Quasi‐Single‐Mode Polaritonic Crystal for Hyperbolic Phonon‐Polaritons

ABSTRACT Polaritonic crystals for hyperbolic phonon‐polaritons (PhP) in van der Waals materials are burdened by the intermode scattering due to the inherently multimodal nature of PhP. So far, polaritonic crystals have been demonstrated within the Type‐II Reststrahlen band (RB‐II) where the in‐plane components of the dielectric permittivity tensor are negative, while the out‐of‐plane component is positive. By contrast, a Type‐I Reststrahlen band (RB‐I) is characterized by the negative out‐of‐plane and positive in‐plane permittivity, and consequently, the inversion of PhP field symmetry, negative phase velocity, and anomalous dispersion. Such an operation regime remains unexplored for periodic structures supporting collective polaritonic modes. Here, we employ a biaxial crystal α‐MoO 3 and near‐field imaging to study mid‐infrared polaritonic Bloch modes in a 1‐D Fourier crystal within the RB‐I. Surprisingly, we observe a manifestation of Bloch waves as a dispersionless near‐field pattern across the first Brillouin zone, in contrast to previous works exclusively focused on the RB‐II. Our results demonstrate the possibility to realize a quasi‐single‐mode operation regime of polaritonic crystal for the inherently multimode hyperbolic PhP, and reveal the importance of field symmetry for polaritonic crystals in general and for the emerging field of Fourier crystals in particular, promising new ways to manipulate the nanolight.