Selective excitation and imaging of ultraslow phonon polaritons in thin hexagonal boron nitride crystals

Abstract

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 mid-infrared 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.

Probing Polaritons in 2D Materials

“…e) The PiFM signal intensity of h‐BN as a function of the distance from the edge and the illuminating light frequency. Reproduced with permission . Copyright 2018, Springer Nature.…”

“…As shown in Figure e, PiFM can effectively map the modes in the lower Reststrahlen band of h‐BN that cannot be obtained by s‐SNOM due to the lack of continuous wave laser sources . Ambrosio et al selectively excited h‐BN with the more confined modes in the lower and upper Reststrahlen bands by PiFM. Figure f is a plot of the PiFM signal intensity from h‐BN showing the fringe periodicity as a function of the illuminating light wavelength, from which the first mode in the lower Reststrahlen band can be inferred.…”

Phonon Polaritons and Hyperbolic Response in van der Waals Materials

“…At the end of this section, we remark that, in addition to detecting polariton standing waves from scattering field off the s‐SNOM tip, the s‐SNOM setup can be implemented with photocurrent, photoinduced force or peak force setup, as photon‐free imaging schemes for polariton waves.…”

Probing Polaritons in 2D Materials with Synchrotron Infrared Nanospectroscopy

“…Most of those observations stem from real‐space images of exciton‐polaritons in transition metal dichalcogenides, plasmon‐polaritons in graphene, phonon‐polaritons in polar vdW crystals, and hybrid plasmon‐phonon polaritons in vdW heterostructures . For these studies, the use of advanced optical methods such as scattering scanning near‐field optical microscopy (s‐SNOM), synchrotron infrared nanospectroscopy (SINS), and, more recently, photoinduced force microscopy (PiFM) have been paramount. Those nanoscopic techniques use the strongly concentrated optical fields confined at the nanometer‐sized apex of an atomic force microscopy (AFM) metallic tip, typically with radius ≈25 nm, to excite and detect such polaritons.…”

“…Accordingly, type I and type II HP 2 s exist in the abovementioned spectral ranges. HP 2 waves have been observed in hBN‐based systems by s‐SNOM and photoinduced force microscopy (PiFM) . They are typically launched or emitted by edges and features of the hBN crystal or by metallic antennas .…”