Benjamin Ofori Okai

All Publications

• Ultrafast multi-cycle terahertz measurements of the electrical conductivity in strongly excited solids. Nature communications Chen, Z., Curry, C. B., Zhang, R., Treffert, F., Stojanovic, N., Toleikis, S., Pan, R., Gauthier, M., Zapolnova, E., Seipp, L. E., Weinmann, A., Mo, M. Z., Kim, J. B., Witte, B. B., Bajt, S., Usenko, S., Soufli, R., Pardini, T., Hau-Riege, S., Burcklen, C., Schein, J., Redmer, R., Tsui, Y. Y., Ofori-Okai, B. K., Glenzer, S. H. 2021; 12 (1): 1638

  Abstract

  Key insights in materials at extreme temperatures and pressures can be gained by accurate measurements that determine the electrical conductivity. Free-electron laser pulses can ionize and excite matter out of equilibrium on femtosecond time scales, modifying the electronic and ionic structures and enhancing electronic scattering properties. The transient evolution of the conductivity manifests the energy coupling from high temperature electrons to low temperature ions. Here we combine accelerator-based, high-brightness multi-cycle terahertz radiation with a single-shot electro-optic sampling technique to probe the evolution of DC electrical conductivity using terahertz transmission measurements on sub-picosecond time scales with a multi-undulator free electron laser. Our results allow the direct determination of the electron-electron and electron-ion scattering frequencies that are the major contributors of the electrical resistivity.

  View details for DOI 10.1038/s41467-021-21756-6

  View details for PubMedID 33712576

• Dynamics of a Persistent Insulator-to-Metal Transition in Strained Manganite Films PHYSICAL REVIEW LETTERS Teitelbaum, S. W., Ofori-Okai, B. K., Cheng, Y., Zhang, J., Jin, F., Wu, W., Averitt, R. D., Nelson, K. A. 2019; 123 (26)

  View details for DOI 10.1103/PhysRevLett.123.267201

  View details for Web of Science ID 000504869100008

• Toward quasi-DC conductivity of warm dense matter measured by single-shot terahertz spectroscopy Ofori-Okai, B. K., Descamps, A., Lu, J., Seipp, L. E., Weinmann, A., Glenzer, S. H., Chen, Z. AMER INST PHYSICS. 2018: 10D109

  Abstract

  We present an experimental setup capable of measuring the near DC conductivity of laser generated warm dense matter using single-shot terahertz time-domain spectroscopy. The setup uses a reflective echelon and balanced detection to record THz waveforms with a minimum detectable signal of 0.2% in a single laser pulse. We describe details of the experimental setup and the data analysis procedure and present single-shot terahertz transmission data on aluminum that has been laser heated to an electron temperature of 0.5 eV.

  View details for DOI 10.1063/1.5038944

  View details for Web of Science ID 000449144500044

  View details for PubMedID 30399773

• A terahertz pump mega-electron-volt ultrafast electron diffraction probe apparatus at the SLAC Accelerator Structure Test Area facility JOURNAL OF INSTRUMENTATION Ofori-Okai, B. K., Hoffmann, M. C., Reid, A. H., Edstrom, S., Jobe, R. K., Li, R. K., Mannebach, E. M., Park, S. J., Polzin, W., Shen, X., Weathersby, S. P., Yang, J., Zheng, Q., Zajac, M., Lindenberg, A. M., Glenzer, S. H., Wang, X. J. 2018; 13

  View details for DOI 10.1088/1748-0221/13/06/P06014

  View details for Web of Science ID 000435599800007

• THz generation using a reflective stair-step echelon OPTICS EXPRESS Ofori-Okai, B. K., Sivarajah, P., Huang, W., Nelson, K. A. 2016; 24 (5): 5057–68

  Abstract

  We present a novel method for THz generation in lithium niobate using a reflective stair-step echelon structure. The echelon produces a discretely tilted pulse front with less angular dispersion compared to a high groove-density grating. The THz output was characterized using both a 1-lens and 3-lens imaging system to set the tilt angle at room and cryogenic temperatures. Using broadband 800 nm pulses with a pulse energy of 0.95 mJ and a pulse duration of 70 fs (24 nm FWHM bandwidth, 39 fs transform limited width), we produced THz pulses with field strengths as high as 500 kV/cm and pulse energies as high as 3.1 μJ. The highest conversion efficiency we obtained was 0.33%. In addition, we find that the echelon is easily implemented into an experimental setup for quick alignment and optimization.

  View details for DOI 10.1364/OE.24.005057

  View details for Web of Science ID 000371435000072

  View details for PubMedID 29092334

• High deuteron and neutron yields from the interaction of a petawatt laser with a cryogenic deuterium jet FRONTIERS IN PHYSICS Jiao, X., Curry, C. B., Gauthier, M., Chou, H. J., Fiuza, F., Kim, J. B., Phan, D. D., McCary, E., Galtier, E. C., Dyer, G. M., Ofori-Okai, B. K., Labun, L., Labun, O. Z., Schoenwaelder, C., Roycroft, R., Tiwari, G., Glenn, G. D., Treffert, F., Glenzer, S. H., Hegelich, B. M. 2023; 10

  View details for DOI 10.3389/fphy.2022.964696

  View details for Web of Science ID 000921355400001

• Ambient-temperature liquid jet targets for high-repetition-rate HED discovery science PHYSICS OF PLASMAS Treffert, F., Glenn, G. D., Chou, H. J., Crissman, C., Curry, C. B., DePonte, D. P., Fiuza, F., Hartley, N. J., Ofori-Okai, B., Roth, M., Glenzer, S. H., Gauthier, M. 2022; 29 (12)

  View details for DOI 10.1063/5.0097857

  View details for Web of Science ID 000899004900003

• Diamond formation kinetics in shock-compressed C─H─O samples recorded by small-angle x-ray scattering and x-ray diffraction. Science advances He, Z., Rodel, M., Lutgert, J., Bergermann, A., Bethkenhagen, M., Chekrygina, D., Cowan, T. E., Descamps, A., French, M., Galtier, E., Gleason, A. E., Glenn, G. D., Glenzer, S. H., Inubushi, Y., Hartley, N. J., Hernandez, J., Heuser, B., Humphries, O. S., Kamimura, N., Katagiri, K., Khaghani, D., Lee, H. J., McBride, E. E., Miyanishi, K., Nagler, B., Ofori-Okai, B., Ozaki, N., Pandolfi, S., Qu, C., Ranjan, D., Redmer, R., Schoenwaelder, C., Schuster, A. K., Stevenson, M. G., Sueda, K., Togashi, T., Vinci, T., Voigt, K., Vorberger, J., Yabashi, M., Yabuuchi, T., Zinta, L. M., Ravasio, A., Kraus, D. 2022; 8 (35): eabo0617

  Abstract

  Extreme conditions inside ice giants such as Uranus and Neptune can result in peculiar chemistry and structural transitions, e.g., the precipitation of diamonds or superionic water, as so far experimentally observed only for pure C─H and H2O systems, respectively. Here, we investigate a stoichiometric mixture of C and H2O by shock-compressing polyethylene terephthalate (PET) plastics and performing in situ x-ray probing. We observe diamond formation at pressures between 72 ± 7 and 125 ± 13 GPa at temperatures ranging from ~3500 to ~6000 K. Combining x-ray diffraction and small-angle x-ray scattering, we access the kinetics of this exotic reaction. The observed demixing of C and H2O suggests that diamond precipitation inside the ice giants is enhanced by oxygen, which can lead to isolated water and thus the formation of superionic structures relevant to the planets' magnetic fields. Moreover, our measurements indicate a way of producing nanodiamonds by simple laser-driven shock compression of cheap PET plastics.

  View details for DOI 10.1126/sciadv.abo0617

  View details for PubMedID 36054354

• Towards performing high-resolution inelastic X-ray scattering measurements at hard X-ray free-electron lasers coupled with energetic laser drivers JOURNAL OF SYNCHROTRON RADIATION Descamps, A., Ofori-Okai, B. K., Baldwin, J. K., Chen, Z., Fletcher, L. B., Glenzer, S. H., Hartley, N. J., Hasting, J. B., Khaghani, D., Mo, M., Nagler, B., Recoules, Redmer, R., Schoerner, M., Sun, P., Wang, Y. Q., White, T. G., McBride, E. E. 2022; 29: 931-938

  Abstract

  High-resolution inelastic X-ray scattering is an established technique in the synchrotron community, used to investigate collective low-frequency responses of materials. When fielded at hard X-ray free-electron lasers (XFELs) and combined with high-intensity laser drivers, it becomes a promising technique for investigating matter at high temperatures and high pressures. This technique gives access to important thermodynamic properties of matter at extreme conditions, such as temperature, material sound speed, and viscosity. The successful realization of this method requires the acquisition of many identical laser-pump/X-ray-probe shots, allowing the collection of a sufficient number of photons necessary to perform quantitative analyses. Here, a 2.5-fold improvement in the energy resolution of the instrument relative to previous works at the Matter in Extreme Conditions (MEC) endstation, Linac Coherent Light Source (LCLS), and the High Energy Density (HED) instrument, European XFEL, is presented. Some aspects of the experimental design that are essential for improving the number of photons detected in each X-ray shot, making such measurements feasible, are discussed. A careful choice of the energy resolution, the X-ray beam mode provided by the XFEL, and the position of the analysers used in such experiments can provide a more than ten-fold improvement in the photometrics. The discussion is supported by experimental data on 10 µm-thick iron and 50 nm-thick gold samples collected at the MEC endstation at the LCLS, and by complementary ray-tracing simulations coupled with thermal diffuse scattering calculations.

  View details for DOI 10.1107/S1600577522004453

  View details for Web of Science ID 000824201700001

  View details for PubMedID 35787558

  View details for PubMedCentralID PMC9255572

• Investigation of hard x-ray emissions from terawatt laser-irradiated foils at the Matter in Extreme Conditions instrument of the Linac Coherent Light Source JOURNAL OF INSTRUMENTATION Fletcher, L. B., Curry, C. B., Gauthier, M., Glenn, G. D., Chen, Z., Cunningham, E., Descamps, A., Frost, M., Galtier, E. C., Heimann, P., Kim, J. B., Mo, M., Ofori-Okai, B. K., Peebles, J., Seiboth, F., Treffert, F., Dyer, G. M., McBride, E. E., Glenzer, S. H. 2022; 17 (4)

  View details for DOI 10.1088/1748-0221/17/04/T04004

  View details for Web of Science ID 000819866900015

• Ultrafast visualization of incipient plasticity in dynamically compressed matter. Nature communications Mo, M., Tang, M., Chen, Z., Peterson, J. R., Shen, X., Baldwin, J. K., Frost, M., Kozina, M., Reid, A., Wang, Y., E, J., Descamps, A., Ofori-Okai, B. K., Li, R., Luo, S., Wang, X., Glenzer, S. 2022; 13 (1): 1055

  Abstract

  Plasticity is ubiquitous and plays a critical role in material deformation and damage; it inherently involves the atomistic length scale and picosecond time scale. A fundamental understanding of the elastic-plastic deformation transition, in particular, incipient plasticity, has been a grand challenge in high-pressure and high-strain-rate environments, impeded largely by experimental limitations on spatial and temporal resolution. Here, we report femtosecond MeV electron diffraction measurements visualizing the three-dimensional (3D) response of single-crystal aluminum to the ultrafast laser-induced compression. We capture lattice transitioning from a purely elastic to a plastically relaxed state within 5 ps, after reaching an elastic limit of~25 GPa. Our results allow the direct determination of dislocation nucleation and transport that constitute the underlying defect kinetics of incipient plasticity. Large-scale molecular dynamics simulations show good agreement with the experiment and provide an atomic-level description of the dislocation-mediated plasticity.

  View details for DOI 10.1038/s41467-022-28684-z

  View details for PubMedID 35217665

• Observation of a highly conductive warm dense state of water with ultrafast pump-probe free-electron-laser measurements MATTER AND RADIATION AT EXTREMES Chen, Z., Na, X., Curry, C. B., Liang, S., French, M., Descamps, A., DePonte, D. P., Koralek, J. D., Kim, J. B., Lebovitz, S., Nakatsutsumi, M., Ofori-Okai, B. K., Redmer, R., Roedel, C., Schorner, M., Skruszewicz, S., Sperling, P., Toleikis, S., Mo, M. Z., Glenzer, S. H. 2021; 6 (5)

  View details for DOI 10.1063/5.0043726

  View details for Web of Science ID 000681018600001

• High-resolution inelastic x-ray scattering at the high energy density scientific instrument at the European X-Ray Free-Electron Laser (vol 92, 013101, 2021) REVIEW OF SCIENTIFIC INSTRUMENTS Wollenweber, L., Preston, T. R., Descamps, A., Cerantola, V., Comley, A., Eggert, J. H., Fletcher, L. B., Geloni, G., Gericke, D. O., Glenzer, S. H., Goede, S., Hastings, J., Humphries, O. S., Jenei, A., Karnbach, O., Konopkova, Z., Loetzsch, R., Marx-Glowna, B., McBride, E. E., McGonegle, D., Monaco, G., Ofori-Okai, B. K., Palmer, C. J., Plueckthun, C., Redmer, R., Strohm, C., Thorpe, I., Tschentscher, T., Uschmann, I., Wark, J. S., White, T. G., Appel, K., Gregori, G., Zastrau, U. 2021; 92 (3)

  View details for DOI 10.1063/5.0043951

  View details for Web of Science ID 000629698000001

• High-resolution inelastic x-ray scattering at the high energy density scientific instrument at the Free-Electron Laser REVIEW OF SCIENTIFIC INSTRUMENTS Wollenweber, L., Preston, T. R., Descamps, A., Cerantola, V., Comley, A., Eggert, J. H., Fletcher, L. B., Geloni, G., Gericke, D. O., Glenzer, S. H., Goede, S., Hastings, J., Humphries, O. S., Jenei, A., Karnbach, O., Konopkova, Z., Loetzsch, R., Marx-Glowna, B., McBride, E. E., McGonegle, D., Monaco, G., Ofori-Okai, B. K., Palmer, C. J., Plueckthun, C., Redmer, R., Strohm, C., Thorpe, I., Tschentscher, T., Uschmann, I., Wark, J. S., White, T. G., Appel, K., Gregori, G., Zastrau, U. 2021; 92 (1)

  View details for DOI 10.1063/5.0022886

  View details for Web of Science ID 000607199800001

• An approach for the measurement of the bulk temperature of single crystal diamond using an X-ray free electron laser. Scientific reports Descamps, A., Ofori-Okai, B. K., Appel, K., Cerantola, V., Comley, A., Eggert, J. H., Fletcher, L. B., Gericke, D. O., Gode, S., Humphries, O., Karnbach, O., Lazicki, A., Loetzsch, R., McGonegle, D., Palmer, C. A., Plueckthun, C., Preston, T. R., Redmer, R., Senesky, D. G., Strohm, C., Uschmann, I., White, T. G., Wollenweber, L., Monaco, G., Wark, J. S., Hastings, J. B., Zastrau, U., Gregori, G., Glenzer, S. H., McBride, E. E. 2020; 10 (1): 14564

  Abstract

  We present a method to determine the bulk temperature of a single crystal diamond sample at an X-Ray free electron laser using inelastic X-ray scattering. The experiment was performed at the high energy density instrument at the European XFEL GmbH, Germany. The technique, based on inelastic X-ray scattering and the principle of detailed balance, was demonstrated to give accurate temperature measurements, within [Formula: see text] for both room temperature diamond and heated diamond to 500 K. Here, the temperature was increased in a controlled way using a resistive heater to test theoretical predictions of the scaling of the signal with temperature. The method was tested by validating the energy of the phonon modes with previous measurements made at room temperature using inelastic X-ray scattering and neutron scattering techniques. This technique could be used to determine the bulk temperature in transient systems with a temporal resolution of 50fs and for which accurate measurements of thermodynamic properties are vital to build accurate equation of state and transport models.

  View details for DOI 10.1038/s41598-020-71350-x

  View details for PubMedID 32884061

• Macroscopic Ionic Flow in a Superionic Conductor Na+ beta-Alumina Driven by Single-Cycle Terahertz Pulses PHYSICAL REVIEW LETTERS Minami, Y., Ofori-Okai, B., Sivarajah, P., Katayama, I., Takeda, J., Nelson, K. A., Suemoto, T. 2020; 124 (14)

  View details for DOI 10.1103/PhysRevLett.124.147401

  View details for Web of Science ID 000525120400009

• Femtosecond Compression Dynamics and Timing Jitter Suppression in a THz-driven Electron Bunch Compressor PHYSICAL REVIEW LETTERS Snively, E. C., Othman, M. K., Kozina, M., Ofori-Okai, B. K., Weathersby, S. P., Park, S., Shen, X., Wang, X. J., Hoffmann, M. C., Li, R. K., Nanni, E. A. 2020; 124 (5)

  View details for DOI 10.1103/PhysRevLett.124.054801

  View details for Web of Science ID 000510844800008

• Analysis of terahertz generation by beamlet superposition OPTICS EXPRESS Ravi, K., Ofori-Okai, B. K., Nelson, K. A., Kaertner, F. X. 2019; 27 (19): 26547–68

  View details for DOI 10.1364/OE.27.026547

  View details for Web of Science ID 000486373100024

• Dynamical Slowing-Down in an Ultrafast Photoinduced Phase Transition. Physical review letters Zong, A., Dolgirev, P. E., Kogar, A., Ergeçen, E., Yilmaz, M. B., Bie, Y. Q., Rohwer, T., Tung, I. C., Straquadine, J., Wang, X., Yang, Y., Shen, X., Li, R., Yang, J., Park, S., Hoffmann, M. C., Ofori-Okai, B. K., Kozina, M. E., Wen, H., Wang, X., Fisher, I. R., Jarillo-Herrero, P., Gedik, N. 2019; 123 (9): 097601

  Abstract

  Complex systems, which consist of a large number of interacting constituents, often exhibit universal behavior near a phase transition. A slowdown of certain dynamical observables is one such recurring feature found in a vast array of contexts. This phenomenon, known as critical slowing-down, is well studied mostly in thermodynamic phase transitions. However, it is less understood in highly nonequilibrium settings, where the time it takes to traverse the phase boundary becomes comparable to the timescale of dynamical fluctuations. Using transient optical spectroscopy and femtosecond electron diffraction, we studied a photoinduced transition of a model charge-density-wave (CDW) compound LaTe_{3}. We observed that it takes the longest time to suppress the order parameter at the threshold photoexcitation density, where the CDW transiently vanishes. This finding can be captured by generalizing the time-dependent Landau theory to a system far from equilibrium. The experimental observation and theoretical understanding of dynamical slowing-down may offer insight into other general principles behind nonequilibrium phase transitions in many-body systems.

  View details for DOI 10.1103/PhysRevLett.123.097601

  View details for PubMedID 31524450

• Dynamical Slowing-Down in an Ultrafast Photoinduced Phase Transition PHYSICAL REVIEW LETTERS Zong, A., Dolgirev, P. E., Kogar, A., Ergecen, E., Yilmaz, M. B., Bie, Y., Rohwer, T., Tung, I., Straquadine, J., Wang, X., Yang, Y., Shen, X., Li, R., Yang, J., Park, S., Hoffmann, M. C., Ofori-Okai, B. K., Kozina, M. E., Wen, H., Wang, X., Fisher, I. R., Jarillo-Herrero, P., Gedik, N. 2019; 123 (9)

  View details for DOI 10.1103/PhysRevLett.123.097601

  View details for Web of Science ID 000483048500015

• Single-Shot Multi-Frame Imaging of Cylindrical Shock Waves in a Multi-Layered Assembly SCIENTIFIC REPORTS Dresselhaus-Cooper, L., Gorfain, J. E., Key, C. T., Ofori-Okai, B. K., Ali, S. J., Martynowych, D. J., Gleason, A., Kooi, S., Nelson, K. A. 2019; 9: 3689

  Abstract

  We demonstrate single-shot multi-frame imaging of quasi-2D cylindrically converging shock waves as they propagate through a multi-layer target sample assembly. We visualize the shock with sequences of up to 16 images, using a Fabry-Perot cavity to generate a pulse train that can be used in various imaging configurations. We employ multi-frame shadowgraph and dark-field imaging to measure the amplitude and phase of the light transmitted through the shocked target. Single-shot multi-frame imaging tracks geometric distortion and additional features in our images that were not previously resolvable in this experimental geometry. Analysis of our images, in combination with simulations, shows that the additional image features are formed by a coupled wave structure resulting from interface effects in our targets. This technique presents a new capability for tabletop imaging of shock waves that can be extended to experiments at large-scale facilities.

  View details for DOI 10.1038/s41598-019-40037-3

  View details for Web of Science ID 000460391500072

  View details for PubMedID 30842469

  View details for PubMedCentralID PMC6403302

• Terahertz-based subfemtosecond metrology of relativistic electron beams PHYSICAL REVIEW ACCELERATORS AND BEAMS Li, R. K., Hoffmann, M. C., Nanni, E. A., Glenzer, S. H., Kozina, M. E., Lindenberg, A. M., Ofori-Okai, B. K., Reid, A. H., Shen, X., Weathersby, S. P., Yang, J., Zajac, M., Wang, X. J. 2019; 22 (1)

  View details for DOI 10.1103/PhysRevAccelBeams.22.012803

  View details for Web of Science ID 000455066400001

• THz-Pump UED-Probe on a Topological Weyl Semimetal Sie, E. J., Nyby, C. M., Pemmaraju, C. D., Park, S., Shen, X., Yang, J., Hoffmann, M. C., Ofori-Okai, B. K., Li, R., Reid, A. H., Weathersby, S., Mannebach, E., Finney, N., Rhodes, D., Chenet, D., Antony, A., Balicas, L., Hone, J., Devereaux, T. P., Heinz, T. F., Wang, X., Lindenberg, A. M., IEEE IEEE. 2019

  View details for Web of Science ID 000482226301297

• An ultrafast symmetry switch in a Weyl semimetal. Nature Sie, E. J., Nyby, C. M., Pemmaraju, C. D., Park, S. J., Shen, X. n., Yang, J. n., Hoffmann, M. C., Ofori-Okai, B. K., Li, R. n., Reid, A. H., Weathersby, S. n., Mannebach, E. n., Finney, N. n., Rhodes, D. n., Chenet, D. n., Antony, A. n., Balicas, L. n., Hone, J. n., Devereaux, T. P., Heinz, T. F., Wang, X. n., Lindenberg, A. M. 2019; 565 (7737): 61–66

  Abstract

  Topological quantum materials exhibit fascinating properties1-3, with important applications for dissipationless electronics and fault-tolerant quantum computers4,5. Manipulating the topological invariants in these materials would allow the development of topological switching applications analogous to switching of transistors6. Lattice strain provides the most natural means of tuning these topological invariants because it directly modifies the electron-ion interactions and potentially alters the underlying crystalline symmetry on which the topological properties depend7-9. However, conventional means of applying strain through heteroepitaxial lattice mismatch10 and dislocations11 are not extendable to controllable time-varying protocols, which are required in transistors. Integration into a functional device requires the ability to go beyond the robust, topologically protected properties of materials and to manipulate the topology at high speeds. Here we use crystallographic measurements by relativistic electron diffraction to demonstrate that terahertz light pulses can be used to induce terahertz-frequency interlayer shear strain with large strain amplitude in the Weyl semimetal WTe2, leading to a topologically distinct metastable phase. Separate nonlinear optical measurements indicate that this transition is associated with a symmetry change to a centrosymmetric, topologically trivial phase. We further show that such shear strain provides an ultrafast, energy-efficient way of inducing robust, well separated Weyl points or of annihilating all Weyl points of opposite chirality. This work demonstrates possibilities for ultrafast manipulation of the topological properties of solids and for the development of a topological switch operating at terahertz frequencies.

  View details for PubMedID 30602749

• An Ultrafast Symmetry Switch in a Weyl Semimetal Nature Sie, E. J., et al 2019; 565, 61

  View details for DOI 10.1038/s41586-018-0809-4

• Setup for meV-resolution inelastic X-ray scattering measurements and X-ray diffraction at the Matter in Extreme Conditions endstation at the Linac Coherent Light Source (vol 89, 10F104, 2018) REVIEW OF SCIENTIFIC INSTRUMENTS McBride, E. E., White, T. G., Descamps, A., Fletcher, L. B., Appel, K., Condamine, F., Curry, C. B., Dallari, F., Funk, S., Galtier, E., Gamboa, E. J., Gauthier, M., Goede, S., Kim, J. B., Lee, H. J., Ofori-Okai, B. K., Oliver, M., Rigby, A., Schoenwaelder, C., Sun, P., Tschentscher, T., Witte, B. L., Zastrau, U., Gregori, G., Nagler, B., Hastings, J., Glenzer, S. H., Monaco, G. 2018; 89 (12): 129901

  View details for DOI 10.1063/1.5084054

  View details for Web of Science ID 000454631900095

  View details for PubMedID 30599579

• Setup for meV-resolution inelastic X-ray scattering measurements and X-ray diffraction at the Matter in Extreme Conditions endstation at the Linac Coherent Light Source McBride, E. E., White, T. G., Descamps, A., Fletcher, L. B., Appel, K., Condamine, F. P., Curry, C. B., Dallari, F., Funk, S., Galtier, E., Gauthier, M., Goede, S., Kim, J. B., Lee, H. J., Ofori-Okai, B. K., Oliver, M., Rigby, A., Schoenwaelder, C., Sun, P., Tschentscher, T., Witte, B. L., Zastrau, U., Gregori, G., Nagler, B., Hastings, J., Glenzer, S. H., Monaco, G. AMER INST PHYSICS. 2018: 10F104

  Abstract

  We describe a setup for performing inelastic X-ray scattering and X-ray diffraction measurements at the Matter in Extreme Conditions (MEC) endstation of the Linac Coherent Light Source. This technique is capable of performing high-, meV-resolution measurements of dynamic ion features in both crystalline and non-crystalline materials. A four-bounce silicon (533) monochromator was used in conjunction with three silicon (533) diced crystal analyzers to provide an energy resolution of ∼50 meV over a range of ∼500 meV in single shot measurements. In addition to the instrument resolution function, we demonstrate the measurement of longitudinal acoustic phonon modes in polycrystalline diamond. Furthermore, this setup may be combined with the high intensity laser drivers available at MEC to create warm dense matter and subsequently measure ion acoustic modes.

  View details for DOI 10.1063/1.5039329

  View details for Web of Science ID 000449144500093

  View details for PubMedID 30399942

• Two-Dimensional Spectroscopy at Terahertz Frequencies TOPICS IN CURRENT CHEMISTRY Lu, J., Li, X., Zhang, Y., Hwang, H. Y., Ofori-Okai, B. K., Nelson, K. A. 2018; 376 (1): 6

  Abstract

  Multidimensional spectroscopy in the visible and infrared spectral ranges has become a powerful technique to retrieve dynamic correlations and couplings in wide-ranging systems by utilizing multiple correlated light-matter interactions. Its extension to the terahertz (THz) regime of the electromagnetic spectrum, where rich material degrees of freedom reside, however, has been progressing slowly. This chapter reviews some of the THz-frequency two-dimensional (2D) spectroscopy techniques and experimental results realized in recent years. Examples include gas molecule rotations, spin precessions in magnetic systems, and liquid molecular dynamics studied by 2D THz or hybrid 2D THz-Raman spectroscopy techniques. The methodology shows promising applications to different THz-frequency degrees of freedom in various chemical systems and processes.

  View details for DOI 10.1007/s41061-018-0185-4

  View details for Web of Science ID 000426497900006

  View details for PubMedID 29362935

• Modeling of THz Pump Induced Plasmonic Oscillations in Silicon Membranes Wang, N., Nanni, E. A., Shen, X., Li, R. K., Hoffmann, M., Ofori-Okai, B. K., Zheng, Q., Yang, J., Wang, X. J., IEEE IEEE. 2018

  View details for Web of Science ID 000449683700106

• Rapid and precise determination of zero-field splittings by terahertz time-domain electron paramagnetic resonance spectroscopy CHEMICAL SCIENCE Lu, J., Ozel, I., Belvin, C. A., Li, X., Skorupskii, G., Sun, L., Ofori-Okai, B. K., Dinca, M., Gedik, N., Nelson, K. A. 2017; 8 (11): 7312–23

  Abstract

  Zero-field splitting (ZFS) parameters are fundamentally tied to the geometries of metal ion complexes. Despite their critical importance for understanding the magnetism and spectroscopy of metal complexes, they are not routinely available through general laboratory-based techniques, and are often inferred from magnetism data. Here we demonstrate a simple tabletop experimental approach that enables direct and reliable determination of ZFS parameters in the terahertz (THz) regime. We report time-domain measurements of electron paramagnetic resonance (EPR) signals associated with THz-frequency ZFSs in molecular complexes containing high-spin transition-metal ions. We measure the temporal profiles of the free-induction decays of spin resonances in the complexes at zero and nonzero external magnetic fields, and we derive the EPR spectra via numerical Fourier transformation of the time-domain signals. In most cases, absolute values of the ZFS parameters are extracted from the measured zero-field EPR frequencies, and the signs can be determined by zero-field measurements at two different temperatures. Field-dependent EPR measurements further allow refined determination of the ZFS parameters and access to the g-factor. The results show good agreement with those obtained by other methods. The simplicity of the method portends wide applicability in chemistry, biology and material science.

  View details for DOI 10.1039/c7sc00830a

  View details for Web of Science ID 000413532800003

  View details for PubMedID 29163882

  View details for PubMedCentralID PMC5672788

• Self-referenced single-shot THz detection OPTICS EXPRESS Russell, B. K., Ofori-Okai, B. K., Chen, Z., Hoffmann, M. C., Tsui, Y. Y., Glenzer, S. H. 2017; 25 (14): 16140–50

  Abstract

  We demonstrate a self-referencing method to reduce noise in a single-shot terahertz detection scheme. By splitting a single terahertz pulse and using a reflective echelon, both the signal and reference terahertz time-domain waveforms were measured using one laser pulse. Simultaneous acquisition of these waveforms significantly reduces noise originating from shot-to-shot fluctuations. We show that correlation function based referencing, which is not limited to polarization dependent measurements, can achieve a noise floor that is comparable to state-of-the-art polarization-gated balanced detection. Lastly, we extract the DC conductivity of a 30 nm free-standing gold film using a single THz pulse. The measured value of σ0 = 1.3 ± 0.4 × 107 S m-1 is in good agreement with the value measured by four-point probe, indicating the viability of this method for measuring dynamical changes and small signals.

  View details for DOI 10.1364/OE.25.016140

  View details for Web of Science ID 000407815100049

  View details for PubMedID 28789123

• Coherent Two-Dimensional Terahertz Magnetic Resonance Spectroscopy of Collective Spin Waves PHYSICAL REVIEW LETTERS Lu, J., Li, X., Hwang, H. Y., Ofori-Okai, B. K., Kurihara, T., Suemoto, T., Nelson, K. A. 2017; 118 (20): 207204

  Abstract

  We report a demonstration of two-dimensional (2D) terahertz (THz) magnetic resonance spectroscopy using the magnetic fields of two time-delayed THz pulses. We apply the methodology to directly reveal the nonlinear responses of collective spin waves (magnons) in a canted antiferromagnetic crystal. The 2D THz spectra show all of the third-order nonlinear magnon signals including magnon spin echoes, and 2-quantum signals that reveal pairwise correlations between magnons at the Brillouin zone center. We also observe second-order nonlinear magnon signals showing resonance-enhanced second-harmonic and difference-frequency generation. Numerical simulations of the spin dynamics reproduce all of the spectral features in excellent agreement with the experimental 2D THz spectra.

  View details for DOI 10.1103/PhysRevLett.118.207204

  View details for Web of Science ID 000401663500010

  View details for PubMedID 28581810

• Broadband terahertz generation with a stair-step echelon Ravi, K., Ofori-Okai, B. K., Sivarajah, P., Huang, W., Kaertner, F. X., Nelson, K. A., IEEE IEEE. 2017

  View details for Web of Science ID 000427296202478

• Nonlinear two-dimensional terahertz photon echo and rotational spectroscopy in the gas phase PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Lu, J., Zhang, Y., Hwang, H. Y., Ofori-Okai, B. K., Fleischer, S., Nelson, K. A. 2016; 113 (42): 11800-11805

  Abstract

  Ultrafast 2D spectroscopy uses correlated multiple light-matter interactions for retrieving dynamic features that may otherwise be hidden under the linear spectrum; its extension to the terahertz regime of the electromagnetic spectrum, where a rich variety of material degrees of freedom reside, remains an experimental challenge. We report a demonstration of ultrafast 2D terahertz spectroscopy of gas-phase molecular rotors at room temperature. Using time-delayed terahertz pulse pairs, we observe photon echoes and other nonlinear signals resulting from molecular dipole orientation induced by multiple terahertz field-dipole interactions. The nonlinear time domain orientation signals are mapped into the frequency domain in 2D rotational spectra that reveal J-state-resolved nonlinear rotational dynamics. The approach enables direct observation of correlated rotational transitions and may reveal rotational coupling and relaxation pathways in the ground electronic and vibrational state.

  View details for DOI 10.1073/pnas.1609558113

  View details for Web of Science ID 000385610400060

  View details for PubMedID 27702903

• Transient terahertz photoconductivity measurements of minority-carrier lifetime in tin sulfide thin films: Advanced metrology for an early stage photovoltaic material (vol 119, 035101, 2016) JOURNAL OF APPLIED PHYSICS Jaramillo, R., Sher, M., Ofori-Okai, B. K., Steinmann, V., Yang, C., Hartman, K., Nelson, K. A., Lindenberg, A. M., Gordon, R. G., Buonassisi, T. 2016; 119 (24)

  View details for DOI 10.1063/1.4954931

  View details for Web of Science ID 000379163800063

• What is the Brillouin zone of an anisotropic photonic crystal? PHYSICAL REVIEW B Sivarajah, P., Maznev, A. A., Ofori-Okai, B. K., Nelson, K. A. 2016; 93 (5)

  View details for DOI 10.1103/PhysRevB.93.054204

  View details for Web of Science ID 000369726400002

• The impact of sodium contamination in tin sulfide thin-film solar cells APL MATERIALS Steinmann, V., Brandt, R. E., Chakraborty, R., Jaramillo, R., Young, M., Ofori-Okai, B. K., Yang, C., Polizzotti, A., Nelson, K. A., Gordon, R. G., Buonassisi, T. 2016; 4 (2)

  View details for DOI 10.1063/1.4941713

  View details for Web of Science ID 000371814500005

• Transient terahertz photoconductivity measurements of minority-carrier lifetime in tin sulfide thin films: Advanced metrology for an early stage photovoltaic material JOURNAL OF APPLIED PHYSICS Jaramillo, R., Sher, M., Ofori-Okai, B. K., Steinmann, V., Yang, C., Hartman, K., Nelson, K. A., Lindenberg, A. M., Gordon, R. G., Buonassisi, T. 2016; 119 (3)

  View details for DOI 10.1063/1.4940157

  View details for Web of Science ID 000369287900019

• Improvement of minority-carrier lifetime in tin monosulfide via substrate engineering Chakraborty, R., Steinmann, V., Hempel, M., Rekemeyer, P., Ofori-Okai, B. K., Hartman, K., Youssef, A., Akey, A., Nelson, K. A., Gradecak, S., Kong, J., Buonassisi, T., IEEE IEEE. 2016: 415–18

  View details for Web of Science ID 000399818700088

• Circumventing Limitations of Tilted-pulse-front Terahertz Generation Using a Stair-step Echelon Ravi, K., Ofori-Okai, B. K., Sivarajah, P., Huang, W., Kartner, F. X., Nelson, K. A., IEEE IEEE. 2016: 3917–18

  View details for Web of Science ID 000400013903163

• Visualization of guided and leaky wave behaviors in an indium tin oxide metallic slab waveguide OPTICS EXPRESS Teo, S. M., Werley, C. A., Wang, C., Fan, K., Ofori-Okai, B. K., Zhang, X., Averitt, R. D., Nelson, K. A. 2015; 23 (11): 14876–96

  Abstract

  We explored the use of the optically transparent semiconductor indium tin oxide (ITO) as an alternative to optically opaque metals for the fabrication of photonic structures in terahertz (THz) near-field studies. Using the polaritonics platform, we confirmed the ability to clearly image both bound and leaky electric fields underneath an ITO layer. We observed good agreement between measured waveguide dispersion and analytical theory of an asymmetric metal-clad planar waveguide with TE and TM polarizations. Further characterization of the ITO revealed that even moderately conductive samples provided sufficiently high quality factors for studying guided and leaky wave behaviors in individual transparent THz resonant structures such as antennas or split ring resonators. However, without higher conductive ITO, the limited reflection efficiency and high radiation damping measured here both diminish the applicability of ITO for high-reflecting, arrayed, or long path-length elements.

  View details for DOI 10.1364/OE.23.014876

  View details for Web of Science ID 000356902400136

  View details for PubMedID 26072845

• Invited Article: Single-shot THz detection techniques optimized for multidimensional THz spectroscopy REVIEW OF SCIENTIFIC INSTRUMENTS Teo, S. M., Ofori-Okai, B. K., Werley, C. A., Nelson, K. A. 2015; 86 (5): 051301

  Abstract

  Multidimensional spectroscopy at visible and infrared frequencies has opened a window into the transfer of energy and quantum coherences at ultrafast time scales. For these measurements to be performed in a manageable amount of time, one spectral axis is typically recorded in a single laser shot. An analogous rapid-scanning capability for THz measurements will unlock the multidimensional toolkit in this frequency range. Here, we first review the merits of existing single-shot THz schemes and discuss their potential in multidimensional THz spectroscopy. We then introduce improved experimental designs and noise suppression techniques for the two most promising methods: frequency-to-time encoding with linear spectral interferometry and angle-to-time encoding with dual echelons. Both methods, each using electro-optic detection in the linear regime, were able to reproduce the THz temporal waveform acquired with a traditional scanning delay line. Although spectral interferometry had mediocre performance in terms of signal-to-noise, the dual echelon method was easily implemented and achieved the same level of signal-to-noise as the scanning delay line in only 4.5% of the laser pulses otherwise required (or 22 times faster). This reduction in acquisition time will compress day-long scans to hours and hence provides a practical technique for multidimensional THz measurements.

  View details for DOI 10.1063/1.4921389

  View details for Web of Science ID 000355923700001

  View details for PubMedID 26026507

• The homogenization limit and waveguide gradient index devices demonstrated through direct visualization of THz fields NEW JOURNAL OF PHYSICS Sivarajah, P., Ofori-Okai, B. K., Teo, S. M., Werley, C. A., Nelson, K. A. 2015; 17

  View details for DOI 10.1088/1367-2630/17/1/013013

  View details for Web of Science ID 000348758000013

• Direct experimental visualization of waves and band structure in 2D photonic crystal slabs NEW JOURNAL OF PHYSICS Ofori-Okai, B. K., Sivarajah, P., Werley, C. A., Teo, S. M., Nelson, K. A. 2014; 16

  View details for DOI 10.1088/1367-2630/16/5/053003

  View details for Web of Science ID 000335383800003

• Imaging of Terahertz Fields and Responses Ofori-Okai, B. K., Sivarajah, P., Teo, S. M., Werley, C. A., Nelson, K. A., Liu, Z., Khoo, I. C., Psaltis, D. SPIE-INT SOC OPTICAL ENGINEERING. 2014

  View details for DOI 10.1117/12.2063350

  View details for Web of Science ID 000344551000023

• Chemically assisted femtosecond laser machining for applications in LiNbO3 and LiTaO3 APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING Sivarajah, P., Werley, C. A., Ofori-Okai, B. K., Nelson, K. A. 2013; 112 (3): 615–22

  View details for DOI 10.1007/s00339-013-7833-x

  View details for Web of Science ID 000322669000014

• High-Resolution, Low-Noise Imaging in THz Polaritonics IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY Werley, C. A., Teo, S. M., Ofori-Okai, B. K., Sivarajah, P., Nelson, K. A. 2013; 3 (3): 239–47

  View details for DOI 10.1109/TTHZ.2013.2250580

  View details for Web of Science ID 000318771300005

• Spin properties of very shallow nitrogen vacancy defects in diamond PHYSICAL REVIEW B Ofori-Okai, B. K., Pezzagna, S., Chang, K., Loretz, M., Schirhagl, R., Tao, Y., Moores, B. A., Groot-Berning, K., Meijer, J., Degen, C. L. 2012; 86 (8)

  View details for DOI 10.1103/PhysRevB.86.081406

  View details for Web of Science ID 000307441900005