Hyperbolic nanomaterials have recently attracted interest due to the alternative lensing mechanism that their unique crystal structure can accommodate. This lensing technique uses a subatomic particle called a phonon-polariton to capture evanescent light rays before they decay1. Using polaritons allows this lensing mechanism to retain resolution that is normally lost in traditional lenses, producing a high resolution lens known as a super-lens2.
Phonon-polaritons are particles caused by the oscillation of polar bonds in polar-covalent crystals as they interact with light1. When you shine light on a polar crystal, the interactions between the electric component of the photon’s electromagnetic field and the polar bonds in the crystal shifts the electron cloud density in the bond according to the frequency of the light ray1. The particles resulting from the oscillation of this polar bond, retain the frequency of the incident light ray, which allows them to retain image information1,2. Due to coulombic interactions, the shift in the electron density of one bond will also oscillate the electron cloud density in adjacent bonds. In this fashion, the polariton can propagate all the way through the crystal and be re-emitted as a photon of light on the other side.
Although this lensing mechanism promises a way to capture evanescent light rays before they decay, the inherent instability of polaritons creates undesirable high loss optics that destroys image information before it can be processed by an observer2. Super-lensing thus requires a special breed of polaritons called hyperbolic-phonon-polaritons in order to efficiently image objects that are smaller than the diffraction limit.
Boron-Nitride is the first natural material that was observed to accommodate these polaritons due to its special crystal configuration2,3.
This configuration, called birefringent, results from an unusual bonding structure where a crystal has weak bonds along one axis and strong bonds along the orthogonal axis3. In the case of Boron-Nitride, the polar-covalent sections make up the strong bond axis and several layers of crystal, held together by Van-der-Waals forces, make up the weak bond axis2,3. Since polaritons are created by the oscillation of polar-bonds, it's easier for polaritons to propagate along the stronger, polar-covalent bonds and more difficult for them propagate along weaker bonds. In the case of Boron-Nitride, the weak bonds are so incredibly weak that they prevent any polariton from propagating through the crystal and cause all light on this axis to reflect away from the crystal. This is a very extreme case of birefringence, known as hyperbolicity, where one axis has incredibly weak bonds (called the negative permittivity axis) and one axis easily propagates polaritons (called the positive permittivity axis). Since the polaritons can only propagate in the positive permittivity direction, they are restricted to moving in a linear fashion once they couple into the crystal. This phenomenon prevents the polaritons from scattering by prohibiting them from moving in the orthogonal direction (along the negative permittivity axis). The lack of scattering extends the lifetime of hyperbolic polaritons when compared to that of the evanescent rays and lends hyperbolic crystals the ability of acting as a super-lens.
Tomadin, A.; Principi, A.; Song, C.W..; Levitov, L.S.; Polini, M. Accessing Phonon Polaritons in Hyperbolic Crystals by Angle-Resolved Photoemission Spectroscopy. Phys Rev Lett USA 2015, 98, 87401-87406.
Li, P.; Lewin, M.; Kretinin, A.V.; Caldwell, J.D.; Novoselov, K.S.; Taniguchi, T.; Watanabe, K.; Gaussmann, F.; Taubner, T. Hyperbolic phonon-polaritons in boron nitride for near-field optical imaging and focusing Nat. Commun USA 2015, 6, 7507-7513.
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