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 upon 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 (CPVs). The concept involves conventional focusing to concentrate a large area of intercepted sunlight onto a smaller area containing a cell. In this way, CPVs 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 CVPs 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 including atmospheric nucleation, the heating of interstellar cosmic-dust clouds by secondary electron emission from interstellar grains, and the charging of regolith of airless space-bodies. Photoelectron emission (PE) is 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.
We studied the photoemission of electrons from sodium chloride nanoparticles that are 50-500 nm in size illuminated by vacuum ultraviolet light with energy ranging from 9.4-10.9 eV. The discrete dipole approximation (DDA) was used to simulate 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) cubic particles, the field intensity is enhanced on the back side of the particle relative to the incident radiation for nonabsorbent particle and is suppressed for absorbing particles. 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 measurements performed at the Lawrence Berkeley National Laboratory. More information is available [here].
Fourier optics in electromagnetic scattering
In the traditional application of electromagnetic 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 discussed above. As an example, the fractal-like morphology of carbon aggregates that form in hydrocarbon-fuel 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 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 above) following a mirror with a through-hole (SFM). A lens (L4) projects the scattering pattern onto a sheet of 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).
Video: 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 convective flow 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].