Ultra-short flashes of light illuminate a possible path to future
beyond-CMOS electronics

Date:
October 5, 2021
Source:
ARC Centre of Excellence in Future Low-Energy Electronics
Technologies
Summary:
Researchers have demonstrated that ultra-short pulses of light,
down to 34 millionths of a billionth of a second, elicit the same
response as continuous illumination. The experiment harnessed
interactions between real and virtual states to 'switch'
the electronic state of an atomically-thin (2D) material,
tungsten-disulfide, aiding the search for future low-energy
electronics based on exotic topological materials.



FULL STORY ========================================================================== Ultrashort pulses of light are proven indistinguishable from continuous illumination, in terms of controlling the electronic states of
atomically-thin material tungsten disulfide (WS2).


==========================================================================
A new, Swinburne-led study proves that ultrashort pulses of light can
be used to drive transitions to new phases of matter, aiding the search
for future Floquet-based, low-energy electronics.

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 resulting temporary states known as Floquet-Bloch states are
interesting from a pure research standpoint as well as for a proposed
new class of transistor based on Floquet topological insulators (FTIs).

In an important finding, 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, an important question
that, until now, had been largely ignored.

A CONTINUOUS WAVE OR ULTRASHORT-PULSES: THE PROBLEM WITH TIME Floquet
physics, which has been used to predict how an insulator can be
transformed into an FTI, is predicated on a purely sinusoidal field,
ie continuous, monochromatic (single wavelength) illumination that has
no beginning or end.



==========================================================================
To observe this phase transition, however, only ultrashort pulses offer sufficient peak intensities to produce a detectable effect. And there's
the rub.

Turning even the purest light source on or off introduces a wide range
of additional frequencies to the light's spectrum; the more abrupt the switching, the more broadband the spectrum. As a result, ultrashort
pulses like those used here don't conform to the assumptions upon which
Floquet physics is based.

"Ultrashort pulses are about as far as you can possibly get from a monochromatic wave," says Dr Stuart Earl at Swinburne University of
Technology (Australia).

"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." PUMP-PROBE SPECTROSCOPY OF ATOMIC MONOLAYER
ELICITS AN INSTANTANEOUS RESPONSE Dr Earl, with collaborators from the Australian National University and the ARC Centre for Future Low-Energy Electronic Technologies (FLEET), subjected an atomic monolayer of tungsten-disulfide (WS2) to light pulses of varying length but the same
total energy, altering the peak intensity in a controlled manner.



==========================================================================
WS2 is a transition metal dichalcogenide (TMD), 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 due to the optical Stark effect
(the simplest realisation of Floquet physics). Thanks to their use of
a sub-bandgap pump pulse, the signal they measured, which persisted
only for as long as the pulse itself, was due to interactions between equilibrium and photon-dressed virtual states within the sample.

"It might sound odd that we can harness virtual states to manipulate
a real transition" says Dr Earl. "But because we used a sub-bandgap
pump pulse, no real states were populated." "The WS2 responded instantaneously, but more significantly, 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"
explains Professor Jeff Davis, also at Swinburne University of Technology.

"This was an exciting finding for our team. Despite the pulses being
extremely short, the states of the system remained coherent" An adiabatic perturbation is one that is introduced extremely slowly, so that the
states of the system have time to adapt, a crucial requirement for FTIs.

While ultrashort pulses shouldn't be compatible with this requirement,
this result provides clear evidence that for these atomic monolayers, they
do. This now enables the team to attribute any evidence of non-adiabatic behaviour to the sample, rather than to their experiment.

These findings now enable the FLEET team to explore Floquet-Bloch states
in these materials with an above-bandgap pulse, which, theoretically,
should drive the material into the exotic phase known as a Floquet
topological insulator.

Understanding this process should then help researchers to incorporate
these materials into a new generation of low-energy, high-bandwidth,
and potentially ultrafast, transistors.

Systems exhibiting dissipationless transport when driven out of
equilibrium are studied within FLEET's Research theme 3, seeking new,
ultra-low energy electronics to address the rising, unsustainable energy consumed by computation (already 8% of global electricity, and doubling
every decade).

========================================================================== Story Source: Materials provided by ARC_Centre_of_Excellence_in_Future_Low-Energy_Electronics
Technologies. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. S. K. Earl, M. A. Conway, J. B. Muir, M. Wurdack, E. A. Ostrovskaya,
J.

O. Tollerud, J. A. Davis. Coherent dynamics of Floquet-Bloch states
in monolayer WS2 reveals fast adiabatic switching. Physical Review
B, 2021; 104 (6) DOI: 10.1103/PhysRevB.104.L060303 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211005101910.htm

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