Time-domain separation of optical properties from structural transitions in resonantly bonded materials

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 owing to a change in crystallinity. Here we show that the connection between optical properties and structure can be broken. Using a 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 fs owing 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.

Ultrafast optical response of the amorphous and crystalline states of the phase change materialGe2Sb2Te5

“…Therefore, we use these data to obtain the complex dielectric function by the transfer matrix method as done previously 12 . The optical properties of the Si 3 N 4 layer and SiO 2 substrate were held constant at literature values, and only the dielectric properties of the GST layer were allowed to change.…”

“…For example, it is known that the lattice thermalization time in the crystalline state is 2.2 ps regardless of fluence 12 . Here, lattice thermalization most closely corresponds to τ 2 .…”

“…However, out of equilibrium this does not have to be the case. Recent measurements of the time-resolved optical and structural changes in GST demonstrated that the some of the desirable optical properties of the crystalline phase change materials can be switched faster than the time required for structural amorphization 12 .…”

Optical fiber-based photocathode

“…Ultrafast electron diffraction (UED) is a highly successful technique for structural dynamics investigation of phase transitions, electron-phonon coupling, and chemical reactions. [1][2][3][4][5][6][7][8][9] Metal photocathodes are currently the most commonly employed electron sources due to their robustness, ultrafast temporal response, and compatibility with high electric fields. They provide sub-picosecond electron pulses with coherence lengths up to a few nanometers.…”

“…Finally, we embedded our fiber-based photocathode inside a bulk cathode structure which is optimized for use in high extraction fields as typically employed in compact DC guns (typically in the keV to hundreds of keV regimes [7][8][9]22,28 ) or radio-frequency (RF) guns ranging into MeV energies. 27 The feasibility of this arrangement was tested in a DC electron gun at an acceleration field of 7 MV/m, and beam energy of 70 kV (see the supplementary material for details).…”