Fig. 4: PbI2 as an HPhP cavity on different substrates and in comparison to other phonon-polaritonic media.
From: High quality-factor terahertz phonon-polaritons in layered lead iodide
a, b AFM and near-field amplitude images of a 492 nm-thick PbI2 crystal on a 130 nm-thick PbI2 crystal on Si measured at 2.52 THz. The near-field image reveals a high-contrast standing-wave pattern of HPhPs within the triangular crystal. Scale bar: 10 μm. c, d AFM and s-SNOM amplitude images of a 370 nm-thick triangular PbI2 flake on Au, highlighting an interference pattern formed by HPhP waves launched by both the tip and crystal edges. Scale bar: 5 μm. e Experimental near-field amplitude profile (open circles) extracted along the white dashed line in (d), fitted by a phenomenological function (red line) that accounts for HPhPs launched by the edge (orange line) and the tip (blue line). f Expected polariton wavelengths as a function of PbI2 thickness when transferred onto an Au substrate for 2.52 THz illumination. The experimental data point (open circle) is obtained from the analysis in (e) (Supplementary Information, Section 3.4). g PbI2 in the context of other selected 2D materials supporting HPhPs across the mid-IR, far-IR, and THz ranges, organized by their key parameters FOM and Lyddane–Sachs–Teller (LST) relation. νTO represents the frequency of the transverse optical (TO) phonon mode. ε0 and ε∞ represent PbI2 dielectric constants at the low and high frequency limits, respectively.
