About

Michael Gehm is a Professor of Electrical and Computer Engineering at Duke University and serves as the Director of Graduate Studies in the Department of Electrical and Computer Engineering. He holds a secondary appointment in Physics and is a Fellow of both SPIE and Optica. His educational background includes a B.S. in Mechanical Engineering from Washington University in St. Louis, and both an M.S. and Ph.D. in Physics from Duke University. Gehm's research primarily focuses on computational and compressive sensing and measurement across various modalities, with particular emphasis on electromagnetic and optical systems ranging from RF to x-ray frequencies, as well as all forms of mass spectrometry. His side interests include optical physics, high-performance x-ray simulation, and rapid-prototyping for creating advanced electromagnetic structures. Throughout his career, he has held academic positions at Duke University and the University of Arizona, where he was promoted to Professor before returning to Duke. His contributions to the field have been recognized through his election as a Fellow of SPIE and Optica.

Research topics

• Physics
• Optics
• Chemistry
• Atomic physics
• Optoelectronics
• Chromatography
• Nanotechnology
• Acoustics
• Engineering
• Mechanical engineering
• Materials science

Selected publications

• A super-resolution coded aperture miniature mass spectrometer proof-of-concept for planetary science

  International Journal of Mass Spectrometry · 2024-11-10 · 2 citations

  article
  PublisherDOI

• Toward One‐Way Smoke: Synthesis of Copper‐Based Microclubs with Asymmetric Scattering and Absorption

  Advanced Functional Materials · 2024-03-17 · 2 citations

  articleOpen accessCorresponding

  Abstract The ultimate goal of this work is to create an engineered aerosol that acts as one‐way smoke, i.e. it creates an asymmetric vision environment in which the ability to image objects depends on the viewing direction. To this end, a rapid, one‐pot synthesis of copper‐based microclubs is developed that consists of a Cu 2 O octahedron attached to a Cu 2 O@Cu shaft. Millions of synthesized particles are analyzed in minutes with a FlowCam to provide a robust statistical analysis of their geometry, and rapidly elucidate the roles of the reaction constituents on the particle shape and yield. The combination of asymmetry in both shape and composition introduces a 30% difference in scattering of light propagating parallel to the microclub axis from opposing directions. This work represents a first step toward the creation of an asymmetric imaging environment with an aerosol consisting of acoustically aligned microclubs.

  PublisherOA PDFDOI

• Further development of asymmetric illumination x-ray differential phase contrast imaging

  2024-06-07

  articleSenior author

  X-ray phase contrast imaging, which measures the change in phase through an object rather than change in attenuation, excels at distinguishing between similarly attenuating materials when compared to conventional transmission imaging. Most methods of phase contrast imaging, however, are challenging to implement and are limited in application to smaller objects. Previously, we proposed an alternative x-ray phase contrast imaging method based on the asymmetric illumination approach to optical differential phase contrast imaging, which requires only the addition of a single anti-scatter grid. This anti-scatter grid acts as an angular filter that translates asymmetry in the Fourier domain into intensity differences on the detector. Previously, we presented an analytical framework for the method as well as simulation results showing that such a system should be able to obtain phase information. Here, we report on our progress as we continue to advance the experimental implementation and validation of the system. We determine the effect of anti-scatter grid design parameters on the system’s sensitivity to phase change in order to move towards fabrication of a customized anti-scatter grid that utilizes 3D printing capabilities.

  PublisherDOI

• A Super-Resolution Coded Aperture Miniature Mass Spectrometer Proof-of-Concept for Planetary Science

  SSRN Electronic Journal · 2024-01-01

  preprintOpen access
  PublisherDOI

• Spectral Reconstruction Improvement in a Cycloidal Coded-Aperture Mass Spectrometer

  Journal of the American Society for Mass Spectrometry · 2024-04-16 · 2 citations

  article

  Spatial aperture coding is a technique used to improve throughput without sacrificing resolution both in optical spectroscopy and sector mass spectrometry (MS). Previous work demonstrated that aperture coding combined with a position-sensitive array detector in a miniature cycloidal mass spectrometer was successful in providing high-throughput, high-resolution measurements. However, due to poor alignment and field nonuniformities, reconstruction artifacts were present. Recently, significant progress was made in eliminating most of the reconstruction artifacts with improved field uniformity and alignment. However, artifacts as large as 1/3 of the main peak were still observed at low mass (<17 u). Such artifacts will reduce accuracy in identification and quantification of analytes, reducing the impact of the throughput advantage gained by using a coded aperture. The artifacts were hypothesized to be a result of a mass dependent in curvature of ions in the ion source. Ions with higher mass (m/z > 17 u) and a larger curvature did not pass through all slits in the coded aperture. Therefore, when reconstructing with a system response derived from the aperture image from a higher mass m/z = 32 u ion, reconstruction artifacts appeared for m/z < 17 u. In this work, two methods were implemented to significantly reduce the presence of artifacts in reconstructed data. First, we modified the reconstruction algorithm to incorporate a mass-dependent system response function across the mass range (10–110 u). This method reduced the size of the artifacts by 82%. Second, to validate the hypothesis that the mass-dependent system response function was a result of differences in curvature of ions in the ion source, we modified the design of the ion source by shifting the coded aperture slits relative to the center of the ionization volume. This method resulted in ions of all masses passing through all slits in the coded aperture, a constant system response function across the entire mass range. Artifacts were reduced by 94%.

  PublisherDOI

• Spectrally responsive edge-illumination (SREI) x-ray phase contrast imaging (XPCI)

  2024-06-07

  articleSenior author

  As opposed to transmission imaging, X-ray Phase Contrast Imaging (XPCI) produces images with higher contrast and allows us to distinguish between materials that are weakly attenuating or between materials that possess similar attenuation values. Edge Illumination (EI), a type of XPCI, utilizes spatial variation to uncover information about an object’s phase properties, such as the index of refraction. Instead of spatial variation, we previously proposed an alternative EI method, Spectrally Responsive Edge Illumination (SREI), which relies on energy variation. Prior SREI experimental efforts struggled to meet the necessary component performance requirements, so, as an intermediate step, we are currently focused on developing an energy resolving x-ray refractometer and a related database of materials. In this paper we will share our theory and initial proof of concept experimental results, as well as our next steps.

  PublisherDOI

• Asymmetric imaging in the presence of a bright interferent

  Applied Optics · 2024-08-16 · 1 citations

  articleSenior author

  Given two aerosol plumes—one scattering and one absorbing—in the presence of a bright interferent, it has been shown that the image contrast ratio through the pair of plumes is different in the two opposing view directions [ Appl. Opt. 54 , 12 ( 2015 ) APOPAI 0003-6935 10.1364/AO.54.000012 ]. Here, we generalize those earlier findings to plumes with mixed absorbing and scattering properties. By using the path integral solution to the radiative transfer equation, we incorporate both path length dependence and redirection of light due to multiple scattering and allow for arbitrary interferent location. Within this expanded model, we demonstrate that these effects can significantly alter the asymmetry predicted by the previous work.

  PublisherDOI

• Design and solution-based testing of a synthetic aerosol particle with asymmetric scattering behavior

  2024-11-19 · 1 citations

  articleSenior author

  We have modeled, synthesized, and performed preliminary testing of a synthetic aerosol particle with the intent to affect an asymmetric (“one-way”) vision environment when deployed as an airborne plume and aligned in real time via an applied acoustic field. The first aerosol particle iteration under test, the microclub, features asymmetry in particle geometry and material composition to cause asymmetric scattering behavior, dependent on the propagation direction of incident mid-infrared light. Despite this asymmetric scattering behavior, the microclub has been shown to maintain electromagnetic reciprocity in computational simulations, exhibiting a consistent extinction cross section with respect to forward and backward propagation directions of incident light. We expect this asymmetric scattering behavior will ultimately cause vision asymmetry when deployed as an airborne plume incorporated into an imaging path. Before proceeding to in-air testing of the microclub, we have performed an intermediate investigation of the microclub while suspended in a solution of water and polyvinyl alcohol (PVA) to test both the particle’s rotation response to an applied acoustic field and the particle rotation’s impact on optical transmission. Here we present the results of this preliminary investigation and we discuss the impact on visibility and next steps.

  PublisherDOI

• A high resolution miniature electron energy spectrometer

  Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment · 2024-04-19 · 1 citations

  articleSenior authorCorresponding
  PublisherDOI

• Simulation and information-theoretic analysis of hybrid CT+XRD imaging system

  2023-06-14

  articleSenior author

  Simulation is a valuable tool for designing and evaluating the performance of x-ray imaging systems. In previous work, a hybrid CT+XRD imaging system was developed for improved identification of threat objects in checked baggage. Through large-scale simulations of this hybrid CT+XRD system, we can investigate the impact of various parameters on system performance. These parameters include varying energy resolution, multi-energy acquisitions, and additional system views. We will report on our findings and evaluate the system performance resulting from these and other variations of the simulated system as well as discuss how these findings may inform future system design.

  PublisherDOI

Recent grants

• Gradient-index mm-Wave/Terahertz Components via Polymer-jetting Rapid Prototyping

  NSF · $329k · 2009–2012

Frequent coauthors

• David J. Brady

  63 shared

• Hao Xin

  Zhengzhou University

  40 shared

• J. E. Thomas

  North Carolina State University

  34 shared

• K. M. O’Hara

  Pennsylvania State University

  29 shared

• Joel A. Greenberg

  28 shared

• Wei‐Ren Ng

  25 shared

• D. R. Golish

  Planetary Science Institute

  25 shared

• Esteban Vera

  Pontificia Universidad Católica de Valparaíso

  23 shared

Labs

• Duke Electrical & Computer EngineeringPI

Awards & honors

• Fellow. SPIE. 2022
• Fellow. Optica (formerly, OSA). 2020