In recent years, the investigation of phonon polaritons (PhPs) – pseudo-particles formed by the coupling of photons and phonons in polar materials – has become increasingly important due to their potential applications in nano-optics; i.e, PhPs possess long lifetimes and the ability to confine light in small volumes. Recently, strongly anisotropic PhPs were discovered in polar van der Waals (vdW) materials such as α-MoO₃ or α-V₂O₅ in specific spectral bands, these materials behave optically like metals along one direction of the crystal and, as conventional dielectrics along the others. This allows for in-plane hyperbolic propagation of PhPs, which provides great control over their direction of propagation and a high density of optical states. In this trend, the design of nanoresonators based on PhPs has opened new possibilities for achieving stronger field confinement, higher quality factors, and the potential to enhance the photonic density of states at mid-infrared frequencies, where numerous molecular vibrations reside. Nanoresonators are crucial elements in various nanotechnological applications due to their ability to confine light at the nanoscale and present diverse applications ranging from spectroscopy to molecular sensing, emission, or photodetection. In this work, we combine the recently discovered in-plane hyperbolic PhPs with phononic nanoresonators to introduce a new class of mid-IR nanoresonators based on hyperbolic PhPs, that incorporates, in addition to all the properties mentioned, a new degree of freedom: twist-tuning.

The sample design is based on a pristine slab of the vdW crystal α-MoO₃ - which supports in-plane hyperbolic PhPs - on top of an electromagnetic-engineered substrate made of patterned Au ribbons. Such a design avoids degradation of the optical properties of the material due to a direct fabrication and creates nanoresonators by defining different out-of-plane interfaces α-MoO₃/air and α-MoO₃/Au. Thus, in-plane propagating PhP modes can bounce back and forth between the air/Au boundaries forming Fabry-Pérot resonances. Furthermore, the in-plane anisotropic propagation of PhPs in α-MoO₃ implies that the polariton supports a different wavelength for each in-plane direction, meaning that using different orientations of the crystal axes with respect to the metal grating allows us to tune the resonances by simply twisting the same flake. This way to tune the properties of the nanoresonators and control the spectral response by simply rotating the same piece of material is a significant breakthrough that provides new opportunities to achieve more efficient and precise control of the confinement of light at the nanoscale and permits the design of low-loss and tunable IR nanotechnologies, which are fundamental requirements for their implementation in molecular sensing, emission, or photodetection applications. 

The far-field resonances are measured by Fourier-transform IR spectroscopy (FTIR) with the incident illumination polarized across the ribbons. Due to the mode’s refractive index steps defined by electromagnetic engineering of the substrate, Fabry-Pérot resonances arise from the multiple reflections at the air/Au boundaries. A theoretical mode analysis is performed to disentangle the peaks observed and confirmed by near-field imaging with scattering-type scanning near-field optical microscopy (s-SNOM). We observe the excited propagating PhP modes, which are dipolar-like at the region α-MoO₃/air, and high-order-like at the region α-MoO₃/Au. We use the s-SNOM tip to scan the α-MoO₃ slab and record the electromagnetic signal as a function of the tip position, composing the near-field images (Fig.1).

Fig.1 | Near-field measurements of twist-tunable PhP nanoresonators. a, Schematic of the near-field measurements by s-SNOM, in which a metal tip is illuminated by p-polarized mid-IR light, and both the amplitude and phase of the tip-scattered field are recorded as a function of the tip position. b, Simulated polariton field distribution, Re(E_z), and experimental near-field images, Re(s₃)  in both the α-MoO₃/air and α-MoO₃/metal regions.

The main resonant peak in the far-field spectra corresponds to the dipolar mode at the interface α-MoO₃/air as confirmed by the near-field imaging and the theoretical mode analysis. After the analysis, we checked that, as expected, by twisting the same slab of α-MoO₃ the main resonant peak shifts to lower frequencies with increasing the angle between the crystallographic axis of the material and the ribbons axis, due to hyperbolic propagation of PhPs at this spectral range. This is confirmed with a theoretical analysis based on full-wave numerical simulations of the fabricated structure that shows a good agreement with the experimental results (Fig.2). Moreover, the resonances coincide with the anisotropic Fabry-Pérot condition.

Fig.2 | Analysis of the PhP resonances and their twist tuning. a, Measured relative reflection spectra δR as a function of frequency, ω for different twist angles φ=0,15,30 and 45º of the same α-MoO₃ slab (red, blue, black, and green curves, respectively). b, Simulated δR as a function of frequency, ω and twist angle, φ.

In conclusion, we demonstrate a broad spectral tuning of the polaritonic resonances up to 32cm-¹, which is around 6 times their full width at half maximum (FWHM) of approximately 5cm-¹, by an in-plane rotation of the same α-MoO₃ slab from 0 to 45º. Thanks to the proposed design we maintain the low-loss nature of our nanoresonators with quality factors (Q) up to 200. The ability to control the spectral response of the nanoresonators by simply rotating the constituent material opens new possibilities for the design of low-loss, tunable IR nanotechnologies, which allows for the precise control of electromagnetic fields at the nanoscale, enabling the creation of nanoresonators with extraordinary properties.