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)
  • 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
  • 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

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

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