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Interacting lasers shed light on topological defects

10 Jul 2017 Hamish Johnston
Spot on: 10 interacting lasers (left) mostly synchronize their phases, while 20 produce more topological defects

A new way of using a laser cavity to study the emergence of topological defects has been unveiled by researchers in Israel.

Topological defects emerge when a system makes a rapid transition from a disordered to an ordered phase – a process called quenching because it often involves rapid cooling. In the case of magnetic order, quenched magnetic moments form small domains in which the moments point in the same direction. Moments in neighbouring domains can point in different directions and the interfaces between domains are called topological defects.

These defects can occur in a wide range of systems, from atomic gases to the rapidly cooling early universe. Understanding how to eliminate topological defects could even be exploited to solve hard computational problems.

Multiple lasers

How topological defects emerge can be very tricky to study in the laboratory because controlling the rapidly changing temperature throughout a sample can be very difficult. In this latest study, Vishwa Pal, Nir Davidson and colleagues at the Weizmann Institute in Israel have used a set of up to 30 coupled laser beams to create a system with topological defects that can be studied more easily.

Their system comprises a laser cavity containing a mask with a number of holes arranged in a circular pattern. Each hole produces its own laser beam, which overlaps a bit with its two neighbours – leading to an interaction between beams. The laser cavity is pumped by an external light pulse and the interaction causes the laser beams to undergo rapid phase oscillations before settling into a steady state that is then measured by the team.

The laser cavity contains about 1000 modes and this provides the system with a large number of initial phase relationships between the laser beams. In most cases the beams synchronize, but occasionally the system gets locked into a state in which there are phase differences between the beams. These states can be described as topological defects, and the team found that their number increased as the number of holes is increased from 10 to 30 – and also when the intensity of the pump pulse is increased.

Pump intensity

The team reckons that when the pump intensity is high, the system reaches equilibrium much faster than when the pump intensity is low. This timescale is analogous to the cooling rate in a quenched system, in which more topological defects occur when the temperature drop is more abrupt. The ring is essentially a 1D system, and the team now plans to extend its work to a 2D system.

The study is described in Physical Review Letters.

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