Selective excitation and imaging of ultraslow phonon polaritons in thin hexagonal boron nitride crystals
A. Ambrosio, M. Tamagnone, K. Chaudhary, L. A. Jauregui, P. Kim, W. L. Wilson and F. Capasso
Light: Science & Applications 7, 27 (2018)
We selectively excite and study two new types of phonon-polariton guided modes that are found in hexagonal boron
nitride thin flakes on a gold substrate. Such modes show substantially improved confinement and a group velocity
that is hundreds of times slower than the speed of light, thereby providing a new way to create slow light in the midinfrared
range with a simple structure that does not require nano-patterning. One mode is the fundamental mode in
the first Restrahlen band of hexagonal boron nitride thin crystals on a gold substrate; the other mode is equivalent to
the second mode of the second Restrahlen band of hexagonal boron nitride flakes that are suspended in vacuum.
The new modes also couple efficiently with incident light at the hexagonal boron nitride edges, as we demonstrate
experimentally using photo-induced force microscopy and scanning near-field optical microscopy. The high
confinement of these modes allows for Purcell factors that are on the order of tens of thousands directly above boron
nitride and a wide band, with new perspectives for enhanced light-matter interaction. Our findings demonstrate a
new approach to engineering the dispersion of polaritons in 2D materials to improve confinement and light-matter
interaction, thereby paving the way for new applications in mid-infrared nano-optics.