Active and Passive Tuning of ultra-narrow Resonances in Polaritonic Nanoantennas

J. Duan, F. J. Alfaro-Mozaz, J. Taboada-Gutiérrez, I. Dolado, G. Álvarez-Pérez, Elena Titova, A. Bylinkin, A. I. F. Tresguerres-Mata, J. Martín-Sánchez, S. Liu, J. H. Edgar, D. A. Bandurin, P. Jarillo-Herrero, R. Hillenbrand, A. Y. Nikitin, P. Alonso-González

Advanced Materials, Early View (2021)
Optical nanoantennas are of great importance for photonic devices and spectroscopy due to their capability to squeeze light at the nanoscale and enhance light-matter interactions. Among them, nanoantennas made of polar crystals supporting phonon polaritons (phononic nanoantennas) exhibit the highest quality factors. This is due to the low optical losses inherent in these materials, which, however, hinder the spectral tuning of the nanoantennas due to their dielectric nature. Here we realize active and passive tuning of ultra-narrow resonances in phononic nanoantennas over a wide spectral range (∼35 cm–1, being the resonance linewidth ∼9 cm–1), monitored by near-field nanoscopy. To do that, we modify the local environment of a single nanoantenna made of hexagonal boron nitride (h-BN) by placing it on different polar substrates, such as quartz (SiO2) and silicon carbide (4H-SiC), or covering it with layers of a high refractive-index van der Waals crystal (WSe2). Importantly, we demonstrate active tuning of the nanoantenna polaritonic resonances by placing it on top of a gated graphene monolayer in which we vary the Fermi energy. Our work presents the realization of tunable polaritonic nanoantennas with ultra-narrow resonances, which could find applications in active nanooptics and (bio)-sensing.