James M Baxter

All Publications

• Coexistence of Incoherent and Ultrafast Coherent Exciton Transport in a Two-Dimensional Superatomic Semiconductor. The journal of physical chemistry letters Baxter, J. M., Koay, C. S., Xu, D., Cheng, S. W., Tulyagankhodjaev, J. A., Shih, P., Roy, X., Delor, M. 2023: 10249-10256

  Abstract

  Fully leveraging the remarkable properties of low-dimensional semiconductors requires developing a deep understanding of how their structure and disorder affect the flow of electronic energy. Here, we study exciton transport in single crystals of the two-dimensional superatomic semiconductor CsRe6Se8I3, which straddles a photophysically rich yet elusive intermediate electronic-coupling regime. Using femtosecond scattering microscopy to directly image exciton transport in CsRe6Se8I3, we reveal the rare coexistence of coherent and incoherent exciton transport, leading to either persistent or transient electronic delocalization depending on temperature. Notably, coherent excitons exhibit ballistic transport at speeds approaching an extraordinary 1600 km/s over 300 fs. Such fast transport is mediated by J-aggregate-like superradiance, owing to the anisotropic structure and long-range order of CsRe6Se8I3. Our results establish superatomic crystals as ideal platforms for studying the intermediate electronic-coupling regime in highly ordered environments, in this case displaying long-range electronic delocalization, ultrafast energy flow, and a tunable dual transport regime.

  View details for DOI 10.1021/acs.jpclett.3c02286

  View details for PubMedID 37938804

• Optical Imaging of Ultrafast Phonon-Polariton Propagation through an Excitonic Sensor. Nano letters Cheng, S. W., Xu, D., Su, H., Baxter, J. M., Holtzman, L. N., Watanabe, K., Taniguchi, T., Hone, J. C., Barmak, K., Delor, M. 2023

  Abstract

  Hexagonal boron nitride (hBN) hosts phonon polaritons (PhP), hybrid light-matter states that facilitate electromagnetic field confinement and exhibit long-range ballistic transport. Extracting the spatiotemporal dynamics of PhPs usually requires "tour de force" experimental methods such as ultrafast near-field infrared microscopy. Here, we leverage the remarkable environmental sensitivity of excitons in two-dimensional transition metal dichalcogenides to image PhP propagation in adjacent hBN slabs. Using ultrafast optical microscopy on monolayer WSe2/hBN heterostructures, we image propagating PhPs from 3.5 K to room temperature with subpicosecond and few-nanometer precision. Excitons in WSe2 act as transducers between visible light pulses and infrared PhPs, enabling visible-light imaging of PhP transport with far-field microscopy. We also report evidence of excitons in WSe2 copropagating with hBN PhPs over several micrometers. Our results provide new avenues for imaging polar excitations over a large frequency range with extreme spatiotemporal precision and new mechanisms to realize ballistic exciton transport at room temperature.

  View details for DOI 10.1021/acs.nanolett.3c02897

  View details for PubMedID 37852205

• Optical control of ultrafast structural dynamics in a fluorescent protein. Nature chemistry Hutchison, C. D., Baxter, J. M., Fitzpatrick, A., Dorlhiac, G., Fadini, A., Perrett, S., Maghlaoui, K., Lefevre, S. B., Cordon-Preciado, V., Ferreira, J. L., Chukhutsina, V. U., Garratt, D., Barnard, J., Galinis, G., Glencross, F., Morgan, R. M., Stockton, S., Taylor, B., Yuan, L., Romei, M. G., Lin, C., Marangos, J. P., Schmidt, M., Chatrchyan, V., Buckup, T., Morozov, D., Park, J., Park, S., Eom, I., Kim, M., Jang, D., Choi, H., Hyun, H., Park, G., Nango, E., Tanaka, R., Owada, S., Tono, K., DePonte, D. P., Carbajo, S., Seaberg, M., Aquila, A., Boutet, S., Barty, A., Iwata, S., Boxer, S. G., Groenhof, G., van Thor, J. J. 2023

  Abstract

  The photoisomerization reaction of a fluorescent protein chromophore occurs on the ultrafast timescale. The structural dynamics that result from femtosecond optical excitation have contributions from vibrational and electronic processes and from reaction dynamics that involve the crossing through a conical intersection. The creation and progression of the ultrafast structural dynamics strongly depends on optical and molecular parameters. When using X-ray crystallography as a probe of ultrafast dynamics, the origin of the observed nuclear motions is not known. Now, high-resolution pump-probe X-ray crystallography reveals complex sub-angstrom, ultrafast motions and hydrogen-bonding rearrangements in the active site of a fluorescent protein. However, we demonstrate that the measured motions are not part of the photoisomerization reaction but instead arise from impulsively driven coherent vibrational processes in the electronic ground state. A coherent-control experiment using a two-colour and two-pulse optical excitation strongly amplifies the X-ray crystallographic difference density, while it fully depletes the photoisomerization process. A coherent control mechanism was tested and confirmed the wave packets assignment.

  View details for DOI 10.1038/s41557-023-01275-1

  View details for PubMedID 37563326

• Ultrafast imaging of polariton propagation and interactions NATURE COMMUNICATIONS Xu, D., Mandal, A., Baxter, J. M., Cheng, S., Lee, I., Su, H., Liu, S., Reichman, D. R., Delor, M. 2023; 14 (1): 3881

  Abstract

  Semiconductor excitations can hybridize with cavity photons to form exciton-polaritons (EPs) with remarkable properties, including light-like energy flow combined with matter-like interactions. To fully harness these properties, EPs must retain ballistic, coherent transport despite matter-mediated interactions with lattice phonons. Here we develop a nonlinear momentum-resolved optical approach that directly images EPs in real space on femtosecond scales in a range of polaritonic architectures. We focus our analysis on EP propagation in layered halide perovskite microcavities. We reveal that EP-phonon interactions lead to a large renormalization of EP velocities at high excitonic fractions at room temperature. Despite these strong EP-phonon interactions, ballistic transport is maintained for up to half-exciton EPs, in agreement with quantum simulations of dynamic disorder shielding through light-matter hybridization. Above 50% excitonic character, rapid decoherence leads to diffusive transport. Our work provides a general framework to precisely balance EP coherence, velocity, and nonlinear interactions.

  View details for DOI 10.1038/s41467-023-39550-x

  View details for Web of Science ID 001022893500004

  View details for PubMedID 37391396

  View details for PubMedCentralID PMC10313693

• Observation of Cation Chromophore Photoisomerization of a Fluorescent Protein Using Millisecond Synchrotron Serial Crystallography and Infrared Vibrational and Visible Spectroscopy JOURNAL OF PHYSICAL CHEMISTRY B Baxter, J. M., Hutchison, C. M., Maghlaoui, K., Cordon-Preciado, V., Aller, P., Butryn, A., Axford, D., Horrell, S., Owen, R. L., Storm, S. S., Devenish, N. E., van Thor, J. J. 2022; 126 (45): 9288-9296

  Abstract

  The chromophores of reversibly switchable fluorescent proteins (rsFPs) undergo photoisomerization of both the trans and cis forms. Concurrent with cis/trans photoisomerisation, rsFPs typically become protonated on the phenolic oxygen resulting in a blue shift of the absorption. A synthetic rsFP referred to as rsEospa, derived from EosFP family, displays the same spectroscopic behavior as the GFP-like rsFP Dronpa at pH 8.4 and involves the photoconversion between nonfluorescent neutral and fluorescent anionic chromophore states. Millisecond time-resolved synchrotron serial crystallography of rsEospa at pH 8.4 shows that photoisomerization is accompanied by rearrangements of the same three residues as seen in Dronpa. However, at pH 5.5 we observe that the OFF state is identified as the cationic chromophore with additional protonation of the imidazolinone nitrogen which is concurrent with a newly formed hydrogen bond with the Glu212 carboxylate side chain. FTIR spectroscopy resolves the characteristic up-shifted carbonyl stretching frequency at 1713 cm-1 for the cationic species. Electronic spectroscopy furthermore distinguishes the cationic absorption band at 397 nm from the neutral species at pH 8.4 seen at 387 nm. The observation of photoisomerization of the cationic chromophore state demonstrates the conical intersection for the electronic configuration, where previously fluorescence was proposed to be the main decay route for states containing imidazolinone nitrogen protonation. We present the full time-resolved room-temperature X-ray crystallographic, FTIR, and UV/vis assignment and photoconversion modeling of rsEospa.

  View details for DOI 10.1021/acs.jpcb.2c06780

  View details for Web of Science ID 000884768900001

  View details for PubMedID 36326150

  View details for PubMedCentralID PMC9677427

• Light activation of Orange Carotenoid Protein reveals bicycle-pedal single-bond isomerization NATURE COMMUNICATIONS Chukhutsina, V. U., Baxter, J. M., Fadini, A., Morgan, R. M., Pope, M. A., Maghlaoui, K., Orr, C. M., Wagner, A., van Thor, J. J. 2022; 13 (1): 6420

  Abstract

  Orange Carotenoid protein (OCP) is the only known photoreceptor which uses carotenoid for its activation. It is found exclusively in cyanobacteria, where it functions to control light-harvesting of the photosynthetic machinery. However, the photochemical reactions and structural dynamics of this unique photosensing process are not yet resolved. We present time-resolved crystal structures at second-to-minute delays under bright illumination, capturing the early photoproduct and structures of the subsequent reaction intermediates. The first stable photoproduct shows concerted isomerization of C9'-C8' and C7'-C6' single bonds in the bicycle-pedal (s-BP) manner and structural changes in the N-terminal domain with minute timescale kinetics. These are followed by a thermally-driven recovery of the s-BP isomer to the dark state carotenoid configuration. Structural changes propagate to the C-terminal domain, resulting, at later time, in the H-bond rupture of the carotenoid keto group with protein residues. Solution FTIR and UV/Vis spectroscopy support the single bond isomerization of the carotenoid in the s-BP manner and subsequent thermal structural reactions as the basis of OCP photoreception.

  View details for DOI 10.1038/s41467-022-34137-4

  View details for Web of Science ID 000886265500028

  View details for PubMedID 36307413

  View details for PubMedCentralID PMC9616832

• The smfBox is an open-source platform for single-molecule FRET NATURE COMMUNICATIONS Ambrose, B., Baxter, J. M., Cully, J., Willmott, M., Steele, E. M., Bateman, B. C., Martin-Fernandez, M. L., Cadby, A., Shewring, J., Aaldering, M., Craggs, T. D. 2020; 11 (1): 5641

  Abstract

  Single-molecule Förster Resonance Energy Transfer (smFRET) is a powerful technique capable of resolving both relative and absolute distances within and between structurally dynamic biomolecules. High instrument costs, and a lack of open-source hardware and acquisition software have limited smFRET's broad application by non-specialists. Here, we present the smfBox, a cost-effective confocal smFRET platform, providing detailed build instructions, open-source acquisition software, and full validation, thereby democratising smFRET for the wider scientific community.

  View details for DOI 10.1038/s41467-020-19468-4

  View details for Web of Science ID 000591592300009

  View details for PubMedID 33159061

  View details for PubMedCentralID PMC7648814

• Democratizing Single-Molecule FRET: An Open-Source Microscope for Measuring Precise Distances and Biomolecular Dynamics Ambrose, B., Baxter, J., Cully, J., Willmott, M., Bateman, B. C., Steele, E., Cadby, A. J., Shewring, J., Aaldering, M., Craggs, T. D. CELL PRESS. 2020: 614A

  View details for Web of Science ID 000513023204074