Fig. 3: THz nano-TDS analysis.
From: High quality-factor terahertz phonon-polaritons in layered lead iodide
a Schematic of the THz s-SNOM TDS experimental setup that employs a fs near-infrared (NIR) source to pump a transmitter (TX) and a receiver (RX) whose signal is digitized by an analog-to-digital converter (ADC). b Near-field complex spectra (2nd harmonic) measured at the center of a PbI2 flake featuring an absorption peak consistent with the Eu(TO) mode. c s-SNOM TDS spectral-linescan across the PbI2 edge (see inset scheme) displaying spatial-spectral amplitude modulations for a 298 nm-thick flake. Two amplitude maxima are recorded as the tip moves away from the edge (position 0 μm), showing a dispersive trend. White dashed lines guide the eyes for the visualization of the amplitude dispersion and the νEu(TO) frequency. d Amplitude profiles extracted from (c) at four different frequencies and fitted to a parameters-retrieval model (Supplementary Information, Section 3.4). e Dispersion diagram of the HPhPs (color map: imaginary part of the Fresnel reflection coefficient rp, white dots: experimental points). f PbI2 HPhPs figure-of-merit (FOM) versus frequency (solid and dashed lines: theoretical FOM; circles and star: extracted from experimental data). Experimental uncertainty bars were calculated via standard error propagation in the fitting procedure described in the Supplementary Information, Section 3.4. Γ is the damping parameter that represents the dielectric losses in the dielectric function (see Supplementary Information, Eq. S4).
