Matthew J. Berg
Condensed Matter Physics
Aerosols, i.e., particulates suspended in air, are ubiquitous and there is tremendous interest in characterizing their physical form and the manner in which they scatter light. For example, understanding climate change is perhaps the foremost scientific challenge, a major component of which is to quantify how aerosols contribute to the solar radiative forcing of the atmosphere. According to the 2013 Intergovernmental Panel on Climate Change, the estimated aerosol forcing is comparable to all other factors, including greenhouse gasses. Yet, the uncertainty in aerosol forcing is nearly as large as the forcing value itself, making their influence on climate change the least understood of the forcing effects. What is missing in this regard are accurate in situ observations of the real atmospheric particles. With such knowledge, the aerosol component of climate models could be vastly improved. Aerosol-particle sensing and characterization is also highly important, e.g., in defense contexts for the detection of biological weapons agents and in public-health applications where the sensing and tracking of aerosolized pathogens, such as bacteria, is greatly needed. Broadly, the development of new computational and experimental techniques utilizing electromagnetic (EM) scattering to enable these aerosol observations is the primary objective of my research program.
Microscopy may seem ideal for aerosol characterization, yet particles can fragment, aggregate, or become distorted upon collection; all leading to an inaccurate picture of the true aerosolized particle-morphology. In some cases, the morphology may even be completely destroyed as with liquid and frozen particles. Thus, only a contact-free, or in situ, technique will suffice, and EM scattering offers this capability. The way a particle scatters incident light into other directions forms a scattering pattern, which depends on the particle's morphology, composition, and orientation. Proper interpretation of a measured pattern can, in principle, be useful to infer the desired properties of an unknown particle. The problem is that no general unambiguous relationship between a measured pattern and a particle's characteristics exists, a difficulty that is known as the inverse problem.
My recent efforts have been aimed at overcoming this inverse problem using digital holography. This technique can provide an unambiguous image of a particle while retaining all of the in situ advantages of conventional EM scattering. First, a particle is illuminated with coherent light and the intensity pattern resulting from the interference of this light with that scattered by the particle is recorded digitally with an optoelectronic sensor. This pattern constitutes the hologram, from which an image of the particle is then reconstructed computationally. Holography is not only useful for imaging. Our group has recently developed new theory that relates optical observables to a particle's hologram. These observables include the scattering pattern, total cross sections, and single-scattering albedo, all of which are important to properly quantify a particle's role in radiative forcing.
- National Science Foundation (CAREER)
- US Army Research Office (YIP & DURIP)
- US Defense Threat Reduction Agency
R. Giri, C. Morello, Y. W. Heinson, O. Kemppinen, G. Videen, M. J. Berg, "Generation of aerosol-particle light-scattering patterns from digital holograms," Opt. Lett. 44(4) p. 819-822 (2019)
M. J. Berg, Y. W. Heinson, O. Kemppinen, "Imaging aerosols with digital holography," Physics Today, 71(3), p. 70-71. Republished in Japanese in Parity (パリティ), 33(9), pg. 38-40 (2018)
M. J. Berg, C. M. Sorensen, "A review and reassessment of diffraction, scattering, and shadows in electrodynamics," J. Quant. Spectrosc. Radiat. Transfer210, pg. 225-239 (2018).
M. J. Berg, Y. W. Heinson, O. Kemppinen, S. Holler, “Solving the inverse problem for coarse-mode aerosol particle morphology with digital holography” Nat. Sci. Rep.7:9400 (2017).
M. J. Berg, N. R. Subedi, P. A. Anderson, “Measuring extinction with digital holography: Nonspherical particles and experimental validation” Opt. Lett.42 (5), pg. 1011-1014 (2017)
M. J. Berg, C. M. Sorensen, and A. Chakrabarti, "A new explanation of the extinction paradox," J. Quant. Spectrosc. Radiat. Transfer112, pg. 1170-1181 (2011).