All-angle negative refraction of highly squeezed plasmon and phonon polaritons in graphene–boron nitride heterostructures

Abstract

A fundamental building block for nanophotonics is the ability to achieve negative refraction of polaritons, because this could enable the demonstration of many unique nanoscale applications such as deep-subwavelength imaging, superlens, and novel guiding. However, to achieve negative refraction of highly squeezed polaritons, such as plasmon polaritons in graphene and phonon polaritons in boron nitride (BN) with their wavelengths squeezed by a factor over 100, requires the ability to flip the sign of their group velocity at will, which is challenging. Here we reveal that the strong coupling between plasmon and phonon polaritons in graphene-BN heterostructures can be used to flip the sign of the group velocity of the resulting hybrid (plasmon-phonon-polariton) modes. We predict allangle negative refraction between plasmon and phonon polaritons and, even more surprisingly, between hybrid graphene plasmons and between hybrid phonon polaritons. Graphene-BN heterostructures thus provide a versatile platform for the design of nanometasurfaces and nanoimaging elements.negative refraction | plasmon polariton | phonon polariton | graphene-boron nitride heterostructure P olaritons with high spatial confinement, such as plasmon polaritons in graphene (1-5) and phonon polaritons in a thin hexagonal boron nitride (BN) slab (6-15), enable control over the propagation of light at the extreme nanoscale, due to their in-plane polaritonic wavelength that can be squeezed by a factor over 100. Henceforth we use the term squeezing factor (or confinement factor) to define the ratio between the wavelength in free space and the in-plane polaritonic wavelength. The combination of tunability, low losses, and ultraconfinement (1,2,8,10,11,15) makes them superior alternatives to conventional metal plasmons and highly appealing for nanophotonic applications (3-5, 10-13, 15). Their extreme spatial confinement, however, also limits our ability to tailor their dispersion relations.Unlike the case of 2D plasmons, the coupling between metal plasmons in a metal-dielectric-metal structure dramatically changes their dispersion relation and can even flip the sign of their group velocities (16,17). This has led to exciting applications by tailoring the in-plane plasmonic refraction, giving flexibility in controlling the energy flow of light. Specifically, by flipping the sign of the group velocity of metal plasmons, plasmonic negative refraction has been predicted (16) and demonstrated (17). The negative refraction has also been extensively explored in metamaterials, metasurfaces, and photonic crystals (18-26), but they become experimentally very challenging to realize when dealing with polaritons with high squeezing factors. In contrast to metal plasmons, the group velocity of graphene plasmons (2,11,27) and all other 2D plasmons (28-32) is always positive, including that in graphene-based multilayer structures (33). This has made the in-plane negative refraction for highly squeezed 2D plasmon polaritons seem impossible to achieve.Contrary to 2D plas...

Normal Doppler Frequency Shift in Negative Refractive‐Index Systems

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