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Hyperbolic phonon-polaritons in boron nitride for near-field optical imaging and focusing

: Li, P.; Lewin, M.; Kretinin, A.V.; Caldwell, J.D.; Novoselov, K.S.; Taniguchi, T.; Watanabe, K.; Gaussmann, F.; Taubner, T.

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Nature Communications 6 (2015), Art. 7507, 9 pp.
ISSN: 2041-1723
Journal Article, Electronic Publication
Fraunhofer ILT ()

Hyperbolic materials exhibit sub-diffractional, highly directional, volume-confined polariton modes. Here we report that hyperbolic phonon polaritons allow for a flat slab of hexagonal boron nitride to enable exciting near-field optical applications, including unusual imaging phenomenon (such as an enlarged reconstruction of investigated objects) and sub-diffractional focusing. Both the enlarged imaging and the super-resolution focusing are explained based on the volume-confined, wavelength dependent propagation angle of hyperbolic phonon polaritons. With advanced infrared nanoimaging techniques and state-of-art mid-infrared laser sources, we have succeeded in demonstrating and visualizing these unexpected phenomena in both Type I and Type II hyperbolic conditions, with both occurring naturally within hexagonal boron nitride. These efforts have provided a full and intuitive physical picture for the understanding of the role of hyperbolic phonon polaritons in near-field optical imaging, guiding, and focusing applications.