Metamaterials, or engineered materials with rationally designed, subwavelength-scale building blocks, allow us to control the behavior of physical fields in optical, microwave, radio, acoustic, heat transfer, and other applications with flexibility and performance that are unattainable with naturally available materials. In turn, metasurfaces-planar, ultrathin metamaterials-extend these capabilities even further. Optical metasurfaces offer the fascinating possibility of controlling light with surface-confined, flat components. In the planar photonics concept, it is the reduced dimensionality of the optical metasurfaces that enables new physics and, therefore, leads to functionalities and applications that are distinctly different from those achievable with bulk, multilayer metamaterials. Here, we review the progress in developing optical metasurfaces that has occurred over the past few years with an eye toward the promising future directions in the field.

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Planar Photonics with Metasurfaces

Optical generation of hot electrons in metallic structures and its potential as an alternative to conventional electron–hole separation in semiconductor devices are reviewed. The possibilities for realizing high conversion efficiencies with low fabrication costs are discussed along with challenges in terms of the materials, architectures and fabrication methods

Plasmon-induced hot-electron generation at nanoparticle/metal-oxide interfaces for photovoltaic and photocatalytic devices

Materials research plays a vital role in transforming breakthrough scientific ideas into next-generation technology. Similar to the way silicon revolutionized the microelectronics industry, the proper materials can greatly impact the field of plasmonics and metamaterials. Currently, research in plasmonics and metamaterials lacks good material building blocks in order to realize useful devices. Such devices suffer from many drawbacks arising from the undesirable properties of their material building blocks, especially metals. There are many materials, other than conventional metallic components such as gold and silver, that exhibit metallic properties and provide advantages in device performance, design flexibility, fabrication, integration, and tunability. This review explores different material classes for plasmonic and metamaterial applications, such as conventional semiconductors, transparent conducting oxides, perovskite oxides, metal nitrides, silicides, germanides, and 2D materials such as graphene. This review provides a summary of the recent developments in the search for better plasmonic materials and an outlook of further research directions.

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Alternative Plasmonic Materials: Beyond Gold and Silver

We report on the synthesis of the first two-dimensional transition metal nitride, Ti4N3-based MXene. In contrast to the previously reported MXene synthesis methods – in which selective etching of a MAX phase precursor occurred in aqueous acidic solutions – here a molten fluoride salt is used to etch Al from a Ti4AlN3 powder precursor at 550 °C under an argon atmosphere. We further delaminated the resulting MXene to produce few-layered nanosheets and monolayers of Ti4N3Tx, where T is a surface termination (F, O, or OH). Density functional theory calculations of bare, non-terminated Ti4N3 and terminated Ti4N3Tx were performed to determine the most energetically stable form of this MXene. Bare and functionalized Ti4N3 are predicted to be metallic. Bare Ti4N3 is expected to show magnetism, which is significantly reduced in the presence of functional groups.

Synthesis of two-dimensional titanium nitride Ti4N3 (MXene)

A high-temperature stable broadband plasmonic absorber is designed, fabricated, and optically characterized. A broadband absorber with an average high absorption of 95% and a total thickness of 240 nm is fabricated, using a refractory plasmonic material, titanium nitride. This absorber integrates both the plasmonic resonances and the dielectric-like loss. It opens a path for the interesting applications such as solar thermophotovoltaics and optical circuits.

Refractory plasmonics with titanium nitride: broadband metamaterial absorber.

The search for alternative plasmonic materials with improved optical properties, easier fabrication and integration capabilities over those of the traditional materials such as silver and gold could ultimately lead to real-life applications for plasmonics and metamaterials. In this work, we show that titanium nitride could perform as an alternative plasmonic material in the visible and near-infrared regions. We demonstrate the excitation of surface-plasmon-polaritons on titanium nitride thin films and discuss the performance of various plasmonic and metamaterial structures with titanium nitride as the plasmonic component. We also show that titanium nitride could provide performance that is comparable to that of gold for plasmonic applications and can significantly outperform gold and silver for transformation-optics and some metamaterial applications in the visible and near-infrared regions.

https://engineering.purdue.edu/~aeb/pubs/Titanium%20nitride%20as%20a%20plasmonic%20material%20for%20visible%20and%20near-infrared%20wavelengths.pdf

Titanium nitride as a plasmonic material for visible and near-infrared wavelengths

A light emitting diode having a metallized silicon substrate including a silicon base, a buffer layer disposed on the silicon base, a metal layer disposed on the buffer layer, and light emitting layers disposed on the metal layer. The buffer layer can be AlN, and the metal layer ZrN. The light emitting layers can include GaN and InGaN. The metallized silicon substrate can also include an oxidation prevention layer disposed on the metal layer. The oxidation prevention layer can be AlN. The light emitting diode can be formed using an organometallic vapor phase epitaxy process. The intermediate ZrN/AlN layers enable epitaxial growth of GaN on silicon substrates using conventional organometallic vapor phase epitaxy. The ZrN layer provides an integral back reflector, ohmic contact to n-GaN. The AlN layer provides a reaction barrier, thermally conductive interface layer, and electrical isolation layer.

Metallized silicon substrate for indium gallium nitride light-emitting diode

A thermoelectric segment and a method for fabricating. The fabricating includes forming structures by depositing thin-film metal-semiconductor multilayers on substrates and depositing metal layers on the multilayers, joining metal bonding layers to form dual structures with combined bonding layers; and removing at least one of the substrates; and using the dual structure to form a thermoelectric segments. The method can include dicing the dual structures before or after removing the substrates. The method can include depositing additional bonding layers and joining dual structures to make thermoelectric segments of different thicknesses. Each multilayer can be about 5-10 μm thick. Each bonding layer can be about 1-2 μm thick. The bonding layers can be made of a material having high thermal and electrical conductivity. The multilayers can be (Hf,Zr,Ti,W)N/(Sc,Y,La,Ga,In,Al)N superlattice layers. Metal nitride layers can be deposited between each of the bonding layers and multilayers.

Laminated thin film metal-semiconductor multilayers for thermoelectrics

Templated synthesis of thermoelectric nanowires in porous anodic alumina (PAA) have potential for enhanced performance relative to bulk materials. A significant challenge is the template material, which can serve as a thermal shunt. In this work, an approach for creating a branched PAA template is described. The process utilizes localized self-heating to destabilize the planar anodization front, yielding branched and interconnected pores growing at a rate of 300 μm/h. The template is selectively etched after electrodeposition of desired materials, yielding self-supporting nanowire arrays with thicknesses up to about 300 μm, thereby eliminating the thermal shunt through the template.

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Self-supporting nanowire arrays templated in sacrificial branched porous anodic alumina for thermoelectric devices

In this work, we investigate the possibility of using pulsed laser annealing to locally tailor the physical properties of Si1-xGex (18% < × < 90%) prepared by low pressure chemical vapor deposition (LPCVD) at 400°C. Films which were amorphous as deposited showed, after laser annealing, strong {111} texture and a columnar grain microstructure and an average resistivity of 0.7 mΩ.cm. AFM showed that the first few laser pulses result in a noticeable reduction in surface roughness, which is proportional to the pulse energy. However, a large number of successive pulses dramatically increases the surface roughness.

Jeremy L. Schroeder

Papers

Titanium nitride as a plasmonic material for visible and near-infrared wavelengths

Metallized silicon substrate for indium gallium nitride light-emitting diode

Laminated thin film metal-semiconductor multilayers for thermoelectrics

Self-supporting nanowire arrays templated in sacrificial branched porous anodic alumina for thermoelectric devices

Pulsed Laser Annealing of Silicon-Germanium Films