Highly Tunable Phonon Polaritons via Metal Intercalation

TL;DR

This work demonstrates that zerovalent metal intercalation into alpha-MoO3 provides a practical, non-mechanical route to reconfigure phonon polaritons in van der Waals crystals. Tin intercalation yields a substantial dispersion shift (up to 38.5% at 910 cm^-1) while preserving PhP lifetimes, and silver intercalation can suppress PhP signals above ~880 cm^-1; these effects are attributed to increases in static dielectric permittivity as shown by DFT and captured in an analytical dispersion model. The combination of PiFM measurements, FDTD simulations, and first-principles calculations reveals a consistent mechanism linking intercalation-induced permittivity changes to PhP dispersion, enabling tunable mid-infrared nanophotonic devices. This approach paves the way for programmable planar optics and scalable infrared nanophotonics in layered oxide materials.

Abstract

Phonon polaritons in van der Waals crystals offer mid-infrared light confinement deep below the diffraction limit, making them promising for nanophotonics applications. However, the practical use of phonon polaritons remains limited, in part due to the lack of precise control over the phonon polariton dispersion, as crystal lattice vibrations are often inert to external stimuli. Here, we address this challenge by zerovalent metal intercalation of $α$-MoO$_3$. Photo-induced force microscopy shows that introducing tin into the van der Waals gap modulates the phonon polariton dispersion by up to $38.5\pm0.5\%$, which is the highest amount of tunability among non-mechanical modulation approaches, to the best of our knowledge. Intercalation with various metal species preserves the phonon polariton lifetimes, while modulating the dielectric permittivity in agreement with the density functional theory and analytical calculations. Our results establish metal intercalation as a practical route to reconfigurable mid-infrared nanophotonics.

Figures (9)

• Figure 1: Intercalation tuning of phonon polaritons (PhPs) in $\alpha$-MoO$_3$. a, Schematic of tin intercalation of $\alpha$-MoO$_3$ and PiFM measurements of PhP propagation. b, Topography and PiFM images of a 120-nm-thick pristine $\alpha$-MoO$_3$ at $\omega$=885 cm$^{-1}$ excitation. c, Topography and PiFM images of a 120-nm-thick Sn-MoO$_3$ at $\omega$=885 cm$^{-1}$ excitation. Scale bars are 5 $\mu$m. d, PiFM intensity profiles extracted from b and c. $\lambda_{\mathrm {PhP}}$ is the PhP wavelength. e, Shift in PhP dispersion after tin intercalation of $\alpha$-MoO$_3$. f, PhP group velocities extracted from the fitting results in e. g, Comparison of the PhPs lifetime in pristine (black) and Sn-MoO$_3$ (red).
• Figure 1: EDX spectra of 1 atm% Sn-MoO$_3$ (red) and 115-nm-thick 0.2 atm% Sn-MoO$_3$ (blue).
• Figure 2: Concentration-dependent tuning of PhP in Sn-MoO$_3$. a, Topography and PiFM images at 885 cm$^{-1}$ excitation of 115 nm-thick 0.2 atm% Sn-MoO$_3$. Scale bars are 5 $\mu$m. b-e, Comparison of PhP dispersion (b), group velocity (c), propagation length (d) and lifetime (e) of 120-nm-thick 1 atm% Sn-MoO$_3$ (red) and 115-nm-thick 0.2 atm% Sn-MoO$_3$ (blue).
• Figure 2: a, PhP dispersion, b, group velocity, c, propagation length, d, lifetime of 120-nm-thick $\alpha$-MoO$_3$ (black), 1 atm% Sn-MoO$_3$ (red) and 115-nm-thick 0.2 atm% Sn-MoO$_3$ (blue).
• Figure 3: Analysis of intercalation species effect on PhP. a, Topography and PiFM images at 860 cm$^{-1}$ excitation of 55 nm-thick $\alpha$-MoO$_3$. b, FDTD simulated out-of-plane electric field distribution at 860 cm$^{-1}$ frequency of 55-nm-thick pristine $\alpha$-MoO$_3$. c, Experimental, analytical and simulated PhP dispersion of $\alpha$-MoO$_3$. d, Topography and PiFM images at 860 cm$^{-1}$ excitation of 63-nm-thick Ag-MoO$_3$. e, FDTD simulated out-of-plane electric field distribution at 860 cm$^{-1}$ frequency of 63-nm-thick pristine $\alpha$-MoO$_3$. f, Comparison of PhP dispersion of Ag-MoO$_3$ and simulated pristine $\alpha$-MoO$_3$. g, Spectral map along [100] of $\alpha$-MoO$_3$. h, Spectral map along [100] of Ag-MoO$_3$. i, Topography and PiFM images at 890 cm$^{-1}$ excitation of 140-nm-thick Cu-MoO$_3$. j, FDTD simulated out-of-plane electric field distribution at 890 cm$^{-1}$ frequency of 140-nm-thick pristine $\alpha$-MoO$_3$. k, Comparison of PhP dispersion of Cu-MoO$_3$ and simulated pristine $\alpha$-MoO$_3$. Scale bars are 3 $\mu$m.