Deep in the bowels of Cambridge lies a lab only accessible to a select few. There is no source of light down here, except the eerie green glow of a laser powerful enough to blind in an instant. Creaking open the door, a researcher enters and carefully places a sample in the path of a laser beam that has been painstakingly guided through metres of mirrors, lenses, and filters. After weeks of meticulous alignment, the experiment is ready to run: Two ultra fast laser pulses will be fired at the sample to probe its most enigmatic properties, revealing secrets only accessible at inconceivably small scales – one hundred thousand times smaller than the width of a human hair and so fast that ten trillion pulses could occur in one human blink.
This is the premise of my midi project. I will investigate phenomena such as charge transport, the lifetimes of excited states, and quasiparticle interactions in thin materials.
Specifically, the materials considered in this project are topological insulators. Such materials support suppressed-scattering conductive states upon their surfaces while remaining insulating in the bulk. These two distinct states, conductive and insulating, arise due to an inversion of the band structure between the internal bulk insulator and external insulating vacuum. The suppressedscattering is an intriguing property because it is inherent to the topology of the insulator. This causes the currents to be long-lasting and stable against defects, reducing electrical resistance and thus energy losses.
In addition to the suppressed-scattering, topological insulators also exhibit spinmomentum locking – a phenomenon where the electron spin is correlated to the direction in which it moves, again a consequence of the topology of the insulator. This property allows us to cherry-pick the spin.
Both spintronics and quantum-information technology can benefit massively from these properties as we have both robust and manipulable spin currents. These advantages of the used materials thus offer a promising platform for nextgeneration devices.
By probing the ultra-fast dynamics of thin topological insulators with a laser and combining this with theoretical predictions based upon simulation, we aim to resolve their response to different conditions, such as cryogenic temperatures and external magnetic fields.
To ensure that our laser can pass through the sample and be detected, we first create very thin samples of materials via exfoliation, a technique where large crystals are made thinner by the action of mechanically separating the layers (often with sticky tape!).
The laser itself consists of two rapid pulses, a pump and a probe, fired in rapid succession at the exfoliated crystal. Both pulses are fine-tuned to ultra-short timescales, typically femtoseconds to picoseconds. Without these ultra-short timescales, this technique could not capture the exotic physics within these crystals.
Initially, the pump-pulse excites our material up from the ground state. This triggers a myriad of different processes in the material, at which point the probepulse passes through the sample. A camera subsequently captures the probepulse on the other side, allowing comparisons between the ground state and the excited state.
By firing the laser we shine a spotlight upon the quantum phenomena determined by the topological characteristics of our materials, revealing the quantum dance unfolding on the smallest of stages.
NanoDTC PhD Student, c2022