Localization of coherent light into photons in a single-crystalline material

TL;DR

This study demonstrates that individual photons can localize their energy into nanometer-sized regions within a crystalline material, causing localized phase transitions despite the overall energy constraints.

Contribution

It provides experimental and simulation evidence that photon energy can be localized at nanoscales in a crystal, challenging traditional views of light absorption.

Findings

Nanometer-sized regions of phase change match photon count

Optical experiments confirm photon localization effects

Simulations reproduce experimental results

Abstract

The absorption of light by materials is one of the most fundamental processes in optics and condensed-matter physics. 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 for short times 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 of single absorbed photons reproduce all measurements results. Although laser light and Bloch electrons are extended quantum objects, the energy of the individual photons is localized into nanometer dimensions, enabling local consequences at substantially higher energy than average.