Recent research


Broadly, my research interests involve experimental and theoretical electromagnetic (EM) scattering applied to the characterization of man-made and naturally occurring aerosol particles. Through my work in this area, I have pioneered a digital holographic method to image single particles on-the-fly in an aerosol stream. The success of this work has spurred a new large-scale study to characterize atmospheric coarse-mode aerosol (CMA) particle morphology. Consisting primarily of mineral dust and biological particles, CMAs are an important component of the global aerosol and quantitative knowledge of the particle morphology is greatly needed. As the absence of such knowledge accounts for major uncertainties in global climate models, this work has promise to substantially improve our understanding for aerosols' role in climate change. Below are descriptions of ongoing research efforts that are expanding our investigative abilities of aerosol particles.  


Digital holographic imaging of aerosol particles


Aerosols are ubiquitous and there is tremendous interest in characterizing their physical form and light-scattering behavior. 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 [1-3]. According to the 2013 Intergovernmental Panel on Climate Change (IPCC), the estimated aerosol forcing is comparable to all other factors, including greenhouse gasses [1-4]. 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 effects [3,5-7]. What is missing in this regard are accurate in situ observations of the real atmospheric particles, and with such knowledge, the aerosol component of climate models could be vastly improved [8]. The development of new computational and experimental techniques to enable these observations is the primary objective of my research program. 

Traditional microscopy may seem ideal for particle characterization, yet it often gives an inaccurate picture of true particle-morphology due to the inherent need to collect the particles. In such cases, particles can fragment, aggregate, or become distorted; all leading to an inaccurate picture of the aerosolized particle-morphology. In some cases, the morphology may be completely destroyed as with liquid and frozen particles. Thus, only a contact-free, or in situ, imaging technique will suffice. Electromagnetic 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 [9]. 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 research efforts have been aimed at overcoming this inverse problem using digital holography. This technique can provide an unambiguous image of a particle while retaining the in situ advantages of conventional EM scattering. Fundamentally, this is a two-step process: 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. This pattern constitutes the hologram, from which an image of the particle is reconstructed.  Traditionally, holograms are recorded with photographic film due to the film’s high resolution, which is required to capture the finer features of the interference pattern. The subsequent chemical development of the film is costly and time consuming, and this greatly limits the practical utility of the technique. For this reason, Charged Coupled Device (CCD) detectors are now used to record the hologram pattern. The resulting "digital hologram" can then be computationally processed, rather than chemically, to reconstruct an image of the particle. Yet, holography is not only useful for imaging. Our group has recently developed a new theory that relates optical observables to a particle's hologram; see here for more information. These include the scattering pattern, total cross sections, and single-scattering albedo, all of which are important to quantify a particle's role in radiative forcing.


Shown above is the experimental apparatus we have built to capture holographic images of flowing aerosol particles. The arrangement consists of two subsystems: aerosol-particle sensing and hologram recording. An aerosol stream is delivered via a nozzle to the measurement volume where an optical trigger, consisting of crossed (red) diode-laser beams, is used to sense the presence of a particle. These lasers have different wavelengths of 635 and 670 nm and intersect near the nozzle outlet. When a flowing particle enters this intersection it scatters both wavelengths of light simultaneously. The scattered light is received by two photomultiplier (PMT) modules, each sensitive to only one of the two wavelengths. A series of signal-analysis units determines if the signals produced by the PMT modules are coincident. If so, this indicates the presence of a particle at the trigger laser-beam intersection and a fire signal is sent to a pulsed (green) laser for the hologram recording. An example of a recorded hologram is shown in the inset. To see a reconstruction of a particle image from its hologram, refer to this paper



Multi-wavelength digital holography and the inverse problem

As mentioned, there are two elements to light scattering analysis for aerosol particle characterization  First, the one- or two-dimensional (2D) scattered intensity pattern produced by an illuminated particle is measured. This can be done very well on the single and multi-particle level using a variety of techniques. Second, the pattern must then be interpreted to infer the desired particle characteristics, and herein lies much difficulty. This is because there is no general mathematical relationship between a measured pattern and the (unknown) particle characteristics. Thus, the interpretation step must involve a priori information, or make strong assumptions that often relate to the particle shape. There is generally no guarantee that the particle size and shape one associates with a pattern is in fact the correct one, which is the essence of the inverse problem.

With digital holography, however, the particle's size, shape, and orientation may be known unambiguously provided that the image resolution of the sensor is not exceeded. The drawback is that the scattering pattern is not readily known in holography since what is measured derives from the superposition of the unscattered and scattered light rather than the scattered light alone. Knowledge of the scattering pattern is nevertheless important as it describes how the particle redistributes light, which, e.g., is key to quantifying the radiative impacts of atmospheric aerosols. The pattern also contains information about the particle's material composition, which is not directly available from the holographic image.

In recent work, we have achieved a proof-of-principle experiment where the digital hologram of a single particle is measured simultaneous with the particle's 2D light-scattering pattern around the forward direction. This is done by illuminating the particle with light at two wavelengths, red and green, and recording the hologram and pattern on a color CCD sensor. Following generation of the particle image, the pattern can then be unambiguously and quantitatively associated with the size, shape, and orientation. Thus, in a sense, this work constitutes a laboratory-based solution to the inverse problem at least as it relates to size and shape determination. While there are examples of seemingly similar experiments in the literature, this work is unique in that the pattern and image are obtained from the same particle at the same time. With this capability, the validity of scattering-pattern-only characterization techniques and could be directly assessed and their capabilities improved. For example, many optical particle-counters/sizers assume that the observed particles are spherical so that Mie theory can be used to relate the instrument response to particle size. This is done out of necessity as knowledge of the true particle morphology is very often unavailable; a shortcoming that could be directly addressed by this new capability.

Figure: Pattern measurement and holographic imaging of a NaCl crystal cluster. The scale bar in (e) is 100 micrometers. See here for more.



Renewable energy: Concentrated solar photovoltaics

In the ongoing effort to advance the viability of solar power, two technical challenges can be identified: raising the conversion efficiency of solar-to-electrical energy, and improving the process of light collection and delivery to a photovoltaic (PV)  cell. The former challenge generally involves the properties of semiconductor materials and is traditionally the focus of much effort in research contexts. Indeed, this attention is well justified as the overall expense and manufacturing-related environmental impact of solar power rests largely on the semiconductor materials involved. There is also promise, however, in the latter research challenge, i.e., collection and delivery of light to the cell. This is traditionally accomplished with so-called concentrator photovoltaics (CPV). The concept involves conventional focusing to concentrate a large area of received sunlight onto a smaller area containing a cell. In this way, CPV have the potential to enhance solar power-production by requiring less cell material for the same power yield. Our group has investigated a new concept in the physics of CVP based on EM scattering, and has assessed its potential to improve solar power technology, see here.  

Photo: High school student Matthew Steed (left) and graduate student Nava Subedi testing a prototype of the new concentrator concept.


Photoelectron emission from nanoparticles

The interaction of ionizing radiation with nanoparticles is important to understanding a variety of phenomena ranging from atmospheric nucleation to the heating of dust clouds by secondary electron emission from interstellar grains. Photoelectron emission (PE) is currently being used as an analytical probe of soot formation within flames, the detection of diesel emission, as spectroscopic probes of micron-sized droplet surfaces, sub-nanometer particles, and bio-aerosols. A number of fundamental studies of nanoparticles, 5-200 nm in size, have revealed both unexpectedly large PE quantum yields and circular dichroism, the magnitude of which depends upon particle size. In many of these studies, the role that particle size and shape play in both the electromagnetic absorption and PE remains an active area of research.

In this work, we studied the photoemission of electrons from sodium chloride nanoparticles that are 50-500 nm in size and illuminated by vacuum ultraviolet light with energy ranging from 9.4-10.9 eV. The discrete dipole approximation was used to calculate the electromagnetic field inside the particles, from which the two-dimensional angular distribution of emitted electrons was simulated. As can be seen in the figure above, which displays the electric field magnitude on nonabsorbing (left) and absorbing (right) particles, the field intensity is enhanced on the back side of the particle, i.e., relative to the incident radiation. Consequently, the majority of emission is found to favor the particle’s geometrically illuminated side for absorbing particles. This modeled emission-asymmetry compared well to previous measurements performed at the Lawrence Berkeley National Laboratory. More information is available here.


Fourier optics in electromagnetic scattering

In the traditional application of EM scattering, the physical characteristics of single and multiple particle systems are investigated from the scattering pattern directly, i.e.,  without the advantages of digital holography. As an example, the fractal-like morphology of carbon aggregates that form in hydrocarbon combustion can be estimated from the aggregates' far-field scattering pattern alone. This morphology can be examined as the aggregates form and evolve in the flame; such observations would not be possible using optical or electron microscopy since collection of the particles would disturb their structure. 
In this work, a new technique was developed to measure the scattering pattern from single or multiple particles over a 2D angular range extending close to the forward direction. The  basic optical arrangement is shown above and allows one to obtain images of a particle's profile simultaneously with the 2D scattering pattern. This is done by splitting the scattered light with a beam splitter (BS) following a mirror with a through-hole (SFM). A lens (L4) projects the scattering pattern onto a sheet of copy paper, which acts as an observation screen.  The other portion of the scattered light is used to image the particles onto the paper screen via lenses (L1, L2, L3). 

A quartz cuvette containing a dilute solution of polystyrene latex microspheres in water is placed at the front focal plane of lens L1.  The microspheres are 20 micrometers in diameter. A commercial digital camera is used to capture a video of the images appearing on the paper screen. The video here shows the particle images on the left and the simultaneous 2D scattering patterns on the right.  The particles move due to convection in the cuvette. As they move, they pass in-and-out of the object plane and hence fade in-and-out of focus.  The scattering pattern is not a single-particle pattern, but rather is the accumulated pattern due to all particles passing through the illumination beam during the course of the video. The pattern shows well-defined fringes indicative of a single-particle because the particle size-distribution is highly monodisperse and because the solution is dilute, thus minimizing multiple scattering effects.  For more information, see this paper.


References:

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[9] K.B. Aptowicz, R.G. Pinnick, S.C. Hill, Y.L. Pan, R.K. Chang, “Optical scattering patterns from single urban aerosol particles at Adelphi, Maryland, USA; a classification relating to particle morphologies,” J. Geophys. Res. 111, D12212 (2006).
[10] M. J. Berg, G. Videen, “Digital holographic imaging of aerosol particles in flight,” J. Quant. Spectrosc. Ratiat. Transfer 112, p. 1776-83 (2011).
[11] M. J. Berg, N. R. Subedi, P. A. Anderson, N. B. Fowler, “Using holography to measure extinction,” Opt. Lett. 39, p. 3993-96 (2014).


We are grateful to the following agencies for supporting our work: