Twist-tunable polaritonic nanoresonators in a van der Waals crystal

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 α-MoO3, and realize a dielectric engineering of a pristine α-MoO3 slab placed on top of metal ribbon grating, which preserves the high quality of the polaritonic resonances. By simple rotating the α-MoO3 slab in the plane (from 0 to 45º), 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 to ~ 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