Image polaritons in boron nitride for extreme polariton confinement with low losses

Abstract

Polaritons in two-dimensional materials provide extreme light confinement that is difficult to achieve with metal plasmonics. However, such tight confinement inevitably increases optical losses through various damping channels. Here we demonstrate that hyperbolic phonon polaritons in hexagonal boron nitride can overcome this fundamental trade-off. Among two observed polariton modes, featuring a symmetric and antisymmetric charge distribution, the latter exhibits lower optical losses and tighter polariton confinement. Far-field excitation and detection of this high-momenta mode become possible with our resonator design that can boost the coupling efficiency via virtual polariton modes with image charges that we dub 'image polaritons'. Using these image polaritons, we experimentally observe a record-high effective index of up to 132 and quality factors as high as 501. Further, our phenomenological theory suggests an important role of hyperbolic surface scattering in the damping process of hyperbolic phonon polaritons.

Real-space imaging of acoustic plasmons in large-area graphene grown by chemical vapor deposition

“…Inside such a narrow dielectric spacer, the AGP wavevector can be about two orders of magnitude larger than that of free space light, which grants access to quantum and non-local phenomena in graphene 5 , 11 , 12 , and allows for the AGP localization in nanostructures 13 with a stunning mode volume confinement factor 4 of ~10 10 . This ultimate capability to compress MIR light outperforms that of other polaritonic species in van der Waals materials 9 , including graphene surface plasmon (GSP) 14 , 15 , and is similar to the case of image phonon-polaritons in boron nitride 16 . For that reason, AGP is promising for applications that require strong light–matter interaction such as molecular sensing 6 , 17 – 20 , polaritonic dispersion engineering in van der Waals crystals 21 , 22 , and dynamic light manipulation by graphene-based active metasurfaces 23 – 26 .…”

Spatiotemporal imaging of 2D polariton wave packet dynamics using free electrons

“…Over the past decade, research on propagating optical polaritons in 2D materials progressed from a promising concept (1, 2) to a platform for demonstrating rich physical phenomena (3)(4)(5)(6)(7), now showing an impact on emerging opto-electronics (8,9) and nanophotonic technologies (10). These polaritons exhibit relatively low loss and long propagation distances, simultaneous with extreme confinement factors (5,(11)(12)(13)(14), facilitating their unique light-matter interactions (2,3,(15)(16)(17)(18)(19).…”

Bound in the continuum modes in indirectly-patterned hyperbolic media

“…Another technological barrier to overcome has to do with near-field microscopy itself. As it stands, the field profile of the cavities in our experiments is already more than 200 times smaller than vacuum wavelength, which is the highest compression seen in an infrared near-field measurement 55 . Such extreme contraction implies that the coupling efficiency into the smaller nanocavity is extremely poor, because the area of the tip is several times larger than the cavity.…”

“…The hBIC cavities we propose consist of a sharply defined hole (with nanometer sharp corners) in a gold film covered by a thin flake of hexagonal Boron Nitride (hBN), which supports PhPs in its Restrahlen bands [53][54][55][56][57] . For a mode to form, the ray needs to acquire an integer multiple of 2 phase, in addition to the aforementioned geometric condition (the ray bouncing from one corner to the other).…”