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Following Fermat's principle, it is intuitively assumed that a pulse always slows down while traveling from low index to high index materials. Here we show that structuring a wave-packet both spatially and temporally challenges this well-established intuition, unveiling anomalous refractive phenomena – group velocity of a pulse increases while traversing denser media. We present a theoretical formulation as well as an experimental demonstration of this remarkable behavior by making use of 'space-time' wave-packets – propagation-invariant pulsed beam endowed with tight correlations between spatial and temporal degrees-of-freedom. We observe the boost of group velocity at the interface of various optical materials (BK7, MgF2, and Sapphire) with air.
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Murat Yessenov, Basanta Bhaduri, Ayman F. Abouraddy, "Anomalous refraction of spatio-temporally structured wave-packets (Conference Presentation)," Proc. SPIE 11297, Complex Light and Optical Forces XIV, 1129703 (9 March 2020); https://doi.org/10.1117/12.2556663