Optical nanoresonators are crucial for various nanotechnological applications, particularly spectroscopy, due to their ability to confine light at the nanoscale. Recently, phonon polariton (PhP)-based nanoresonators in polar crystals like SiC and h-BN have gained significant attention because of their strong field confinement, high quality factors, and enhancement of the photonic density of states in the mid-infrared (mid-IR) frequency range. This mid-IR range is particularly important because it encompasses numerous molecular vibrations. However, creating tunable PhP nanoresonators remains a challenge. This research presents a novel class of mid-IR nanoresonators that addresses this challenge by introducing a new degree of freedom: twist tuning. The spectral response of these resonators can be controlled by rotating the constituent α-MoO3 van der Waals (vdW) crystal, which supports in-plane hyperbolic PhPs. A pristine α-MoO3 slab is placed on an array of metallic ribbons, a design that avoids degrading the crystal's optical properties during fabrication. The different refractive indices of PhP modes in the α-MoO3/air and α-MoO3/metal regions create resonances, and the orientation of the crystal axes relative to the metal grating acts as a tuning mechanism. This allows for broad spectral tuning (up to 32 cm⁻¹) simply by rotating the α-MoO3 slab. Both far-field (Fourier-transform infrared spectroscopy, FTIR) and near-field (scattering-type scanning near-field optical microscopy, s-SNOM) techniques were used to characterize the tunability of these nanoresonators. Theoretical analysis supports the experimental findings and helps to understand the observed resonant PhP modes.

The excitation of phonon polaritons (PhPs) in van der Waals (vdW) crystals has emerged as a promising method for manipulating infrared (IR) light at deeply subwavelength scales. Materials like α-MoO3 and α-V2O5, exhibiting long lifetimes and in-plane hyperbolic propagation of PhPs, are particularly attractive for creating low-loss optical nanoresonators. However, structuring vdW crystals for nanoresonator fabrication often degrades their optical properties and PhP lifetimes. Unlike easily tunable plasmonic resonances in 2D materials via external gates, tunability in PhP nanoresonators is more difficult. Previous research has explored polaritonic nanoresonators using h-BN and graphene layers, but the twist-tunable approach presented in this study offers a novel advancement in controlling their spectral response.

The fabrication process involved creating arrays of metallic ribbons (Au or Al) on a CaF2 substrate using electron beam lithography. For Al ribbons, a 70 nm thick Al layer was deposited, followed by photoresist application, electron beam lithography, development, reactive-ion etching, and photoresist removal. Au ribbons were fabricated using a similar process, with the deposition of 5 nm Cr and 50 nm Au. α-MoO3 flakes were mechanically exfoliated from bulk crystals and transferred onto the metal gratings using a dry transfer technique involving polydimethylsiloxane (PDMS) and polycarbonate (PC). The flakes were twisted using a micromanipulator to achieve the desired angle. Far-field optical characterization was performed using Fourier-transform infrared spectroscopy (FTIR) with a Varian 620-IR microscope coupled to a Varian 670-IR spectrometer and a MCT detector. Reflectance spectra were collected with a 2 cm⁻¹ spectral resolution. Near-field measurements were carried out using scattering-type scanning near-field optical microscopy (s-SNOM) with a Neaspec system and a quantum cascade laser. Metal-coated AFM tips were used, and the 3rd harmonic of the tip frequency was demodulated for background suppression. Full-wave numerical simulations using COMSOL software (finite-element method) were used to model the structure and analyze the far-field reflection and near-field electric field distributions. These simulations helped to corroborate experimental observations and understand the Fabry-Pérot resonances.

The fabricated α-MoO3 nanoresonators exhibited sharp resonance peaks within the Reststrahlen bands of α-MoO3. High quality factors (Q) up to 200 were observed. The experimental reflection spectra showed good agreement with full-wave simulations. Analysis of the simulations revealed that the resonances were Fabry-Pérot resonances (FPRs) arising from multiple reflections of PhP waveguide modes. The spectral position of the resonances was strongly dependent on the twist angle between the α-MoO3 slab and the metallic ribbons, with the main resonant peak shifting to lower frequencies with increasing twist angle. Near-field s-SNOM measurements confirmed the excitation of PhPs and showed fringe patterns consistent with the FPR model, with the number of oscillations matching the FPR order. The experimental data allowed for the reconstruction of the three-dimensional anisotropic dispersion surfaces of the PhPs. By varying the air gap width and twist angle, the experimental data points matched closely with the calculated isofrequency curves, confirming the Fabry-Pérot origin of the resonances.

The results demonstrate a novel approach for creating twist-tunable, low-loss mid-IR nanoresonators. The unique tunability, achieved without compromising the crystal quality, is a significant advancement in PhP-based nano-optics. The ability to reconstruct the PhP dispersion surfaces from near- and far-field measurements provides a powerful new tool for characterizing anisotropic materials. The high Q factors and tunability make these nanoresonators promising candidates for applications in molecular sensing, narrow-band filtering, and other mid-IR technologies.

This work successfully demonstrated twist-tunable Fabry-Pérot nanoresonators exhibiting high quality factors by utilizing a pristine α-MoO3 vdW crystal slab on a metal grating. The tunability achieved through simple rotation provides a new degree of freedom for controlling the spectral response of polaritonic devices. The ability to reconstruct the PhP dispersion surfaces offers an alternative characterization method for vdW materials. Future research could explore the integration of these nanoresonators into more complex devices and the expansion of this approach to other anisotropic vdW materials.

The study focused primarily on a specific geometry and material. Further research is needed to investigate the scalability and robustness of this approach across different geometries and materials. The analysis assumed specific values for the reflection phase of the PhP modes, which could be refined through more advanced simulations. The influence of fabrication imperfections on the resonator performance wasn't thoroughly analyzed.