The 2DNANOPTICA project has successfully completed most of the planned milestones. These achievements constitute a step forward in nanoscale light control using 2D materials and promise light-matter experiments with unprecedented capabilities. Listed below are up to 5 major results/achievements obtained during the project:
1.- We discovered polaritons in α-MoO3 [1] with strong in-plane anisotropic propagation. Dependent on the frequency, these polaritons show either elliptical or hyperbolic in-plane dispersion. They are not only exciting from fundamental aspects but also for applications such as thermal heat management or quantum optics, owing to the enhanced density of states provided by hyperbolic polaritons and their directional propagation, which was one of the main objectives of 2DNANOPTICA.
[1] W. Ma, et al. Nature, 562, 557–562, (2018).
2.- Phonon polaritons (PhPs) – light coupled to lattice vibrations – constituted basic building blocks of 2DNANOPTICA since they exhibit strong field confinement and long propagation. However, a main drawback of PhPs is the lack of tunability of the spectral range where they can exist – the so-called Reststrahlen Band (RB) –, which is narrow and material-specific, therefore severely limiting their implementation in nanophotonics technologies. During the second part of the project, we partially solved the drawback by demonstrating a broad spectral tuning of low-loss PhPs [1]. This was achieved by intercalating atoms (Na) into a van der Waals material (-V2O5), which allowed inducing a spectral shift of the RB of aprox. 60% of its initial bandwidth while keeping their lifetimes in the ps range.
[2] J. Taboada-Gutiérrez, et al. Nature Materials 19, 964-968 (2020).
3.- We demonstrated that the “twisting” principle can be extended to the optics realm [3], introducing new degree of control for the propagation of light on the nanoscale (main aim of 2DNANOPTICA). In particular, we demonstrated an efficient manipulation of the direction of propagation of in-plane hyperbolic PhPs in a twisted structure made out of two α-MoO3 slabs. At wavelength-dependent critical twisting angles, the PhPs showed directional propagation along extremely narrow channels opened in specific in-plane directions – so-called canalization regime - with enlarged propagation lengths. As such, these results demonstrated that the principles used to control electronic bands in “twistronics” can be analogously extended for controlling polaritons at the nanoscale. This allowed us to establish the field of “twistoptics”.
[3] J. Duan, et al. Nano Letters 20, 5323-5329 (2020).
4.- During the last part of the project, we studied strong coupling phenomena between organic molecules and propagating PhPs in unstructured h-BN layers. To do this, we used, for the first time, spectroscopic nanoimaging, which allowed the visualization of the strongly coupled modes in real space [5]. [4] A. Bylinkin,et al. Nature Photonics 15, 197-202 (2021).
5.- We demonstrated that the merits of graphene plasmonics and phononics can be merged by fabricating optical nanoantennas with large quality factors and broad active/passive spectral tunability [5]. In particular, we showed the possibility to: i) passive tuning of ultra-narrow resonances (Q~165) in a phononic nanoantenna made of h-BN by modifying its dielectric environment and ii) active tuning (~35 cm-1, being the resonance linewidth ~9 cm-1) of ultra-narrow resonances (Q~100 that is one order of magnitude larger than typical values (Q~10) in active graphene nanoantennas) in a phononic nanoantenna by placing it on top of gated graphene in which we vary the Fermi energy.
[5] J. Duan, et al. Active and Passive Tuning of Ultranarrow Resonances in Polaritonic Nanoantennas. Advanced. Materials 2104954 (2021).
