Tunable Electrical and Optical Properties of Photonic and Plasmonic Multilayer Metastructures Based on Alternative Plasmonic Materials

The current research in plasmonic nanotechnologies is focusing on nano-systems with intrinsic multifunctionalities, aimed at reaching large tunability of properties to fulfill at once multiple needs, spanning different applications (optoelectronics, biosensing, solar-energy harvesting). The archetypal plasmonic materials, i.e. noble metals, show limited modulation outside the visible spectrum due to the fixed charge carrier density [1]. This justifies the spreading interest for alternatives (transparent conductive oxides TCOs, transition metal nitrides or oxynitrides) and periodically-arranged multi-phase nanostructures, where the high level of control in carrier concentration is achievable directly at synthesis or with an “active” approach (e.g. external bias). Indeed, several nanoarchitectures have been investigated to modulate optical characteristics of plasmonic metals outside the visible, enhance performances, and eventually activate novel functionalities. Hyperbolic metamaterials especially sustain unique “high-k modes” activated by the periodic alternation of conductors (e.g. noble metals) and dielectrics (e.g oxides) of subwavelength thicknesses [2]. Besides, when increasing the characteristic dimension of layers, Bragg’s reflections start to appear and one-dimensional (1D) photonic crystals can be achieved straightforward [3].<br/>Here, a variety of novel multifunctional nano-structures have been developed, involving alternative materials (TCOs, nitrides and oxynitrides) and original design routes (via pulsed laser deposition PLD) with the aim to explore, from a material science perspective, electrical/optical responses resulting from unusual combinations of materials and properties (transparency, conductivity, tunable VIS-IR plasmonics, active modulation).<br/>For instance, novel TiN-TiO₂ plasmonic multilayers have been realized as hyperbolic metamaterial in the visible, with added advantages determined by the good material affinity and the versatility of TiO₂ as a wide-band gap semiconductor. In addition, the resulting features can benefit from the refractory nature and CMOS compatibility of TiN, along with the possibility to modify the plasmonic response through stoichiometry. Then, original transparent conducting multilayers based on the less-explored Tantalum-doped TiO₂ (Ta:TiO₂) TCO have been obtained directly in a simple one-step synthesis, by alternating conductive (compact) and dielectric (nanoporous) layers of the same TCO achieved by varying the deposition pressure (1-6 Pa O₂). Structural and electrical properties have been optimized to control optical/plasmonic outputs as a function of deposition conditions, doping content and geometrical parameters. Future applications are foreseen as hyperbolic platforms or biosensing devices in the IR, while 1D photonic crystals can be accomplished by customizing compact/porous fraction and dimension of the layers. The proper optimization of material properties leads to an intense photonic band gap, spanning from green to red wavelengths in the visible, which in turn can be actively modulated with an external bias. Finally, the investigation of TiON films as tailorable oxynitrides with expected epsilon-near-zero behaviour, has been started by finely controlling the oxygen content directly during PLD synthesis.<br/>Concluding, these multifunctional meta-devices are application-oriented and possess a cross-disciplinary attitude. Current challenges are related to mastering the material properties and designing devices to prove the real applicability of such meta-structures. Hence, the list of practical applications includes, but is not limited to, VIS or mid-IR biosensors, surface enhanced infrared Raman spectroscopy, optoelectronic elements for color manipulation and thermophotovoltaic emitters.<br/>[1] G.V. Naik et al. Advanced Materials 25, 24, 3264-3294 (2013).<br/>[2] Z. Guo et al. Journal of Applied Physics 127, 071101 (2020).<br/>[3] H. Shen et al. RCS Advances, 6, 4505-4520 (2016).