Low-dimensional materials form an emerging platform for exotic light-matter interactions, ideally suited for various photonic technologies. Light can strongly engage matter in these materials to create quasiparticles. These quasiparticles are known as polaritons, supporting deep-subwavelength optical fields with broadband responses. Van der Waals (vdW) crystals can feature these polaritons in the mid-infrared frequency range. Anisotropic vdW crystals are fascinating for nanophotonics because the material has different atomic interactions at the bonding level.
In a new paper published in eLight, a team of scientists led by Professor Andrea Alù of the City University of New York (CUNY) demonstrated the possibility of generating and controlling mid-infrared hyperbolic polariton vortices at the nanoscale. Their paper, “Spin-orbit-locked hyperbolic polariton vortices carrying reconfigurable topological charges,” sought to assess the complex interplay between excitation spin, spiral geometry, and hyperbolic phonon polariton (HP2) dispersion.
Traditional approaches to hyperbolic phenomena in metamaterials have suffered from various imperfections in fabrication. These failures have inevitably hindered their widespread use in commercial applications. However, the natural hyperbolic polaritons in polar vdW crystals can overcome these challenges.
The research team used hexagonal boron nitride (hBN) as a host for HP2. This natural hyperbolic material has been broadly used to explore polaritonic physics. It is due to their low loss optical phonons, tunable polariton dispersion, and mature preparation and nanostructure fabrication processes. In turn, this can facilitate a variety of polaritonic applications.
Their study of spin-orbit interactions revealed unprecedented opportunities for the control of HP2. They also found ways to precisely generate nanoscale vortices with associated controlled topological charges locked to the excitation spin. Their experiment revealed new degrees of freedom and enhanced robustness in HP2 control based on optical spin-orbit interactions and topological charges. These features are ideal for super-resolution sensing and imaging, enhanced light-matter interactions, communications and multiplexing, and particle manipulation at mid-IR frequencies.
The research team demonstrated infrared nanoscale HP2 vortices (HP2Vs) induced in pristine hBN thin flakes. They achieved this demonstration by creating a gold (Au) disk with an Archimedean spiral shape to launch HP2 in hBN. Their experiment showed that they could create broadly reconfigurable topological charges and showcase polaritonic features. The demonstrated polariton vortices were highly tunable. The research could adjust the excitation spin, the polariton launcher geometry, and other hyperbolic features.
The outcome of their research opens unique opportunities to process multiple data streams at mid-IR wavelengths, with enormous potential for super-resolution imaging systems, ultracompact mid-IR sensors, and ultracompact polaritonic devices.
The findings broadly enrich the nanophotonic and polaritonic platforms enabled by vdW nanomaterials. It provides significant opportunities of great interest for various classical and quantum applications.
Light Science & Applications