of room temperature deposited Aluminium doped Zinc Oxide (AZO) thin films by excimer laser annealing

High quality transparent conductive oxides (TCOs) often require a high thermal budget fabrication process. In this study, Excimer Laser Annealing (ELA) at a wavelength of 248 nm has been explored as a processing mechanism to facilitate low thermal budget fabrication of high quality aluminium doped zinc oxide (AZO) thin films. 180 nm thick AZO films were prepared by radio frequency magnetron sputtering at room temperature on fused silica substrates. The effects of the applied RF power and the sputtering pressure on the outcome of ELA at different laser energy densities and number of pulses have been investigated. AZO films deposited with no intentional heating at 180 W, and at 2 mTorr of 0.2% oxygen in argon were selected as the optimum as-deposited films in this work, with a resistivity of 1×10−3 Ω.cm, and an average visible transmission of 85%. ELA was found to result in noticeably reduced resistivity of 5×10−4 Ω.cm, and enhancing the average visible transmission to 90% when AZO is processed with 5 pulses at 125 mJ/cm2. Therefore, the combination of RF magnetron sputtering and ELA, both low thermal budget and scalable techniques, can provide a viable fabrication route of high quality AZO films for use as transparent electrodes