Speaker
Description
The absorption of light is one of the most fundamental processes in condensed-matter physics and optics. Here we investigate whether laser light is absorbed by a crystalline material as an electromagnetic wave or as localized photon energies. We excite the first-order phase transition of vanadium dioxide with laser pulses of sufficient frequency to overcome the band gap but with insufficient pulse energy to overcome the latent heat. According to Maxwell’s equations and Bloch theory, no transition should occur, because nowhere in the material is enough energy. Nevertheless, we observe with ultrafast electron diffraction a disordered crystal geometry with nanometer-sized spots of switched material. Their amount matches approximately to the number of photons in the absorbed laser wave. Two optical experiments confirm this phenomenon, and simulations reproduce all measurements results. Although laser light and Bloch electrons are extended quantum objects, the electromagnetic wave collapses into nanometer-sized excitations when absorbed. This mechanism enables local consequences at substantially higher energy than average and provides insight into the inner quantum-mechanical couplings within complex materials.