The researchers observed directional THz waves that were tightly wedged in a thin semiconductor crystal

May 04, 2023

(Nanowerk News) An international team of scientists from the Basque research center CIC nanoGUNE, Shanghai University of Science and Technology, Fudan University (Shanghai), Brno University of Technology, University of the Basque Country, Center for Materials Physics (CSIC-UPV/EHU), Donostia International Physics Center and Max Planck Institute for Solid Chemical Physics (Dresden) has imaged and analyzed THz waves propagating in the form of polariton plasmons along thin anisotropic semiconductor platelets with a wavelength reduced to 65 times compared to THz waves in free space. What’s more interesting is that the wavelength varies with the direction of propagation. Such THz waves can be applied to investigate the fundamental properties of materials at the nanometer scale and pave the way for the development of ultra-compact on-chip THz devices.

This work has been published in Natural Ingredients (“Real space observations of an in-plane ultra-constrained acoustic anisotropic plasmon polariton”). Artistic illustration of an in-plane acoustic elliptical THz plasmon polariton propagating along thin Ag2Platelets above the Au mirror, excited and examined by a THz-irradiated sharp metal tip. (Image: CIC nanoGUNE).

Polariton is a hybrid state between light and matter that arises from the fusion of light with excitation of matter. Plasmons and polariton phonons are one of the most prominent examples, formed by the coupling of light-to-collective electron oscillations and crystal lattice vibrations, respectively. They play an important role in a wide range of applications, from sub-diffraction optical spectroscopy and ultrasensitive chemical sensors to ultracompact modulators for communications applications. In thin layers, polariton can propagate with wavelengths up to 100 times shorter than the wavelength of the corresponding photon, enabling manipulation of light on a much smaller scale than was previously possible with conventional photonic devices.

While most of these ultra-restricted polaritons have been observed to be in the form of phonon polaritons in the mid-infrared spectral range, the researchers focused on plasmon polarritons, as these can exist in a much wider spectral range.

“On the other hand, polariton plasmons often experience large attenuation, resulting in short propagation lengths. This has challenged the observation of ultra-enclosed plasmon polariton in real space,” said Shu Chen, first author of the publication.

Using a THz nanoscope (more precisely, a THz scattering-type near-field optical microscope, s-SNOM) in Rainer Hillenbrand’s lab at CIC nanoGUNE (San Sebastian, Spain), Chen studied thin platelets from low-symmetry crystalline silver telluride (Ag2Te; Hessite) and obtained the first real-space image of a THz plasmon polariton, whose wavelength decreases by up to 65 times compared to the wavelength of a photon and varies with the direction of propagation.

“Silver telluride is a narrow bandgap semiconductor with a relatively high concentration of moving electrons, which makes the material plasmonic at THz frequencies”, said Pengliang Leng, the co-contributing first author, who made platelets in Faxian Xiu’s lab at Fudan University (Shanghai). , China). “Due to the low symmetry monoclinic crystal structure, the effective electron mass is highly anisotropic along the platelet surface, which explains the anisotropic polariton plasmon propagation,” added Faxian Xiu.

The researchers also demonstrated that the relative propagation length of the THz polariton can be significantly increased by combining it with its mirror image on an adjacent metal substrate. “Due to this fusion, the so-called acoustic plasmon polaritons are formed”, explains Andrea Konečná from the University of Brno (Czech Republic), who theoretically modeled the acoustic polaritons.

“Most importantly, the propagation anisotropy of the polariton was qualitatively preserved, and the long relative propagation length allowed us to clearly verify that the polariton propagates with an elliptic wavefront,” adds Rainer Hillenbrand of nanoGUNE, who led the work.

The relatively long propagation length of the elliptical acoustic plasmon polariton has finally allowed researchers to determine the anisotropic in-plane effective electron mass, establishing a unique method for the nanoscale measurement of the directional effective carrier mass at room temperature.

In addition to exploring fundamental material properties in conventional and novel quantum materials, in-plane ultra-constrained anisotropic acoustic plasmon polaritons could lead to ultra-compact on-chip THz applications. The strong field concentration in the gap between the polaritonic layer and the metal surface can be exploited for field-enhanced molecular sensing or to enhance the coupling of (ultra)strong THz light matter to molecules, classical 2D electron gas, or quantum materials.

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