Time-Domain Separation of Optical Properties From Structural Transitions in Resonantly Bonded Materials (1412.0901v3)

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.

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

The paper investigates the separation of optical properties from structural changes in the phase change material Ge$_2$Sb$_2$Te$_5$ (GST) during photo-induced amorphization using a combination of time-resolved femtosecond optical spectroscopy and femtosecond electron diffraction. Traditionally, the optical contrast between the crystalline and amorphous states of such materials is attributed to changes in crystallinity. However, this research elucidates that optical changes can occur independently of the structural disorder, potentially impacting future developments in ultrafast optoelectronic devices.

Key Findings

The paper provides a direct observation of the electronic and structural dynamics of GST, wherein:

1. Decoupling of Optical and Structural Changes: The dielectric function was found to change by 30% within 100 femtoseconds due to rapid depletion of electrons from resonantly bonded states, while the crystallinity of the lattice remained unaffected with a heating time constant of 2 picoseconds. This result indicates that optical properties, specifically changes in the dielectric function, can emerge over ultrafast timescales without immediate structural transformations.
2. Mechanism: Resonant bonds, crucial to optical contrast, can be disrupted by ultrafast laser excitation. The traditional view suggests that lattice disorder leads to optical switch. However, here, transient optical states are achieved before the lattice structure becomes amorphous, extending the basis of optical manipulation without complete atomic rearrangements.
3. Long-Term Implications: The implications of maintaining optical contrast with minimal structural change are significant. For instance, reversibility, cycle durability in memory devices, and novel applications in high-speed optical modulators are achievable. Such control over resonantly bonded states can substantially enhance device performance in terms of speed and energy efficiency.

Broader Impact and Speculation

The realization that optical properties in phase change materials can be manipulated ultrafast without altering structural configurations paves the way for more efficient non-volatile memory technologies. This finding challenges the conventional understanding of how ultrafast light-matter interaction affects phase change materials. The potential to generate large optical contrasts without structural alterations opens avenues for new design paradigms in sensing, communications, and reconfigurable photonics.

Looking forward, the exploration of these transient states in other resonantly bonded materials could lead to discovering a broad class of materials suitable for ultrafast optical applications. Further integration with nanoscale structures and material interfaces, such as graphene, may enhance the control over energetic pathways and further optimize performance, leading to groundbreaking advancements in optical data processing and storage technologies.

Overall, the paper represents a significant stride in understanding the intricacies of resonant bonding and its pivotal role in the fast optical dynamics of phase change materials, challenging entrenched conceptions and offering a fresh perspective on the fundamental interactions within GST.