Monolayers of hexagonal boron nitride (hBN) have been – besides their high relevance in 2D materials research – traditionally been very difficult to handle due their lack of optical resonances making them essentially invisible in any optical microscope. Making use of the strong infrared resonance associated with a lattice vibration in hBN, the recently developed sum-frequency generation (SFG) microscope was shown to drastically enhance imaging contrast, enabling live-imaging of hBN monolayers. [more...]

The confinement of electromagnetic radiation to sub-wavelength scales relies on strong light–matter interactions. In the infrared and terahertz spectral ranges, phonon polaritons are commonly employed to achieve such subdiffractional light confinement and these optical modes offer much lower losses in compared to plasmon polaritons. Hyperbolic phonon polaritons in anisotropic materials, such as hafnium-based dichalcogenides, offer a promising platform and we report here on ultraconfined phonon polaritons with confinement [more...]