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Brian O'Callahan

Dr. O'Callahan is a chemist in the Structural Biology team. His PhD research at the University of Colorado Boulder focused on using a novel super-resolution technique to achieve nanometer spatial resolution with infrared and optical spectroscopy. Since joining EMSL, he has extended the use of this technique to biological systems, specifically focusing on protein identification and location. Under a U.S. Department of Energy Biological and Environmental Research program bioimaging grant, he developed the first-of-its-kind in-liquid nanoscale Fourier transform infrared spectroscopy (nano-FTIR), which allows imaging of samples immersed in liquid with nanometer spatial resolution and chemical specificity.

Research Interests

Using infrared scattering-type scanning near-field optical microscopy (IR SNOM)/nano-FTIR to perform infrared spectroscopy with nanoscale spatial resolution and sub-ps time resolution. This allows chemical identification of biological systems on their relevant length and time scales.


  • PhD in Physics, University of Colorado Boulder, 2017
  • MS in Physics, University of Colorado Boulder, 2015
  • BS in Physics and Mathematics, University of Washington, 2011

Awards and Recognition

Recognition for Postdoc Appreciation Week 2019, PNNL



Bhattarai A., B.T. O'Callahan, C. Wang, S. Wang, and P.Z. El-Khoury. 2020. "Spatio-Spectral Characterization of Multipolar Plasmonic Modes of Au Nanorods via Tip-Enhanced Raman Scattering." Journal of Physical Chemistry Letters 11, no. 8:2870-2874. PNNL-SA-151304. doi:10.1021/acs.jpclett.0c00485

O'Callahan B.T., A. Bhattarai, Z. Schultz, and P.Z. El-Khoury. 2020. "Power-Dependent Dual Analyte Tip-Enhanced Raman Spectral Imaging." The Journal of Physical Chemistry C 124, no. 28:15454-15459. PNNL-SA-153512. doi:10.1021/acs.jpcc.0c05396

O'Callahan B.T., K. Park, I.V. Novikova, T. Jian, C. Chen, E.A. Muller, and P.Z. El-Khoury, et al. 2020. "In liquid infrared scattering scanning near-field optical microscopy for chemical and biological nanoimaging." Nano Letters 20, no. 6:4497-4504. PNNL-SA-152805. doi:10.1021/acs.nanolett.0c01291

Wang C., B.T. O'Callahan, D. Kurouski, A.V. Krayev, and P.Z. El-Khoury. 2020. "The Prevalence of Anions at Plasmonic Nanojunctions: A Closer Look at P-Nitrothiophenol." Journal of Physical Chemistry Letters 11, no. 10:3809-3814. PNNL-SA-152463. doi:10.1021/acs.jpclett.0c01006

Wang C., Z. Cheng, B.T. O'Callahan, K.T. Crampton, M.R. Jones, and P.Z. El-Khoury. 2020. "Tip-Enhanced Multipolar Raman Scattering." Journal of Physical Chemistry Letters 11, no. 7:2464-2469. PNNL-SA-151145. doi:10.1021/acs.jpclett.0c00559

Wang, C. F., B. T. O'Callahan, D. Kurouski, A. Krayev, Z. D. Schultz, and P. Z. El-Khoury. 2020. “Suppressing Molecular Charging, Nanochemistry, and Optical Rectification in the Tip-Enhanced Raman Geometry.” Journal of Physical Chemistry Letters 11 (15): 5890-5895.


O'Callahan B.T., M. Hentschel, M.B. Raschke, P.Z. El-Khoury, and A. Lea. 2019. "Ultrasensitive Tip- and Antenna-Enhanced Infrared Nanoscopy of Protein Complexes." Journal of Physical Chemistry C 123, no. 28:17505-17509. PNNL-SA-144661. doi:10.1021/acs.jpcc.9b05777


O'Callahan, B. T., J. Yan, F. Menges, E. A. Muller, and M. B. Raschke. 2018. “Photoinduced Tip-Sample Forces for Chemical Nanoimaging and Spectroscopy.” Nano Letters 18 (9): 5499-5505.

O'Callahan B.T., K.T. Crampton, I.V. Novikova, T. Jian, C. Chen, J.E. Evans, and M.M. Raschke, et al. 2018. "Imaging Nanoscale Heterogeneity in Ultrathin Biomimetic and Biological Crystals." Journal of Physical Chemistry C 122, no. 43:24891-24895. PNNL-SA-136203. doi:10.1021/acs.jpcc.8b06681