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

Periodic Reporting for period 3 - 2DNANOPTICA (Nano-optics on flatland: from quantum nanotechnology to nano-bio-photonics)

Reporting period: 2020-01-01 to 2021-06-30

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.
During the first part of 2DNANOPTICA we have successfully implemented important tasks and milestones planned for this time frame. These achievements constitute a step forward in the control of light at the nanoscale using 2D materials and promise light-matter experiments with unprecedented capabilities (as foreseen in the proposal). They are listed below:

a) We have implemented two different deterministic transfer methods that allow us for a routine fabrication of heterostructures of two dimensional materials with low optical losses, including graphene and h-BN.
b) Using these transfer methods in combination with e-beam lithography and reactive ion etching, we have been able to fabricate h-BN nanostructures that act as excellent optical nanoantennas supporting phonon polaritons (PhPs) in the mid-infrared, as characterized by scattering-type near-field optical microscopy (s-SNOM, technique implemented in the host institution within the project). This achievement has been published in [1].
c) As a follow-up result of [1] we have also been able to demonstrate long-propagating h-BN PhPs in nanoscale waveguide structures (manuscript under preparation).
d) We have been able to demonstrate (by far-field measurements) the strong optical coupling of h-BN PhPs localized in h-BN nanoresonators with ensembles of molecular resonators, as published in [2].
e) We have visualized phonon polaritons in α-MoO3 [3] propagating with in-plane anisotropy (dependent on the incident frequency, they show either elliptical or hyperbolic in-plane dispersion).
f) We have demonstrated the first polaritonic crystal (PC) made out of a vdW crystal (h-BN) supporting ultra-confined and long-lived hyperbolic PhPs [4].

[1] F. J. Alfaro-Mozaz, P. Alonso-González, S. Vélez, I. Dolado, M. Autore, S. Mastel, F. Casanova, L. E. Hueso, P. Li, A. Y. Nikitin, and R. Hillenbrand, "Nanoimaging of resonating hyperbolic polaritons in linear boron nitride antennas", Nature Communications, 8, 15624 (2017).
[2] M. Autore, P. Li, I. Dolado, F. J. Alfaro-Mozaz, R. Esteban, A. Atxabal, F. Casanova, L. E. Hueso, P. Alonso-González, J. Aizpurua, A. Y. Nikitin, S. Vélez, and R. Hillenbrand "Boron nitride nanoresonators for phonon-enhanced molecular vibrational spectroscopy at the strong coupling limit", Light: Science & Applications, 7(4), 17172, (2017).
[3] W. Ma, P. Alonso-González, S. Li, A. Y. Nikitin, J. Yuan, J. Martín-Sánchez, J. Taboada-Gutiérrez, I. Amenabar, P. Li, S. Vélez, C. Tollan, Z. Dai, Y. Zhang, S. Sriram, K. Kalantar-Zadeh, S.T. Lee, R. Hillenbrand, and Q. Bao, "In-plane anisotropic and ultra-low-loss polaritons in a natural van der Waals crystal", Nature, 562, 557–562, (2018).
[4] F.J. Alfaro-Mozaz, S.G. Rodrigo, P. Alonso-González, S. Vélez, I. Dolado, F. Casanova, L.E. Hueso, L. Martín-Moreno, R. Hillenbrand, and A. Y. Nikitin, “Deeply subwavelength phonon-polaritonic crystal made of a van der Waals material”, Nature Communications, 10, 1, 42 (2019).
All reported works along the project and summarised in the previous section are beyond the state of the art.
In the second part of the project we expect to develop first functional 2D nanodevices where an active control of the flow of energy at the nanoscale is achieved. This will allow us for studying fundamental light-matter interactions with a small amount of molecular resonators, which is essential for developing nano-chemistry experiments where chemical processes can be controlled optically.
We also envision a fundamental understanding on the propagation of polaritons in highly anisotropic media, which have great potential for a directional control of light and light-matter interactions at the nanoscale.
In-plane anisotropic propagation of highly confined polaritons