Researchers from different countries have observed and analyzed THz waves that travel down thin anisotropic semiconductor platelets as plasmon polaritons, with wavelengths up to 65 times shorter than THz waves in free space.
Even more remarkable is the fact that the wavelengths change depending on the propagation direction. Such THz waves open the door to the creation of incredibly small on-chip THz devices and can be used to probe fundamental material properties at the nanoscale scale. The project was released in Nature Materials.
When light and matter excitations are coupled, polaritons, which are hybrid states of light and matter, are created. Among the most well-known examples are phonon and phonon polaritons, which are produced when light is coupled to crystal lattice vibrations and collective electron oscillations, respectively.
They are essential in a variety of applications, including ultrasensitive chemical sensors, sub-diffraction optical spectroscopy, and ultracompact modulators for communications. It is now possible to manipulate light on a considerably smaller scale than with conventional photonic devices because thin layers allow polaritons to propagate with wavelengths up to 100 times shorter than the corresponding photon wavelength.
The researchers concentrated on plasmon polaritons because they can exist in far wider spectral ranges than phonon polaritons, which are how most of these ultra-confined polaritons have been seen in the mid-infrared spectral region.
Most important, the anisotropy of the polariton propagation is qualitatively preserved, and the long relative propagation lengths allowed us to unambiguously verify that the polaritons propagate with elliptical wavefronts.
Rainer Hillenbrand
“On the other hand, plasmon polaritons often suffer from large damping, resulting in short propagation lengths. This has been challenging the observation of ultra-confined plasmon polaritons in real space,” says Shu Chen, first author of the publication.
Using a THz nanoscope (more precisely, a THz scattering-type scanning near-field optical microscope, s-SNOM) in Rainer Hillenbrand´s lab at CIC nanoGUNE (San Sebastian, Spain), Chen studied thin platelets of the low-symmetry crystal silver telluride (Ag2Te; hessite) and obtained the first real-space images of THz plasmon polaritons, whose wavelengths are up to 65 times reduced compared to the photon wavelength and vary with the propagation direction.
“Silver telluride is a narrow bandgap semiconductor with a relatively high mobile electron concentration, which makes this material plasmonic at THz frequencies,” says Pengliang Leng, equally contributing first author, who fabricated the platelets in Faxian Xiu´s lab at Fudan University (Shanghai, China).
“Because of the low-symmetry monoclinic crystal structure, the effective electron mass is strongly anisotropic along the platelet surface, which explains the anisotropic plasmon polariton propagation,” adds Faxian Xiu.
The researchers also showed that connecting the THz polaritons with their mirror counterpart in a nearby metal substrate can dramatically lengthen their respective propagation lengths.
“Because of this coupling, so-called acoustic plasmon polaritons are formed,” explains Andrea Konečná from Brno University (Czech Republic), who theoretically modeled the acoustic polaritons.
“Most important, the anisotropy of the polariton propagation is qualitatively preserved, and the long relative propagation lengths allowed us to unambiguously verify that the polaritons propagate with elliptical wavefronts,” adds Rainer Hillenbrand from nanoGUNE, who led the work.
Finally, the researchers were able to calculate the in-plane anisotropic effective electron mass, creating a special technique for the nanoscale measurement of directional effective carrier masses at room temperature. This was made possible by the lengthy relative propagation lengths of the elliptical acoustic plasmon polaritons.
Ultra-confined in-plane anisotropic acoustic plasmon polaritons may result in ultra-compact on-chip THz applications in addition to examining fundamental material features in traditional and innovative quantum materials.
The strong field concentration in the space between the polaritonic layer and metal surface can be used to enhance THz light-matter coupling with molecules, conventional 2D electron gases, or quantum materials, or to enhance field-enhanced molecular sensing.
