The science and applications of electronics and optoelectronics have been driven for decades by progress in the growth of semiconducting heterostructures. Many applications in the infrared and terahertz frequency range exploit transitions between quantized states in semiconductor quantum wells (intersubband transitions). However, current quantum well devices are limited in functionality and versatility by diffusive interfaces and the requirement of lattice-matched growth conditions. Here, we introduce the concept of intersubband transitions in van der Waals quantum wells and report their first experimental observation. Van der Waals quantum wells are naturally formed by two-dimensional materials and hold unexplored potential to overcome the aforementioned limitations—they form atomically sharp interfaces and can easily be combined into heterostructures without lattice-matching restrictions. We employ near-field local probing to spectrally resolve intersubband transitions with a nanometre-scale spatial resolution and electrostatically control the absorption. This work enables the exploitation of intersubband transitions with unmatched design freedom and individual electronic and optical control suitable for photodetectors, light-emitting diodes and lasers.
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We acknowledge discussions with A. Tredicucci about the general concept and S. Wall about the experimental measurement technique. We also thank A. Govyadinov for discussions about the thin-film inversion model. P.S. acknowledges financial support by a scholarship from the ‘la Caixa’ Banking Foundation. F.V. acknowledges financial support from Marie-Curie International Fellowship COFUND and ICFOnest programme. M.M. thanks the Natural Sciences and Engineering Research Council of Canada (PGSD3-426325-2012). K.-J.T. acknowledges support from a Mineco Young Investigator Grant (FIS2014-59639-JIN). F.H.L.K. acknowledges financial support from the Government of Catalonia through an SGR grant (2014-SGR-1535), and from the Spanish Ministry of Economy and Competitiveness through the ‘Severo Ochoa’; Programme for Centres of Excellence in R&D (SEV-2015-0522), support by the Fundacio Cellex Barcelona, CERCA Programme/Generalitat de Catalunya and the Mineco grants Ramón y Cajal (RYC-2012-12281) and Plan Nacional (FIS2013-47161-P and FIS2014-59639-JIN). Furthermore, the research leading to these results received funding from the European Union Seventh Framework Programme under grant agreement no. 696656 Graphene Flagship, European Reasearch Council (ERC) Starting grant (307806, CarbonLight) and ERC Synergy Grant Hetero2D. K.S.T. acknowledges financial support from The Center for Nanostructured Graphene sponsored by the Danish National Research Foundation (Project DNRF103) and the ERC under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 773122, LIMA).
R.H. is cofounder of and on the scientific advisory board of Neaspec GmbH, a company that produces scattering-type near-field scanning optical microscope systems, such as the one used in this study. The remaining authors declare no competing interests.
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Schmidt, P., Vialla, F., Latini, S. et al. Nano-imaging of intersubband transitions in van der Waals quantum wells. Nature Nanotech 13, 1035–1041 (2018). https://doi.org/10.1038/s41565-018-0233-9
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