The sparking of grapes in a household microwave has been a popular party trick among curious (and pyrotechnically inclined) physics students for decades. The plasma grabs one’s attention, but the real excitement lies in what happens inside the grape. At the 2.45 GHz frequency of a microwave oven, water has a refractive index of about 9, so the wavelength inside a grape shrinks to little more than a centimetre, roughly the size of the grape itself. The grape becomes a resonator, trapping microwaves much as nanoparticles trap visible light. These Mie resonances were worked out mathematically over a century ago and have since become canonical ‘textbook’ knowledge, yet they are seldom experimentally visualized. Inside a grape, the trapped fields heat the water, allowing a thermal camera to photograph the resonance patterns directly. In this talk I'll show these images alongside calculations and computer simulations, and follow how the patterns change when two grapes touch and form the intense hotspot associated with the spark. Along the way, I'll describe how a question first chased by undergraduates grew into a curiosity-driven research program built by students at every level.
Aaron Slepkov is a Professor of Physics & Astronomy and past Canada Research Chair in the Physics of Biomaterials at Trent University. When he’s not accidentally misaligning his students’ experimental ultrafast laser microscopy setups, Aaron is playing with fruit and a thermal camera in his microwave lab, pondering the physics of birefringence-based art, or studying the use of multiple-choice testing methods in university physics education. Aaron completed a dual undergraduate degree in Physics and Chemistry at Brock University in 1999, followed by a masters (2001) and PhD (2006) in Physics at the University of Alberta. He then moved to Ithaca, NY, to conduct postdoctoral research in few-photon atom-waveguide interactions at Cornell. This was followed by a second postdoc at the National Research Council of Canada in Ottawa, where he fell in love with nonlinear optical microscopy.