arXiv subjects

A. Paarmann

Publications and source records attributed to A. Paarmann.

3 recordsLinked to original sources

arXiv2025

Ultraconfined THz Phonon Polaritons in Hafnium Dichalcogenides

The confinement of electromagnetic radiation to subwavelength scales relies on strong light-matter interactions. In the infrared (IR) and terahertz (THz) spectral ranges, phonon polaritons are commonly employed to achieve extremely subdiffractional light confinement, with much lower losses as compared to plasmon polaritons. Among these, hyperbolic phonon polaritons in anisotropic materials offer a highly promising platform for light confinement, which, however, typically plateaus at values of {\lambda}0/100, with {\lambda}0 being the free-space incident wavelength. In this study, we report on ultraconfined phonon polaritons in hafnium-based dichalcogenides with confinement factors exceeding {\lambda}0/250 in the terahertz spectral range. This extreme light compression within deeply sub-wavelength thin films is enabled by the unprecedented magnitude of the light-matter coupling strength in these compounds, and the natural hyperbolicity of HfSe2 in particular. Our findings emphasize the critical role of light-matter coupling for polariton confinement, which for phonon polaritons in polar dielectrics is dictated by the transverse-longitudinal optic phonon energy splitting. Our results demonstrate transition metal dichalcogenides as an enabling platform for THz nanophotonic applications that push the limits of light control.

physics.optics↗

arXiv2024

Unidirectional Ray Polaritons in Twisted Asymmetric Stacks

The vast repository of van der Waals (vdW) materials supporting polaritons offers numerous possibilities to tailor electromagnetic waves at the nanoscale. The development of twistoptics - the modulation of the optical properties by twisting stacks of vdW materials - enables directional propagation of phonon polaritons (PhPs) along a single spatial direction, known as canalization. Here we demonstrate a complementary type of directional propagation of polaritons by reporting the visualization of unidirectional ray polaritons (URPs). They arise naturally in twisted hyperbolic stacks with very different thicknesses of their constituents, demonstrated for homostructures of $\alpha$-MoO$_3$ and heterostructures of $\alpha$-MoO$_3$ and $\beta$-Ga$_2$O$_3$. Importantly, their ray-like propagation, characterized by large momenta and constant phase, is tunable by both the twist angle and the illumination frequency. Apart from their fundamental importance, our findings introduce twisted asymmetric stacks as efficient platforms for nanoscale directional polariton propagation, opening the door for applications in nanoimaging, (bio)-sensing or polaritonic thermal management.

physics.optics↗

arXiv2018

Ultrafast relaxation dynamics of the antiferrodistortive phase in Ca doped SrTiO3

The ultrafast dynamics of the octahedral rotation in Ca:SrTiO3 is studied by time resolved x-ray diffraction after photo excitation over the band gap. By monitoring the diffraction intensity of a superlattice reflection that is directly related to the structural order parameter of the soft-mode driven antiferrodistortive phase in Ca:SrTiO3, we observe a ultrafast relaxation on a 0.2 ps timescale of the rotation of the oxygen octahedron, which is found to be independent of the initial temperaure despite large changes in the corresponding soft-mode frequency. A further, much smaller reduction on a slower picosecond timescale is attributed to thermal effects. Time-dependent density-functional-theory calculations show that the fast response can be ascribed to an ultrafast displacive modification of the soft-mode potential towards the normal state, induced by holes created in the oxygen 2p states.

cond-mat.mtrl-sci↗