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Ultrafast multi-terahertz nano-spectroscopy with sub-cycle temporal resolution

Abstract

Phase-locked ultrashort pulses in the rich terahertz spectral range1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18 have provided key insights into phenomena as diverse as quantum confinement7, first-order phase transitions8,12, high-temperature superconductivity11 and carrier transport in nanomaterials1,6,13,14,15. Ultrabroadband electro-optic sampling of few-cycle field transients1 can even reveal novel dynamics that occur faster than a single oscillation cycle of light4,8,10. However, conventional terahertz spectroscopy is intrinsically restricted to ensemble measurements by the diffraction limit. As a result, it measures dielectric functions averaged over the size, structure, orientation and density of nanoparticles, nanocrystals or nanodomains. Here, we extend ultrabroadband time-resolved terahertz spectroscopy to the sub-nanoparticle scale (10 nm) by combining sub-cycle, field-resolved detection (10 fs) with scattering-type near-field scanning optical microscopy (s-NSOM)16,17,18,19,20,21,22,23,24,25,26. We trace the time-dependent dielectric function at the surface of a single photoexcited InAs nanowire in all three spatial dimensions and reveal the ultrafast (<50 fs) formation of a local carrier depletion layer.

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Figure 1: Set-up for single-nanowire terahertz spectroscopy.
Figure 2: Dynamics of the oscillating electric near field.
Figure 3: Sub-cycle spectral dynamics.
Figure 4: Femtosecond tomography.

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Acknowledgements

The authors thank M. Furthmeier for technical assistance and J. Lupton, P. Klemm, D. Bougeard, B. Surrer, R. Hillenbrand and F. Keilmann for discussions. This work was supported by the European Research Council through ERC grant 305003 (QUANTUMsubCYCLE), the Deutsche Forschungsgemeinschaft through Graduate Research College GRK 1570, and the Italian Ministry of Education, University, and Research (MIUR) through the Futuro in Ricerca 2010 grant RBFR10LULP (Fundamental Research on Terahertz Photonic Devices). T.L.C. acknowledges the support of the Alexander von Humboldt Foundation.

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Contributions

M.E., T.L.C. and R.H. conceived the study and built the experimental set-up. M.E., T.L.C., M.A.H., M.P. and R.H. carried out the experiment and analysed the data. M.A.H., T.L.C. and M.E. performed simulations. D.E. and L.S. grew the InAs nanowires. L.V. and M.S.V. designed, fabricated and characterized the nanowire samples. T.L.C., M.E., M.A.H. and R.H. wrote the manuscript. All authors contributed to the discussions.

Corresponding author

Correspondence to R. Huber.

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The authors declare no competing financial interests.

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Eisele, M., Cocker, T., Huber, M. et al. Ultrafast multi-terahertz nano-spectroscopy with sub-cycle temporal resolution. Nature Photon 8, 841–845 (2014). https://doi.org/10.1038/nphoton.2014.225

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