Bose–Einstein condensation occurs when a gas of atoms is cooled until the de Broglie wavelength of the atoms becomes comparable to the distance between them. The atoms then collapse into the same quantum ground state and can therefore be described by the same wavefunction. The phenomenon was predicted nearly a century ago by Albert Einstein and Satyendra Nath Bose, and researchers created the first such condensate in 1995 with rubidium atoms.
Since then, Bose–Einstein condensates (BECs) have also been observed in polaritons, photons and magnons, to name but three systems. A team of researchers at Aalto University in Finland has now succeeded in creating the first-ever BEC of light coupled with surface plasmon polaritons (the particle-like collective oscillations that occur when light interacts with a metal’s conduction electrons).
New condensate is very different
“Observing a new type of condensate such as this one is important since it will push the limits of the BEC phenomenon and open the way for new technological applications,” says team leader Päivi Törmä. Surface plasmon polaritons, for example, are expected to play an important role in future photonics devices that would use light instead of electricity to process information. Such devices should be much faster and use less energy than their electronic counterparts.
The new condensate is very different from most of those made before in that it can form at room temperature rather than at near-zero temperatures. It also appears extremely quickly – on the picosecond timescale – and is based on an easy-to-fabricate on-chip nanoparticle lattice whose geometry can readily be tuned to modify the properties of the condensate.
Periodic array of gold nanoparticles
To make their lattice, Törmä and colleagues begin by fabricating a periodic array of gold nanoparticles on a glass slide, using nanofabrication techniques such as electron beam lithography. The particles are separated by around 580–610 nm and the array is 100 x 300 μm2 in size. They then overlay the array with a solution containing dye molecules.
“Using a femtosecond laser, we then ‘pump’ the molecules at a spot located at one end of the array,” explains Törmä. “The molecules then emit light and thereby excite the plasmonic modes of the lattice – that is, they create those particles that will then condense. These particles are mostly photons but they also contain electron plasma oscillations in the gold nanoparticles.”
Tracking how the condensate forms
“The particles start to propagate from the end of the array and interact with the dye molecules by light absorption and emission,” she tells nanotechweb.org. “Between the absorption and emission, the molecules lose some of their energy to vibrations and the energy of the light therefore decreases at each absorption-emission cycle. By monitoring the frequency of the emitted light when the particles propagate in the array, we can thus observe how the condensate forms.”
The condensate in fact forms when the lowest energy state possible in the lattice (the so-called band edge) is reached, she explains. “By monitoring the emitted light as it propagates along the lattice, we can track how the condensate forms over time. This propagation would be extremely difficult to monitor using other techniques since it occurs so quickly – in just one picosecond.”
Lasing or BEC?
By altering the distance between the gold nanoparticles in the lattice, the researchers say they can control whether BEC condensation or ordinary lasing occurs. “The two phenomena are similar and studying the crossover between them will help us better understand how they are related and how they differ,” says Törmä. “Both lasing and BEC produce bright beams of light but the coherences of the light they offer have different properties, which means they might be used in different applications. The new condensate can produce light pulses that are extremely short and so may offer faster speed for information processing and imaging applications.
“As well as looking into such applications, we will also be trying to increase the amount of dye molecules in our system so that we have strong coupling between them and the plasmonic modes, and see how this affects BEC,” she adds.
The present research is detailed in Nature Physics 10.1038/s41567-018-0109-9.