# Nonlinear charge oscillation driven by a single-cycle light field in an organic superconductor

## Abstract

Application of an intense light field to solids produces enormous and ultrafast nonlinear phenomena such as high-harmonic generation1,2 and attosecond charge dynamics3,4. These are distinct from conventional photonics. However, the main targets for investigation have been limited to insulators and semiconductors, although theoretical approaches have also been developed for correlated metals and superconductors5. Here, in a layered organic superconductor, a nonlinear charge oscillation driven by a nearly single-cycle strong electric field of >10 MV cm−1 is observed as a stimulated emission. The charge oscillation is different from a linear response and ascribed to a polar charge oscillation with a period of 6 fs. This nonlinear polar charge oscillation is enhanced by critical fluctuations near a superconducting transition temperature and a critical end-point of first-order Mott transitions. Its observation on an ultrafast timescale of 10 fs clarifies that Coulomb repulsion plays an essential role in the superconductivity of organic superconductors.

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## Acknowledgements

This work was supported by the Japan Science and Technology Agency (Core Research for Evolutional Science and Technology (CREST: Elucidation of elementary dynamics of photoinduced phase transition by using advanced ultrashort light pulses) and Exploratory Research for Advanced Technology (ERATO: JPMJER1301)) and the Japan Society for the Promotion of Science (grant numbers JP15H02100, JP23244062, JP16K13814, JP17K14317, JP26887003, JP16K05459, JP25287080, JP26287070, JP17H02916 and JP16H04140). Part of the work was conducted in the Equipment Development Center (Institute for Molecular Science), supported by the Nanotechnology Platform Program (Molecule and Material Synthesis) of the Ministry of Education, Culture, Sports, Science and Technology, Japan.

## Author information

Authors

### Contributions

Y.K., T.A., Y.Y., Y.A., H.I. and S.Iw. developed the 6 fs light source, carried out the transient reflectivity/transmittance measurements using the 6 fs pulse and, with contributions from H.K., analysed the data. G.K. and H.M.Y. performed the synthesis and characterization of the thin film. K.I., H.K. and T.S. performed the synthesis and characterization of the single crystal. S.Is., Y.T. and K.Y. made theoretical considerations and calculations. S.Iw. devised all of the experiments. Y.K., K.Y. and S.Iw. wrote the paper after discussions with all co-authors.

### Corresponding author

Correspondence to S. Iwai.

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### Competing interests

The authors declare no competing interests.

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## Supplementary information

### Supplementary Information

Supplementary notes and figures.

## Rights and permissions

Reprints and Permissions

Kawakami, Y., Amano, T., Yoneyama, Y. et al. Nonlinear charge oscillation driven by a single-cycle light field in an organic superconductor. Nature Photon 12, 474–478 (2018). https://doi.org/10.1038/s41566-018-0194-4

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