Time-Domain Separation of Optical Properties From Structural Transitions in Resonantly Bonded Materials

TL;DR

This study demonstrates that optical property changes in phase change materials can occur independently of structural transitions, enabling ultrafast light manipulation without lattice crystallinity alterations.

Contribution

The paper reveals that optical contrast in phase change materials can be decoupled from structural changes, showing rapid dielectric function modifications without lattice crystallinity perturbation.

Findings

Dielectric function changes by 30% within 100 femtoseconds.

Optical changes occur without perturbing lattice crystallinity.

Ultrafast optical modulation exceeds silicon capabilities.

Abstract

The extreme electro-optical contrast between crystalline and amorphous states in phase change materials is routinely exploited in optical data storage and future applications include universal memories, flexible displays, reconfigurable optical circuits, and logic devices. Optical contrast is believed to arise due to a change in crystallinity. Here we show that the connection between optical properties and structure can be broken. Using a unique combination of single-shot femtosecond electron diffraction and optical spectroscopy, we simultaneously follow the lattice dynamics and dielectric function in the phase change material Ge2Sb2Te5 during an irreversible state transformation. The dielectric function changes by 30% within 100 femtoseconds due to a rapid depletion of electrons from resonantly-bonded states. This occurs without perturbing the crystallinity of the lattice, which heats with a 2 ps time constant. The optical changes are an order-of-magnitude larger than those achievable with silicon and present new routes to manipulate light on an ultrafast timescale without structural changes.