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Time-domain separation of optical properties from structural transitions in resonantly bonded materials

Abstract

The extreme electro-optical contrast between crystalline and amorphous states in phase-change materials is routinely exploited in optical data storage1 and future applications include universal memories2, flexible displays3, reconfigurable optical circuits4,5, and logic devices6. Optical contrast is believed to arise owing to a change in crystallinity. Here we show that the connection between optical properties and structure can be broken. Using a combination of single-shot femtosecond electron diffraction and optical spectroscopy, we simultaneously follow the lattice dynamics and dielectric function in the phase-change material Ge2Sb2Te5 during an irreversible state transformation. The dielectric function changes by 30% within 100 fs owing to a rapid depletion of electrons from resonantly bonded states. This occurs without perturbing the crystallinity of the lattice, which heats with a 2-ps time constant. The optical changes are an order of magnitude larger than those achievable with silicon and present new routes to manipulate light on an ultrafast timescale without structural changes.

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Figure 1: Below-threshold dynamics in crystalline GST.
Figure 2: Dynamics of the dielectric function during amorphization.
Figure 3: Structural dynamics during amorphization.
Figure 4: Schematic of the ultrafast transformation pathway.

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Acknowledgements

L.W. acknowledges support by the Leibniz graduate school ‘Dynamics in New Light’. T.A.M. acknowledges financial support through the Marie Curie COFUND project and Spanish Ministry of Economy and Competitiveness (MINECO). R.B. thanks the Alexander von Humboldt Foundation for financial support. V.P. acknowledges financial support from MINECO and the ‘Fondo Europeo de Desarrollo Regional’ (FEDER) through grant TEC2013-46168-R. R.E. acknowledges fruitful discussions with M. Wuttig and funding from the Max Planck Society. S.W. acknowledges financial support from Ramon y Cajal program RYC-2013-14838 and Marie Curie Career Integration Grant PCIG12-GA-2013-618487. V.P. and S.W. acknowledge support from Fundació Cellex.

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Contributions

S.W., L.W. and R.E. initiated the project. T.A.M. and S.W. performed the multi-shot optical measurements. S.W., L.W. and T.A.M. performed the single-shot optical measurements. L.W. and R.B. performed the time-resolved diffraction measurements. M.R. fabricated samples, which were characterized by M.R., T.A.M. and J.O. All authors provided input to the interpretation of the data and writing the manuscript.

Corresponding authors

Correspondence to Ralph Ernstorfer or Simon Wall.

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The authors declare no competing financial interests.

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Waldecker, L., Miller, T., Rudé, M. et al. Time-domain separation of optical properties from structural transitions in resonantly bonded materials. Nature Mater 14, 991–995 (2015). https://doi.org/10.1038/nmat4359

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