Ultraconfined Phonon Polaritons and Nonlinear Optical Microscopy to Bridge Spectral Ranges

Niclas S. Mueller

Emmy Noether Group Leader, Department of Physics, Freie Universität Berlin

Phonon polaritons are mixed light-matter quasiparticles arising from the ultrastrong coupling of infrared photons with lattice vibrations [1,2]. They enable the miniaturization of photonic components for processing infrared light on the nanoscale, such as waveguides, coherent thermal light sources, and sensors.

In this talk, I will demonstrate how phonon polaritons in the transition metal dichalcogenides HfSe2 and HfS2 compress THz wavelengths (~60 µm) down to the scale of UV light (~250 nm) [3]. This extreme confinement results from their exceptionally strong light-matter interaction, which approaches the excitation frequency itself.

In the second part, I will present recent advances in sum-frequency generation (SFG) microscopy, which combines ultrafast mid-infrared and visible lasers. Here, a material resonance is excited in the infrared, and upconversion with a visible laser enables imaging with sub-diffractional spatial resolution in a visible light microscope. We employ this technique to visualize the propagation and dispersion of phonon polaritons in mid-IR metasurfaces, enabling strong coupling and directional thermal emission [4]. Moreover, phase-resolved SFG microscopy allows us to image the crystal structure of monolayer hexagonal boron nitride—a 2D material that is usually optically invisible [5].

[1] Abajo, …, NSM et al. Roadmap for Photonics with 2D Materials, arXiv 2504.04558 (2025)

[2] NSM et al. Ultrastrong Light-Matter Coupling in Materials, arXiv 2505.06373 (2025)

[3] Kowalski, NSM et al. Ultraconfined THz Phonon Polaritons in Hafnium Dichalcogenides, arXiv 2502.09909 (2025)

[4] Niemann, NSM et al. Spectroscopic and Interferometric Sum-Frequency Imaging of Strongly Coupled Phonon Polaritons in SiC Metasurfaces, Adv. Mater. 36, 2312507 (2024)

[5] Mueller, Fellows et al. Full Crystallographic Imaging of Hexagonal Boron Nitride Monolayers with Phonon-Enhanced Sum-Frequency Microscopy, arXiv 2504.15939 (2025)