Braun Group People
Andrew Brzezinski
Graduate Student in Materials Science and Engineering
B.A.Sc from the University of Toronto (2004)
I am performing optical characterization of photonic crystals with the goal to better understand the physics so that we can apply this knowledge to fabricating devices with improved functionality. I built an optical diffractometer to measure the angle-resolved wavelength-dependent emission from a fluorescent photonic crystal (Fig. 1). It is possible to engineer the emission profile by changing the geometry in order to utilize the photonic stop bands, due to diffraction from Bragg-planes (Fig. 2), which redistributes the emissive states to other, nearby, wavelengths and directions. One way of engineering the photonic crystal shape is by conformal deposition processes, such as atomic layer deposition. Successive monolayers are of dielectric material are deposited on a template until the desired geometry is achieved. Scanning electron microscopy and reflection measurements can be compared with calculations of the expected geometry (Fig. 3) and finite-difference time-domain (FDTD) simulations of the expected electromagnetic response (Fig. 4).
Figure 1: (a) Cross-section of a photonic crystal made of fluorescent ruthenium tris-bipyridine. (b) Arrangement for angle resolved spectroscopic measurements with plots for the (c) 617 nm and (d) 690 nm wavelengths. The brighter and darker areas are enhancements and suppressions of the fluorescence emission, which correspond to a modified density of electromagnetic states.
Figure 2: The first Brillouin Zone of an FCC colloidal crystal. The sphere represents the momentum of a photon of a particular wavelength and the facets are the Bragg planes that cause the photon to diffract (like a reflection) so that the famous relationship, G = k – k', is satisfied.
Figure 3: The scanning electron micrograph of a focused ion-beam milled cross-section of a photonic crystal infilled with alumina via atomic layer deposition is compared with a calculation. Alumina (yellow in the calculation) shows up lighter than the polymer (blue) template.
Figure 4: Comparisons of the reflection spectra with the FDTD calculated spectra of a photonic crystal infilled with alumina. On the right, the calculation unit cell of the periodic structure is depicted with the polymer template (blue) and alumina (yellow) on a glass substrate (light blue) for 0 nm (top right), 56 nm (middle right), and 129 nm (bottom right) of deposited alumina.
Professor Paul Braun • Phone: +1.217.244.7293 • Fax: +1.217.333.2736 • Email: pbraun@illinois.edu
Department of Materials Science and Engineering • University of Illinois at Urbana-Champaign