As a result of recent developments in nanofabrication techniques, the dimensions of metallic building blocks of plasmonic devices continue to shrink down to nanometer range thicknesses. The optical and electronic properties of ultra-thin plasmonic films are expected to have a strong dependence on the film thickness, composition, and strain, as well as an increased sensitivity to external optical and electrical perturbation. This unique tailorability establishes ultra-thin plasmonic films as an attractive material for the design of tailorable and dynamically switchable metasurfaces. Due to their epitaxial growth on lattice matched substrates, TiN is an ideal material to investigate the tailorable properties of plasmonic films with thicknesses of just a few monolayers. MXenes, a class of two-dimensional (2D) nanomaterials formed of transition metal carbides and carbon nitrides, are yet another promising material platform for tailorable plasmonic metamaterials. MXenes have been widely explored in a variety of applications, such as electromagnetic shielding and SERS. However, investigations of MXenes in the context of nanophotonics and plasmonics have been limited leading to this current exploration of MXenes as building blocks for plasmonic and metamaterial devices. In this study, we investigate these two emerging classes of materials, MXenes and ultra-thin transition metal nitrides, as potential material platforms for tailorable plasmonic metamaterials. We report on the strain and oxidation dependent optical properties of ultrathin TiN. Applications of MXenes as a broadband plasmonic metamaterial absorber and a random laser device are also discussed.
MXenes are a recently discovered family of two-dimensional nanomaterials formed of transition metal carbides and carbon nitrides with the general chemical form Mn+1XnTx, where ‘M’ is a transitional metal, ‘X’ is either C or N, and ‘T’ represents a surface functional group (O, -OH or -F). MXenes are derived from layered ternary carbides and nitrides known as MAX (Mn+1AXn) phases by selective chemical etching of the ‘A’ layers and addition of functional groups ‘T’.
In our work, we focus on one of the most well studied MXene, titanium carbide (Ti3C2Tx). Single to few layer flakes of Ti3C2Tx (in a solution dispersed form) are used to create a continuous film on a desired substrate by using spin coating technique. Losses inherent to the bulk MXene and existence of strong localized SP resonances in Ti3C2Tx disks/pillar-like nanostructures at near-IR frequencies are utilized to design an efficient broadband absorber. For Ti3C2Tx MXene disk array sitting on a bilayer stack of Au/Al2O3, high efficiency (>90%) absorption across visible to near-IR frequencies (bandwidth ~1.55 μm), is observed experimentally.
We also experimentally study random lasing behavior in a metamaterial constructed by randomly dispersing single layer nanosheets of Ti3C2Tx into a gain medium (rhodamine 101, R101). Sharp lasing peaks are observed when the pump energy reaches the threshold value of ~ 0.70 μJ/pulse. This active metamaterial holds a great potential to achieve tunable random lasing by changing the optical properties of Ti3C2Tx flakes.
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