Johnathan Georgaras

All Publications

• 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

  Abstract

  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

• Tailoring Phonon-Driven Responses in α-MoO3 through Isotopic Enrichment. Advanced materials (Deerfield Beach, Fla.) Arnaud, T. S., Spangler, R. W., Georgaras, J. D., Haber, J. B., Hirt, D., Obst, M., Álvarez-Pérez, G., Iii, M. L., Kaps, F. G., Wetzel, J., Ragle, C., Buchner, J. E., Kim, Y., Senarath, A. S., Niemann, R., He, M., Carini, G., Arregui-Leon, U., Behera, A. C., Bangari, R., Sahoo, N., Brumby, N. C., Klopf, J. M., Wolf, M., Eng, L. M., Kehr, S. C., Folland, T. G., Paarmann, A., Hopkins, P. E., Jornada, F., Maria, J. P., Caldwell, J. D. 2026: e73629

  Abstract

  The implementation of polaritonic materials into nanoscale devices requires selective tuning of parameters to realize desired spectral or thermal responses. One robust material, α-MoO3, an orthorhombic crystal boasting three distinct phonon dispersions, provides three polaritonic dispersions of hyperbolic phonon polaritons (HPhPs) across the mid-infrared (MIR). Here, the tunability of both optical and thermal responses in isotopically enriched α-MoO3 (98MoO3, Mo18O3, and 98Mo18O3) is explored. A uniform ∼5% spectral redshift from 18O enrichment is observed in both Raman- and IR-active TO phonons. Both the in- and out-of-plane thermal conductivities for the isotopic variations are reported. Ab initio calculations both replicate experimental findings and analyze the select-mode three-phonon scattering contributions. The HPhPs from each isotopic variation are probed with s-SNOM, and we report an HPhP Q-factor maxima increase in 98Mo18O3 of ∼50% along the [100] in the RB2 and ∼100% along the [001] in the RB3 with respect to 98MoO3. Observations in both real and Fourier space of higher-order HPhP modes propagating in slabs of isotopically enriched α-MoO3 without the use of a subdiffractional surface scatterer are presented here. This work establishes the dual-element isotope enrichment of α-MoO3 as an intrinsic strategy to design optical, thermal, and polaritonic properties.

  View details for DOI 10.1002/adma.73629

  View details for PubMedID 42273746

• Hidden phonon highways promote photoinduced interlayer energy transfer in twisted transition metal dichalcogenide heterostructures. Science advances Johnson, A. C., Georgaras, J. D., Shen, X., Yao, H., Saunders, A. P., Zeng, H. J., Kim, H., Sood, A., Heinz, T. F., Lindenberg, A. M., Luo, D., da Jornada, F. H., Liu, F. 2024; 10 (4): eadj8819

  Abstract

  Vertically stacked van der Waals (vdW) heterostructures exhibit unique electronic, optical, and thermal properties that can be manipulated by twist-angle engineering. However, the weak phononic coupling at a bilayer interface imposes a fundamental thermal bottleneck for future two-dimensional devices. Using ultrafast electron diffraction, we directly investigated photoinduced nonequilibrium phonon dynamics in MoS2/WS2 at 4° twist angle and WSe2/MoSe2 heterobilayers with twist angles of 7°, 16°, and 25°. We identified an interlayer heat transfer channel with a characteristic timescale of ~20 picoseconds, about one order of magnitude faster than molecular dynamics simulations assuming initial intralayer thermalization. Atomistic calculations involving phonon-phonon scattering suggest that this process originates from the nonthermal phonon population following the initial interlayer charge transfer and scattering. Our findings present an avenue for thermal management in vdW heterostructures by tailoring nonequilibrium phonon populations.

  View details for DOI 10.1126/sciadv.adj8819

  View details for PubMedID 38266081

• Bidirectional phonon emission in two-dimensional heterostructures triggered by ultrafast charge transfer. Nature nanotechnology Sood, A., Haber, J. B., Carlström, J., Peterson, E. A., Barre, E., Georgaras, J. D., Reid, A. H., Shen, X., Zajac, M. E., Regan, E. C., Yang, J., Taniguchi, T., Watanabe, K., Wang, F., Wang, X., Neaton, J. B., Heinz, T. F., Lindenberg, A. M., da Jornada, F. H., Raja, A. 2022

  Abstract

  Photoinduced charge transfer in van der Waals heterostructures occurs on the 100 fs timescale despite weak interlayer coupling and momentum mismatch. However, little is understood about the microscopic mechanism behind this ultrafast process and the role of the lattice in mediating it. Here, using femtosecond electron diffraction, we directly visualize lattice dynamics in photoexcited heterostructures of WSe2/WS2 monolayers. Following the selective excitation of WSe2, we measure the concurrent heating of both WSe2 and WS2 on a picosecond timescale-an observation that is not explained by phonon transport across the interface. Using first-principles calculations, we identify a fast channel involving an electronic state hybridized across the heterostructure, enabling phonon-assisted interlayer transfer of photoexcited electrons. Phonons are emitted in both layers on the femtosecond timescale via this channel, consistent with the simultaneous lattice heating observed experimentally. Taken together, our work indicates strong electron-phonon coupling via layer-hybridized electronic states-a novel route to control energy transport across atomic junctions.

  View details for DOI 10.1038/s41565-022-01253-7

  View details for PubMedID 36543882