Light Excites Material Change Despite Lacking Sufficient Energy

A new understanding of light-matter interactions is emerging from observations showing laser light absorbed as discrete photon energies within a crystalline material. Daniel Kazenwadel of the University of Konstanz and colleagues report observing a phase transition in vanadium dioxide under conditions previously thought impossible according to established physics. Ultrafast electron diffraction experiments revealed disordered crystal geometry with nanometer-sized switched regions, the number of which corresponded to the number of photons in the laser wave. Supported by optical experiments and simulations, these findings suggest that despite light and electrons behaving as extended quantum objects, energy from individual photons localises to nanoscale dimensions, triggering effects at unexpectedly high energy levels.

Localised photon energy deposition drives phase transition in vanadium dioxide

Vanadium dioxide underwent a phase transition despite a calculated energy deficit of 85 meV per rutile unit cell, a threshold previously thought insurmountable for initiating such a change without sufficient heat. Conventional models predicted no transition would occur given the laser pulse energy, but ultrafast electron diffraction revealed nanometer-sized regions of switched material forming within the crystal. The approximate number of these switched spots directly correlated with the number of photons within the absorbed laser wave, indicating energy deposition is not widespread but is localised to individual photons.

Two independent optical experiments consistently substantiated these findings, demonstrating this localised energy deposition mechanism. Simulations employing time-dependent density functional theory, performed on crystallographic super-cells, accurately reproduced all measured results when modelling the absorption of single photons. These calculations revealed that each photon could, in principle, transform approximately twenty unit cells if energy dissipated as a wave, yet the observed switching correlated one-to-one with individual photon absorption.

A fundamentally different energy transfer process than previously understood appears to be at play. Breaking a single vanadium-vanadium dimer, a key component of the phase transition, requires around 60 meV, well within the 1.2 eV energy of each photon used in the experiment. While these results definitively show energy concentrating into nanometer-sized regions, they do not yet explain how this localised absorption circumvents the 85 meV energy deficit previously thought necessary for the phase transition, nor do they clarify the pathway to scaling this effect for practical applications like efficient laser material processing.

Time-resolved electron diffraction visualises nanoscale phase transitions in vanadium dioxide

Ultrafast electron diffraction proved key in revealing these localised effects, functioning much like a strobe photograph of atoms moving within a material and allowing observation of changes as they happen. This technique fires a series of extremely short electron pulses at a sample, creating a diffraction pattern that reveals the arrangement of atoms. By varying the time delay between the laser pulse and the electron pulses, researchers created a movie of the material’s structural changes following light absorption.

The resolution achieved with this method, at the nanometer scale, was essential for detecting the small, switched regions within the vanadium dioxide crystal. Laser pulses with 1.2 electron volt photons were used to excite a phase transition in vanadium dioxide crystals, lasting 250 femtoseconds and with energies between 10 and 500 picojoules, focused to a spot of 1.8μm by 1.1μm. Conventional methods lack the capability to observe such fleeting phenomena. This approach allowed scientists to visualise the structural changes following light absorption with unprecedented detail. Less than one electron per pulse was employed to minimise sample damage.

Vanadium dioxide exhibits phase transition defying established energy absorption models

The established understanding of light absorption relies on models like Maxwell’s equations, predicting energy spreads as a wave; however, this work demonstrates a distinctly different mechanism in vanadium dioxide. Pinpointing how this localised energy deposition bypasses the previously understood energy requirements for a phase transition remains elusive, despite exciting progress. The simulations accurately model the observed switching, yet offer no insight into the intermediate steps governing energy confinement, leaving open the possibility of currently unknown physical processes at play.

Acknowledging that simulations currently fail to detail how energy is confined, this discovery remains significant because it challenges a century-old understanding of material interaction with light. Confirming these mechanisms could revolutionise fields reliant on light-matter interactions, from advanced sensors to more efficient energy technologies. The observation that a phase change in vanadium dioxide occurred despite insufficient energy according to conventional physics suggests previously unknown mechanisms are at work, demanding further investigation.

Vanadium dioxide’s response to laser light reveals energy absorption operates differently than predicted by conventional physics. Energy localises to individual photons, resulting in nanometer-scale regions undergoing a phase transition despite insufficient energy for widespread heating, rather than spreading as a wave. This one-to-one relationship between light particles and material change was detected using ultrafast electron diffraction, a technique akin to capturing a strobe image of atomic movement, and confirmed by the direct correspondence between the quantity of switched areas and the number of photons absorbed.

The research demonstrated that vanadium dioxide absorbed laser light not as a wave, but as localised energy from individual photons. This finding matters because it challenges the long-held understanding of how light interacts with materials, suggesting energy can be deposited in a more concentrated manner than previously thought. Researchers observed a phase transition within the material, forming nanometer-sized switched areas corresponding to the number of photons absorbed, despite the overall energy being insufficient for a bulk transition. The authors suggest further investigation is needed to fully understand the mechanisms behind this energy confinement.