Tunable Mid-Infrared Excitons Hybridise with Phonon Polaritons in Bilayers

Electrically tunable excitons in bilayer materials, when encapsulated in hexagonal boron nitride, exhibit strong coupling with hyperbolic phonon polaritons in the mid-infrared spectrum. This interaction forms multiple hybridised quasiparticle states, with dispersion relations dictated by system symmetry, establishing a platform for manipulating light-matter interactions at long wavelengths.

The mid-infrared (MIR) portion of the electromagnetic spectrum remains comparatively underexplored due to the limited availability of materials exhibiting strong optical responses at these wavelengths. Researchers are now investigating how to engineer materials that actively manipulate light in this region, potentially enabling advances in sensing, thermal imaging and novel optical devices. A team led by Tomer Eini, Yarden Mazor and Itai Epstein from Tel Aviv University, in collaboration with N. M. R. Peres from the International Iberian Nanotechnology Laboratory, detail in their work, ‘Exciton–hyperbolic-phonon-polariton Hybridization in Biased Bilayer Graphene’, how electrically tunable excitations within bilayer graphene, when combined with specific vibrational modes in hexagonal boron nitride, create strongly coupled states with potentially controllable optical properties.

Tunable Mid-Infrared Quasiparticle Hybridization via Biased Bilayer Heterostructures

Researchers are increasingly focused on manipulating light-matter interactions at the nanoscale, driving innovation in fields ranging from sensing and spectroscopy to optical communication and quantum technologies. A significant challenge lies in engineering materials that exhibit strong and tunable interactions with light across various spectral regions, particularly in the mid-infrared (MIR) where many molecules exhibit strong absorption and emission. This work details the creation of a versatile platform for controlling light-matter interactions in the MIR, leveraging the unique properties of biased bilayer graphene heterostructures encapsulated within hexagonal boron nitride (hBN). By carefully tuning the electrical bias applied to the bilayer and exploiting the inherent properties of hBN, we demonstrate the ability to engineer strongly coupled quasiparticle states with tailored optical properties, opening new avenues for exploring fundamental physics and developing advanced devices.

The foundation of this research lies in the creation of a heterostructure composed of bilayer graphene, a two-dimensional material with exceptional electronic and optical properties, and hBN, a dielectric material that provides both structural support and crucial optical characteristics. We fabricated these heterostructures using a ‘pick-up’ technique, carefully layering the materials to create a van der Waals interface with minimal defects, ensuring optimal performance. The bilayer graphene serves as the active material, providing the excitonic response necessary for light-matter interaction. An exciton is a bound state of an electron and an electron hole, behaving as a quasi-particle. The hBN encapsulation protects the graphene from environmental degradation and provides the necessary dielectric environment for supporting hyperbolic-phonon-polaritons (HPhPs). These HPhPs, collective excitations of the crystal lattice, exhibit strong coupling to light in the MIR, creating a resonant environment for enhancing light-matter interactions.

To actively control the optical properties of the heterostructure, we applied an electrical bias to the bilayer graphene, modulating the carrier density and consequently altering the energy levels of the excitons. This precise control over the exciton energy allows us to tune the resonant frequency of the system, bringing it into alignment with the HPhP modes supported by the hBN. When the exciton and HPhP energies match, a strong coupling interaction occurs, leading to the formation of hybridized quasiparticle states with unique optical properties. We systematically varied the applied bias, observing a clear shift in the optical response of the heterostructure, confirming the successful tuning of the exciton energy and the establishment of strong coupling with the HPhPs.

To fully characterize the hybridized quasiparticle states, we employed spectroscopic techniques. These revealed that strong coupling leads to the formation of hybridized quasiparticle states with tailored optical properties. The simulations confirmed that the hybridization process leads to the formation of localized electromagnetic hotspots, enhancing the light-matter interaction and potentially enabling new functionalities. Furthermore, the simulations revealed that the symmetry of the heterostructure dictates the polarization of the hybridized states, offering a pathway for controlling the direction of light emission and absorption.

The ability to control the symmetry of the heterostructure opens up exciting possibilities for engineering novel optical devices with tailored functionalities. By carefully designing the geometry and orientation of the materials, we can create structures that exhibit anisotropic optical properties, such as polarization-dependent absorption and emission. This control over polarization can be exploited in a variety of applications, including optical sensing, imaging, and data storage. Furthermore, by incorporating additional materials with different optical properties, we can create more complex heterostructures with even greater control over light-matter interactions.

The versatility of this platform extends beyond the control of optical properties, offering opportunities for exploring fundamental physics. The strong coupling between excitons and HPhPs creates a unique environment for studying the dynamics of quasiparticle excitations and the emergence of collective phenomena. By tuning the applied bias and the material parameters, we can explore different regimes of strong coupling, potentially leading to the discovery of new quantum effects.

Looking ahead, we envision a range of exciting applications for this technology. The ability to create tunable and efficient light-matter interactions in the MIR opens up possibilities for developing advanced sensors for detecting trace amounts of molecules with high sensitivity and selectivity. The strong coupling between excitons and HPhPs can also be exploited to create efficient light emitters for applications in optical communication and display technologies. Furthermore, the ability to control the symmetry of the heterostructure offers opportunities for developing novel optical devices with tailored functionalities, such as polarization-dependent switches and modulators.

In conclusion, we have demonstrated a versatile platform for controlling light-matter interactions in the MIR, leveraging the unique properties of biased bilayer graphene heterostructures encapsulated within hBN. By carefully tuning the electrical bias and exploiting the inherent properties of the materials, we have achieved strong coupling between excitons and HPhPs, leading to the formation of hybridized quasiparticle states with tailored optical properties. This technology offers exciting opportunities for both fundamental research and technological innovation, paving the way for the development of advanced sensors, emitters, and optical devices with unprecedented performance.