Tunable optical materials for multi-resonant plasmonics: from TiN to TiON [Invited]

Alternative plasmonic materials are gaining more and more interest since they deliver a plethora of advantages in designing of optical metadevices. Among other alternatives, titanium nitride (TiN) has shown an exceptional combination of encouraging properties, such as CMOS- and bio-compatibility, high carrier concentration, tunability and outstanding robustness (high mechanical, chemical and temperature durability). Optical constants of TiN can be tuned at the synthesis stage. This allows for the adjustment of the spectral position of a plasmon resonance within the visible and near-infrared (NIR) range in order to match the desired working wavelength of a particular device. Together, these factors made TiN a popular material of choice in a diversity of recent plasmonic applications. Titanium oxynitride (TiON), which can be produced through the oxidation of TiN, have a great potential to build upon the success of TiN. Recently, it has been demonstrated that TiON thin films can exhibit a negative double-epsilon-near-zero (2ENZ) dielectric function. This unusual behavior of the permittivity opens up novel opportunities for the excitation of the plasmon resonance at several distinct frequencies within the visible and NIR region. Multi-resonant plasmonic components are beneficial for applications, where the enhanced light-matter interaction at multiple frequencies is demanded, such as nonlinear optics, up- and down-conversion, wavelength multiplexing and broadband absorption. This work begins with a brief survey of the recent progress in plasmonics made with TiN-based structures. Then we focus on TiON thin films with the 2ENZ behavior by discussing their potential in plasmonics. The experimental approaches useful for characterization of TiON thin films and the corresponding results are analyzed. These results are valuable for the development of 2ENZ plasmonic materials with large figure-of-merits in a diversity of applications. We believe that 2ENZ media is a powerful concept for multi-resonant plasmonics that will augment the functionalities and extend the operation bandwidth of plasmonic devices.