Twist-tunable Polaritonic Nanoresonators in a van der Waals Crystal

TL;DR

This paper introduces twist-tunable phonon polaritonic nanoresonators in van der Waals crystals, enabling spectral control of IR resonances through simple rotation, with high quality factors and broad tunability for nanophotonic applications.

Contribution

It demonstrates a novel twist-tuning mechanism for phonon polaritonic nanoresonators in $ ext{MoO}_3$, combining low-loss hyperbolic propagation with dielectric engineering for spectral control.

Findings

Achieved spectral tuning of polaritonic resonances up to 32 cm$^{-1}$.

Demonstrated high-quality factors up to 200.

Enabled broad tunability through simple in-plane rotation.

Abstract

Optical nanoresonators are fundamental building blocks in a number of nanotechnology applications (e.g. in spectroscopy) due to their ability to efficiently confine light at the nanoscale. Recently, nanoresonators based on the excitation of phonon polaritons (PhPs) $-$ light coupled to lattice vibrations $-$ in polar crystals (e.g. SiC, or h-BN) have attracted much attention due to their strong field confinement, high-quality factors, and potential to enhance the photonic density of states at mid-infrared (IR) frequencies. Here, we go one step further by introducing PhPs nanoresonators that not only exhibit these extraordinary properties but also incorporate a new degree of freedom $-$ twist tuning, i.e. the possibility to be spectrally controlled by a simple rotation. To that end, we both take advantage of the low-loss in-plane hyperbolic propagation of PhPs in the van der Waals crystal $\alpha$-MoO$_3$, and realize dielectric engineering of a pristine $\alpha$-MoO$_3$ slab placed on top of metal ribbon grating, which preserves the high-quality of the polaritonic resonances. By simple rotating the $\alpha$-MoO$_3$ slab in the plane (from 0 to 45$^{\circ}$), we demonstrate via far- and near-field measurements that the narrow polaritonic resonances (with quality factors Q up to 200) can be tuned in a broad range (up to 32 cm$^{-1}$, i.e up 6 ~ times its full width at half maximum, FWHM ~ 5 cm$^{-1}$). Our results open the door to the development of tunable low-loss nanotechnologies at IR frequencies with application in sensing, emission or photodetection.