Substrate-mediated hyperbolic phonon polaritons in MoO₃

Abstract

Hyperbolic phonon polaritons (HPhPs) are hybrid excitations of light and coherent lattice vibrations that exist in strongly optically anisotropic media, including two-dimensional materials (e.g., MoO3). These polaritons propagate through the material’s volume with long lifetimes, enabling novel mid-infrared nanophotonic applications by compressing light to sub-diffractional dimensions. Here, the dispersion relations and HPhP lifetimes (up to ≈12 ps) in single-crystalline α-MoO3 are determined by Fourier analysis of real-space, nanoscale-resolution polariton images obtained with the photothermal induced resonance (PTIR) technique. Measurements of MoO3 crystals deposited on periodic gratings show longer HPhPs propagation lengths and lifetimes (≈2×), and lower optical compressions, in suspended regions compared with regions in direct contact with the substrate. Additionally, PTIR data reveal MoO3 subsurface defects, which have a negligible effect on HPhP propagation, as well as polymeric contaminants localized under parts of the MoO3 crystals, which are derived from sample preparation. This work highlights the ability to engineer substrate-defined nanophotonic structures from layered anisotropic materials.

A guide to nanoscale IR spectroscopy: resonance enhanced transduction in contact and tapping mode AFM-IR

“…In addition to measuring vibrational (in the mid-IR) and electronic (in the visible) excitations, AFM-IR has been also employed to measure plasmonic 23,30,34 and polaritonic 38,39,43,54 modes. Some nano-optics applications, such as mapping surfaceenhanced IR absorption intensity near plasmonic nanostructures 23,30,34 and the characterization of dark plasmonic 23,30 and polaritonic 39,43 modes in metallic and 2D-material nanostructures, have been discussed in earlier reviews 4,5 (and references therein).…”

“…More recently, AFM-IR was used to characterize hyperbolic phonon polaritons (HPhPs), hybrid excitations of light and coherent lattice vibrations in polar crystals such as hBN 38 and MoO 3 . 54 These excitations can be generated within spectral bands delimited by pairs of longitudinal and transverse optical phonon modes and propagate through the material with large, quantized momenta and high optical compressions, l 0 /l HPhP E 100. In AFM-IR absorption images (e.g., Fig.…”

“…During each tapping cycle, frequency mixing between the photothermal excitation of the sample (at f pulse ) and the driven cantilever oscillations (e.g., at f 2 ) induce excitations in the cantilever at the neighboring resonance mode (e.g., f 1 ). Dazzi and coworkers derived an expression 19 for the demodulated (heterodyned) cantilever 54 This journal is The Royal Society of Chemistry 2022 oscillation amplitude (Z) in AFM-IR experiments, which is simplified here as:…”

“…We note that since the incident IR light can penetrate up to a few μm in some samples, regions of high signal contrast evident in AFM-IR absorption images may not necessarily correlate with features evident in the surface topography since they can result from absorption by materials located within 9 or beneath the sample. 54 Changes in the contact resonance frequency can be associated either with changes in the inherent stiffness of the top (≈ 25 nm) sample layer, in the case of heterogeneous samples, or less commonly to softening of the sample caused by heating, as mentioned above. When properly configured, the PLL should track these changes, automatically adjusting the laser pulse rate to match the cantilever resonance as the probe is scanned across the sample.…”

Ultrabroadband Nanocavity of Hyperbolic Phonon–Polaritons in 1D-Like α-MoO3

“…Their photonic properties is, typically, described in terms of the corresponding type I or type II ω– q dispersion relations . In the case of 2D crystals of α-MoO 3 , strong phonons enable HP 2 waves ,,,, with long lifetimes (up to ∼12 ps) in an ultrabroadband frequency range spanning from 250 cm –1 in the THz domain to 1200 cm –1 in the mid-IR.…”