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Surface polaritons in the mid infrared have emerged as a versatile platform for extreme light confinement and tailored nanophotonics [1,2]. These light-matter coupled modes can be hosted on two-dimensional materials such as graphene or hexagonal boron nitride. Whereas surface plasmons have been successfully modulated in gapless graphene, semiconducting layered materials and heterostructures are promising candidates for high-contrast, ultrafast control of polaritons.
Here, we report on the first ultrafast snapshots of a switchable surface polariton mode [3]. In a custom-tailored heterostructure, we sandwich the direct-gap semiconductor black phosphorus [4] (BP) between two SiO$_2$ layers. Upon ultrafast optical interband excitation, surface plasmon polaritons on BP couple to surface phonon polaritons on the adjacent SiO$_2$ layers to form hybrid interface polaritons. Using pump-probe scanning near-field optical microscopy (SNOM) in the mid infrared [5], we resolve the photo-activated polariton in real space as a standing wave pattern. We find that the hybrid polariton appears within ∼50 fs after photo-excitation, as electron-hole pairs are created within the BP layer. The subsequent recombination of the photogenerated carriers results in a decay of the hybrid mode within ∼5 ps. Remarkably, the hybrid polariton’s wavelength is found to be almost constant throughout its entire lifetime. This decoupling of the hybrid polariton’s momentum from the BP carrier density stands in stark contrast to the behavior of conventional plasmon polaritons [1,2].
Moreover, employing nano-spectroscopy, we trace the evolution of the hybrid polariton’s energy and observe that it is strongly confined at a frequency of ∼33.8 THz. Furthermore, this instantaneous mode frequency is almost independent of the pump-probe delay time, which coincides with our theoretical modelling of the polariton hybridization. This switchable polariton mode with constant energy and momentum holds great promise for future ultrafast polaritonic devices because efficient incoupling throughout its entire lifetime can, for example, be provided by a monochromatic laser and a single grating.
[1] D. N. Basov, M. M. Fogler and F. J García de Abajo, Science 354, 195 (2016).
[2] T. Low et al., Nat. Mater. 16, 182 (2016).
[3] M. A. Huber et al., Nat. Nanotechnol. 12, 207 (2017).
[4] X. Ling, H. Wang, S. Huang, F. Xia and M. S. Dresselhaus, Proc. Natl. Acad. Sci. U.S.A. 112, 4523 (2015).
[5] M. Eisele et al., Nat. Photon. 8, 841 (2014).
