All Publications

  • Tunable Phonon Polariton Hybridization in a van der Waals Hetero-Bicrystal. Advanced materials (Deerfield Beach, Fla.) Wehmeier, L., Yu, S. J., Chen, X., Mayer, R. A., Xiong, L., Yao, H., Jiang, Y., Hu, J., Janzen, E., Edgar, J. H., Zheng, X., Heinz, T. F., Basov, D. N., Homes, C. C., Hu, G., Carr, G. L., Liu, M., Fan, J. A. 2024: e2401349


    Phonon polaritons, the hybrid quasiparticles resulting from the coupling of photons and lattice vibrations, have gained significant attention in the field of layered van der Waals heterostructures. Particular interest has been paid to hetero-bicrystals composed of molybdenum oxide (MoO3) and hexagonal boron nitride (hBN), which feature polariton dispersion tailorable via avoided polariton mode crossings. In this work, we systematically study the polariton eigenmodes in MoO3-hBN hetero-bicrystals self-assembled on ultrasmooth gold using synchrotron infrared nanospectroscopy. We experimentally demonstrate that the spectral gap in bicrystal dispersion and corresponding regimes of negative refraction can be tuned by material layer thickness, and we quantitatively match these results with a simple analytic model. We also investigate polaritonic cavity modes and polariton propagation along "forbidden" directions in our microscale bicrystals, which arise from the finite in-plane dimension of the synthesized MoO3 micro-ribbons. Our findings shed light on the unique dispersion properties of polaritons in van der Waals heterostructures and pave the way for applications leveraging deeply sub-wavelength mid-infrared light matter interactions. This article is protected by copyright. All rights reserved.

    View details for DOI 10.1002/adma.202401349

    View details for PubMedID 38657644

  • Quantum control of exciton wave functions in 2D semiconductors. Science advances Hu, J., Lorchat, E., Chen, X., Watanabe, K., Taniguchi, T., Heinz, T. F., Murthy, P. A., Chervy, T. 2024; 10 (12): eadk6369


    Excitons-bound electron-hole pairs-play a central role in light-matter interaction phenomena and are crucial for wide-ranging applications from light harvesting and generation to quantum information processing. A long-standing challenge in solid-state optics has been to achieve precise and scalable control over excitonic motion. We present a technique using nanostructured gate electrodes to create tailored potential landscapes for excitons in 2D semiconductors, enabling in situ wave function shaping at the nanoscale. Our approach forms electrostatic traps for excitons in various geometries, such as quantum dots, rings, and arrays thereof. We show independent spectral tuning of spatially separated quantum dots, achieving degeneracy despite material disorder. Owing to the strong light-matter coupling of excitons in 2D semiconductors, we observe unambiguous signatures of confined exciton wave functions in optical reflection and photoluminescence measurements. This work unlocks possibilities for engineering exciton dynamics and interactions at the nanometer scale, with implications for optoelectronic devices, topological photonics, and quantum nonlinear optics.

    View details for DOI 10.1126/sciadv.adk6369

    View details for PubMedID 38507493

  • Millimeter-Scale Exfoliation of hBN with Tunable Flake Thickness for Scalable Encapsulation ACS APPLIED NANO MATERIALS McKeown-Green, A. S., Zeng, H. J., Saunders, A. P., Li, J., Shi, J., Shen, Y., Pan, F., Hu, J., Dionne, J. A., Heinz, T. F., Wu, S. M., Zheng, F., Liu, F. 2024
  • Valley-Coherent Quantum Anomalous Hall State in AB-Stacked MoTe2/WSe2 Bilayers PHYSICAL REVIEW X Tao, Z., Shen, B., Jiang, S., Li, T., Li, L., Ma, L., Zhao, W., Hu, J., Pistunova, K., Watanabe, K., Taniguchi, T., Heinz, T. F., Mak, K., Shan, J. 2024; 14 (1)
  • Moiré-Assisted Strain Transfer in Vertical van der Waals Heterostructures. Nano letters Hu, J., Yu, L., Chen, X., Lee, W., Mate, C. M., Heinz, T. F. 2023


    Strain provides a powerful method to study 2D monolayers and to tune their properties. The same approach also has great potential for van-der-Waals (vdW) heterostructures. However, we need to understand how strain can be applied to vertically stacked vdW structures, for which strain transfer from one layer to the next remains little explored. In our experiment, we fabricated vertical heterostructures consisting of transition metal dichalcogenides (TMDCs) monolayers that were deposited on a flexible substrate. These TMDC heterostructures allowed us to read out separately the strain in each monolayer by photoluminescence measurements. We find that, in TMDC heterostructures with large twist angles (>5°), strain transfer is limited. However, for aligned heterostructures with small twist angles (≤5°), near unity strain transfer efficiency is observed. We correlate this finding with the moiré domains formed in the aligned heterostructures by reconstruction.

    View details for DOI 10.1021/acs.nanolett.3c03388

    View details for PubMedID 37903015

  • Observation of quadrupolar and dipolar excitons in a semiconductor heterotrilayer. Nature materials Yu, L., Pistunova, K., Hu, J., Watanabe, K., Taniguchi, T., Heinz, T. F. 2023


    Van der Waals (vdW) materials have opened up many avenues for discovery through layer assembly, as epitomized by interlayer dipolar excitons that exhibit electrically tunable luminescence, lasing and exciton condensation. Extending interlayer excitons to more vdW layers, however, raises fundamental questions concerning coherence within excitons and coupling between moiré superlattices at multiple interfaces. Here, by assembling angle-aligned WSe2/WS2/WSe2 heterotrilayers, we demonstrate the emergence of quadrupolar excitons. We confirm the exciton's quadrupolar nature by the decrease in its energy of 12 meV from coherent hole tunnelling between the two outer layers, its tunable static dipole moment under an external electric field and the reduced exciton-exciton interactions. At high exciton density, we also see signatures of a phase of oppositely aligned dipolar excitons, consistent with a staggered dipolar phase predicted to be driven by attractive dipolar interactions. Our demonstration paves the way for discovering emergent exciton orderings for three vdW layers and beyond.

    View details for DOI 10.1038/s41563-023-01678-y

    View details for PubMedID 37857888

  • The Reststrahlen Effect in the Optically Thin Limit: A Framework for Resonant Response in Thin Media. Nano letters Ma, E. Y., Hu, J., Waldecker, L., Watanabe, K., Taniguchi, T., Liu, F., Heinz, T. F. 2022


    Sharp resonances can strongly modify the electromagnetic response of matter. A classic example is the Reststrahlen effect - high reflectivity in the mid-infrared in many polar crystals near their optical phonon resonances. Although this effect in bulk materials has been studied extensively, a systematic treatment for finite thickness remains challenging. Here we describe, experimentally and theoretically, the Reststrahlen response in hexagonal boron nitride across more than 5 orders of magnitude in thickness, down to a monolayer. We find that the high reflectivity plateau of the Reststrahlen band evolves into a single peak as the material enters the optically thin limit, within which two distinct regimes emerge: a strong-response regime dominated by coherent radiative decay and a weak-response regime dominated by damping. We show that this evolution can be explained by a simple two-dimensional sheet model that can be applied to a wide range of thin media.

    View details for DOI 10.1021/acs.nanolett.2c02819

    View details for PubMedID 36112673

  • Structure of the moire exciton captured by imaging its electron and hole. Nature Karni, O., Barre, E., Pareek, V., Georgaras, J. D., Man, M. K., Sahoo, C., Bacon, D. R., Zhu, X., Ribeiro, H. B., O'Beirne, A. L., Hu, J., Al-Mahboob, A., Abdelrasoul, M. M., Chan, N. S., Karmakar, A., Winchester, A. J., Kim, B., Watanabe, K., Taniguchi, T., Barmak, K., Madeo, J., da Jornada, F. H., Heinz, T. F., Dani, K. M. 2022; 603 (7900): 247-252


    Interlayer excitons (ILXs) - electron-hole pairs bound across two atomically thin layered semiconductors - have emerged as attractive platforms to study exciton condensation1-4, single-photon emission and other quantum information applications5-7. Yet, despite extensive optical spectroscopic investigations8-12, critical information about their size, valley configuration and the influence of the moire potential remains unknown. Here, in a WSe2/MoS2 heterostructure, we captured images of the time-resolved and momentum-resolved distribution of both of the particles that bind to form the ILX: the electron and the hole. We thereby obtain a direct measurement of both the ILX diameter of around 5.2nm, comparable with the moire-unit-cell length of 6.1nm, and the localization of its centre of mass. Surprisingly, this large ILX is found pinned to a region of only 1.8nm diameter within the moire cell, smaller than the size of the exciton itself. This high degree of localization of the ILX is backed by Bethe-Salpeter equation calculations and demonstrates that the ILX can be localized within small moire unit cells. Unlike large moire cells, these are uniform over large regions, allowing the formation of extended arrays of localized excitations for quantum technology.

    View details for DOI 10.1038/s41586-021-04360-y

    View details for PubMedID 35264760