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Nano-optics on flatland: from quantum nanotechnology to nano-bio-photonics

Periodic Reporting for period 4 - 2DNANOPTICA (Nano-optics on flatland: from quantum nanotechnology to nano-bio-photonics)

Reporting period: 2021-07-01 to 2021-12-31

With the advent of two-dimensional (2D) materials and their extraordinary optical properties, first visualizations of both low-loss and electrically tunable (active) plasmons in graphene and high optical quality phonons in monolayer and multilayer h-BN nanostructures have been shown in the mid-infrared spectral range, thus introducing a very encouraging arena for scientifically ground-breaking discoveries in nano-optics. Indeed, first proof-of-concept devices permitting to control the propagation of graphene plasmons, such as a lens and a prism, have been demonstrated. Inspired by the extraordinary prospects given by these initial experiments, this ERC project aims to develop the fields of 2D nanoplasmonics (graphene plasmonics) and nanophononics to establish a technological platform that, including coherent sources, waveguides, routers, and efficient detectors, permits an unprecedented active control and manipulation of light and light-matter interactions on the nanoscale and at room temperature, thus laying the foundations of 2D nano-optics. Advances in this direction will have an enormous scientific importance and technological relevance in a variety of fields such as sensing, quantum science, or photochemistry, where active control of fundamental nanoscale light-matter processes are of vital importance.

The aims of the project are separated in two parts:

1. In a first part, the project will focus mainly on the development of 2D nanoplasmonic and nanophononic structures. We will design and fabricate optical elements that allow for generating (sources), guiding or detecting optical fields on the nanoscale. Based on low-loss optical materials, they will constitute a nano-optics toolkit.
2. Building on the basic elements and unique capabilities obtained in the first part, we will focus in a second part of the project on developing the field of 2D nano-optics by demonstrating first functional 2D nanodevices, which we separate into three different domains:
A. Active 2D nano-optics where by combining different building blocks developed in part 1 we aim to demonstrate actively controlled 2D nanodevices that permit an unprecedented manipulation and concentration of optical fields on the nanoscale.
B. Quantum 2D nano-optics where by adding gain to the sources developed in part 1 we aim to realize coherent nanosources of low-loss plasmons and phonons (nano-spasers and nano-”sphasers”, Surface Plasmon/Phonon Amplification by Stimulated Emission of Radiation).
C. Molecular 2D nano-optics where by making use of the nanoantennas developed in Part 1 and the functional active and quantum 2D nanodevices demonstrated in Parts 2A and 2B we aim to fundamentally explore strong light-matter interactions at room temperature, ultimately involving single molecular resonators. The successful achievement of this aim will be a breakthrough in the fields of nano-optics and nano-chemistry.
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).
All reported works summarised in the previous section are beyond the state of the art. In particular, the functional 2D nanodevices developed within the project allowed us to: i) visualize novel forms of the most fundamental optical phenomena, such as reflection and refraction, at the nanoscale, ii) demonstrate exotic canalization phenomena (natural waveguiding), which opened the field of twistoptics, and iii) study strong light-matter interactions with a small amount of molecular resonators, key for developing nanochemistry applications.
Twistoptics: Canalization of nanolight in twisted van der Waals slabs
Focusing of light to the nanoscale in a 2D material
Anomalous refraction of light at the nanoscale
In-plane anisotropic propagation of highly confined polaritons