Ultra-Short or Infinitely Long: It All Looks the Same

Source: Sally Wood

A new, Swinburne-led study proves that ultrashort pulses of light can be used to drive transitions to new phases of matter.

The research aids the search for future Floquet-based, low-energy electronics. Ultrashort pulses of light are indistinguishable from continuous illumination, in terms of controlling the electronic states of atomically-thin material tungsten disulfide (WS2). But there is significant interest in transiently controlling the band-structure of a monolayer semiconductor by using ultra-short pulses of light to create and control exotic new phases of matter. The ultra-short pulses of light necessary for detecting the formation of Floquet states were shown to be as effective in triggering the state as continuous illumination.

A Continuous Wave or UltrashortPulses: The Problem with Time

Floquet physics, which has been used to predict how an insulator can be transformed into a Floquet topological insulator, is predicated on a purely sinusoidal field.

However, only ultrashort pulses offer sufficient peak intensities to produce a detectable effect.

Dr Stuart Earl from Swinburne University of Technology explained this phenomenon “Ultrashort pulses are about as far as you can possibly get from a monochromatic wave.”

“However, we’ve now shown that even with pulses shorter than 15 optical cycles (34 femtoseconds, or 34 millionths of a billionth of a second), that just doesn’t matter,” he said.

Pump-Probe Spectroscopy of Atomic Monolayer Elicits an Instantaneous Response

Dr Earl, alongside a wide collaboration of FLEET researchers subjected a WS2 to light pulses of varying length but the same total energy, which alters the peak intensity in a controlled manner. WS2 is a transition metal dichalcogenide, which is a family of materials investigated for use in future ‘beyond CMOS’ electronics. The team used pump-probe spectroscopy to observe a transient shift in the energy of the ‘A’ exciton of WS2 because of the optical stark effect. “It might sound odd that we can harness virtual states to manipulate a real transition. But because we used a sub-bandgap pump pulse, no real states were populated,” Dr Earl said. Professor Jeff Davis, who is also from Swinburne University of Technology, explained how the WS2 responded instantaneously. “Its response depended linearly on the instantaneous intensity of the pulse, just as if we’d turned on a monochromatic field infinitely slowly, that is, adiabatically.” “This was an exciting finding for our team. Despite the pulses being extremely short, the states of the system remained coherent,” he explained. An adiabatic perturbation is introduced relatively slowly. As such, the states of the system have time to adapt, which is a crucial requirement for Floquet topological insulators. The results provide clear evidence that for these atomic monolayers, ultrashort pulses are compatible with these requirements. This will enable the team to attribute any evidence of non-adiabatic behaviour to the sample, rather than to their experiment. Together, these findings allow the FLEET team to explore Floquet-Bloch states in these materials with an above-bandgap pulse. This is expected to drive the material into the exotic phase, which is known as a Floquet topological insulator. In all, understanding this process should help researchers and industry representatives to incorporate these materials into a new generation of low-energy, high-bandwidth, and potentially ultrafast, transistors. Systems that exhibit dissipationless transport when driven out of equilibrium are studied within FLEET’s third research theme. Under this research theme, researchers are seeking new, ultra-low energy electronics to address the rising, unsustainable energy consumed by computation.

Professor Jeff Davis (Swinburne University of Technology) leads Swinburne’s ultrafast spectroscopy lab.