An exciton can be described as an electron-hole pair in a crystal that is bound in a manner analogous to the electron and proton in a hydrogen atom. Thus, the exciton behaves like an atomic excitation that passes from one atom to another. Exciton energy states involve a band with a particular dispersion law that depends on the crystal properties. In most cases excitons are induced by an electromagnetic wave, a photon.
Interestingly, in some cases the frequencies of vibration of the incoming electromagnetic wave and the resulting exciton are very similar, and they resonate. When they have a strong resonance interaction with each other, a quasi-particle results from the admixture of states of the photon and the exciton. This is called a polariton. In this case there is no longer a sharp difference between the excitons and the photons in the crystal; hence, the speed of light through the crystal depends on the frequency, a characteristic of polaritonic substances. Polaritons are quasiparticles resulting from the strong coupling of photons with a dipole-carrying excitation.
Photonic crystals – periodically structured media with unit cell sizes comparable to the wavelength of light – have been known for several decades, particularly in the visible spectral range. In contrast, van der Waals polaritonic crystals with unit cell sizes that are much smaller than the free-space wavelength have been emerging only in the last few years. They support polaritons, generally at mid-infrared frequencies. But, because the polariton wavelengths are smaller than that of light, conventional spectroscopy does not allow for a complete characterization.
Among the most studied polaritonic crystals are graphene superlattices, graphene on structured substrates, and Moirè patterns in twisted graphene bilayers, all of them hosting plasmon polaritons – the result from the coupling of surface plasmons – quantizations of the oscillations of the free electron gas density – with light. On the other hand, van der Waals polaritonic crystals, supporting long-lived phonon polaritons – result from the coupling of an infrared photon with an optic phonon, i.e., quantizations of mechanical vibrations – have received much less attention, although they may find various exciting applications, such as angle-independent infrared absorption (relevant for photodetectors) or strong coupling with tiny amounts of molecules. So far, only nonspectroscopic imaging by scattering-type scanning near-field optical microscopy has been applied to study phonon polariton crystals.
Now, a team of researchers has demonstrated 1 that hyperspectral infrared nanoimaging based on nanoscale Fourier transform infrared spectroscopy (nano-FTIR) can be used for analyzing the band structure of deeply subwavelength van der Waals polaritonic crystals.
As a proof of concept, a 2D array of infrared near-field spectra (hyperspectral images) of a square lattice of circular holes in an hexagonal boron nitride slab was recorded. The researchers found that flat regions of the bands manifest as peaks in the nano-FTIR spectra. This way, the researchers demonstrate that infrared spectra recorded at individual spatial positions within the unit cell of the van der Waals polaritonic crystal can be associated with its band structure and the local density of photonic states.
This work introduces hyperspectral infrared nanoimaging as a tool for the comprehensive analysis of polaritonic crystals, which could find applications in the reconstruction of complex polaritonic dispersion surfaces in momentum-frequency space or for exploring exotic electromagnetic modes in topological photonic structures.
Author: César Tomé López is a science writer and the editor of Mapping Ignorance
Disclaimer: Parts of this article may have been copied verbatim or almost verbatim from the referenced research paper/s.