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Study of phonon-polaritons (PhPs) in hexagonal boron nitride (hBN) is an important application of the s-SNOM technique. Previously, the research team of Prof. Kourosh Kalantar-Zadeh used a Bruker nanoIR3-s Broadband system to study the nano-FTIR spectroscopy and s-SNOM imaging of PhPs in layered hBN crystals on SiO2/Si wafers (see article here). With the high-power broadband IR laser source on their nanoIR3-s instrument, the authors were able to resolve multiple groups of auxiliary peaks in the nano-FTIR spectra that had not been observed before. In s-SNOM imaging over a variety of different boundary conditions, diﬀerences between the near-ﬁeld auxiliary signatures when the hBN layers were supported or suspended were observed and identified.
In this new work, further measurements were performed on the hBN-based phonon-polaritonic crystal (PPC). The hBN sheets were incorporated into silicon-based substrates with pre-patterned rectangular hole arrays to form a periodically suspended hBN structure. With the correct choice of structural parameters of the hole array, the photonic crystal modes are close to the upper Reststrahlen band of hBN and that PhP waves on SiO2-supported hBN interfere. Under such conditions, the hBN-based PPCs could manifest a range of Archimedean-like tiling patterns. Specifically, two types of repetitive local field distribution patterns were resolved, i.e., dipolar-like field distributions and the more important highly dispersive PhP interference patterns. The distribution patterns are consistent with the results from theoretical studies.
Furthermore, the tunability of PPC band structures by varying the thickness of hBN flakes was demonstrated. s-SNOM amplitude images of PPCs with three different hBN flake thicknesses (20.3, 34.9 and 166.6 nm) were measured, and they all showed Archimedean-like tiling patterns. However, higher frequencies were required to obtain the same pattern for thicker hBN flakes, indicating that the band structure of PPCs is dependent on the thickness of hyperbolic materials. This trend was explained by numerical simulations, which showed that the thickness of hBN, as well as the structural parameters of the rectangular hole array and the thickness of SiO2, have a strong impact on the band structure of PPCs. The imaging pattern was further supported by results from nano-FTIR spectroscopy measurements using the tunable broadband laser source.
Archimedean-like tiling, Hexagonal boron nitride, nano-FTIR, Phonon-polaritonic crystal, s-SNOM, Scattering-type nanoIR