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Subcycle contact-free nanoscopy of ultrafast interlayer transport in atomically thin heterostructures

Abstract

Tunnelling is one of the most fundamental manifestations of quantum mechanics. The recent advent of lightwave-driven scanning tunnelling microscopy has revolutionized ultrafast nanoscience by directly resolving electron tunnelling in electrically conducting samples on the relevant ultrashort length- and timescales. Here, we introduce a complementary approach based on terahertz near-field microscopy to perform ultrafast nano-videography of tunnelling processes even in insulators. The central idea is to probe the evolution of the local polarizability of electron–hole pairs with evanescent terahertz fields, which we detect with subcycle temporal resolution. In a proof of concept, we resolve femtosecond interlayer transport in van der Waals heterobilayers and reveal pronounced variations of the local formation and annihilation of interlayer excitons on deeply subwavelength, nanometre scales. Such contact-free nanoscopy of tunnelling-induced dynamics should be universally applicable to conducting and non-conducting samples and reveal how ultrafast transport processes shape functionalities in a wide range of condensed matter systems.

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Fig. 1: Probing interlayer tunnelling of photo-carriers by their polarizability.
Fig. 2: Out-of-plane and in-plane polarizability of intralayer and interlayer electron–hole pairs.
Fig. 3: Clocking ultrafast interlayer tunnelling using terahertz emission nanoscopy.
Fig. 4: Resolving nanoscale inhomogeneities of tunnelling efficiency and electron–hole pair lifetimes by ultrafast contact-free nanoscopy.

Data availability

The data sets generated during and/or analysed during the current study are available from the corresponding authors upon reasonable request.

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Acknowledgements

We thank M. Furthmeier for technical assistance and T. F. Heinz for fruitful discussions. This work was supported by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)—Project-ID, 314695032—SFB 1277 (Subprojects A05 and B05) and through research grants HU1598/3, HU1598/8 and CO1492/1. P.E.F.J. acknowledges funding by the A. v. Humboldt Foundation and by Capes (Grant No. 99999.000420/2016-06). M.A.H. was supported in part by the US Department of Energy, Office of Science, Basic Energy Sciences (BES), Chemical Sciences, Geosciences, and Biosciences Division, AMOS Program. M.G. acknowledges support by the Ministry of Education, Science, Research and Sport of the Slovak Republic (Grant No. VEGA 1/0105/20). J.L.B. acknowledges support by the A. v. Humboldt Foundation and EPSRC (UK) via project EP/S037438/1.

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Contributions

M.P. and F.M. fabricated the samples. M.P., F.M., T.S., M.Z., F. Sandner, F. Schiegl, S.M., M.A.H., J.L.B., T.L.C. and R.H. conducted the experiments. P.E.F.J., M.G. and J.F. performed the density functional theory calculations, F.M. performed the finite element simulations, and J.F., T.L.C. and R.H. supervised the study. M.P., F.M. and R.H. wrote the manuscript with input from all authors.

Corresponding authors

Correspondence to J. Fabian or R. Huber.

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Peer review information Nature Photonics thanks Pavel Jelinek and Feng Wang for their contribution to the peer review of this work.

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Supplementary information

Supplementary Information

Supplementary Figs. 1–11, discussion and Tables 1–3.

41566_2021_813_MOESM2_ESM.mp4

Supplementary Video 1 Temporal evolution of the emission of electromagnetic fields by a time-dependent point-dipole source on a silicon substrate. a, Out-of-plane current density jz corresponding to interlayer charge transfer with a characteristic tunnelling time τtunnel = 200 fs (compare Fig. 3c). b, Simulated far-field waveform based on the current density depicted in a, accounting for the electro-optic detector response. c, Maps of the out-of-plane electric field Ez as a function of the delay time. The snapshots were obtained by superposition of the respective distributions for different frequencies ranging from 0 to 3 THz (see Supplementary Fig. 6 and the Methods section) calculated by the finite element method.

41566_2021_813_MOESM3_ESM.mp4

Supplementary Video 2 Temporal evolution of the emission of electromagnetic fields by a tip-enhanced dipole emitter source on a silicon substrate. a, Out-of-plane current density jz corresponding to interlayer charge transfer with a characteristic tunnelling time τtunnel = 200 fs (compare Fig. 3c). b, Simulated far-field waveform based on the current density depicted in a, accounting for the tip transfer function and electro-optic detector response. c, Maps of the out-of-plane electric field Ez as a function of the delay time. The snapshots including a near-field probe with realistic dimensions were obtained by superposition of the respective distributions for different frequencies ranging from 0 to 3 THz (see Supplementary Fig. 6 and the Methods section) calculated by the finite element method.

41566_2021_813_MOESM4_ESM.mp4

Supplementary Video 3 Subcycle nano-videography of the interlayer exciton formation and annihilation. The height profile and contour lines represent the topography of the heterostructure recorded by atomic force microscopy. The two-dimensional maps of the interlayer eh pair density were obtained by evaluating a series of snapshot images of the magnitudes of the pump-induced changes in electric field \({\Delta}\hat E_1^{{\mathrm{scat}}}\) for a set of delay times after photoexcitation (compare Fig. 4). For each pixel, the corresponding temporal evolution of \({\Delta}\hat E_1^{{\mathrm{scat}}}\) was fitted by a mono-exponential decay curve, which was then colour-coded onto the height profile, for each delay time (see the main text for further details).

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Plankl, M., Faria Junior, P.E., Mooshammer, F. et al. Subcycle contact-free nanoscopy of ultrafast interlayer transport in atomically thin heterostructures. Nat. Photon. 15, 594–600 (2021). https://doi.org/10.1038/s41566-021-00813-y

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