The emergence of smart glasses is a product of the new era of technology and is widely regarded as a key technology for the future. However, due to technological limitations, applications are also restricted. In addition, if the size of high-efficiency luminescent pixels is reduced to the wavelength of emitted light, their use will also be limited by traditional optics.
Now, physicists at Julius-Maximilians-Universität Würzburg (JMU), Germany, say they have made progress toward luminous miniature displays and, with the help of optical antennas, have created “the world’s smallest pixel to date”. A research group led by Professors Jens Pflaum and Bert Hecht was responsible for the work.
The group has now published their results in Science Advances.
View of a nano light-emitting diode as developed in Würzburg
Display on a square millimeter
“With the help of a metallic contact that allows current injection into an organic light-emitting diode while simultaneously amplifying and emitting the generated light, we have created a pixel for orange light on an area measuring just 300 nm x 300 nm. This pixel is just as bright as a conventional OLED pixel with normal dimensions of 5 µm x 5 µm,” said Bert Hecht, describing the key finding of the study.
This means that a display or projector with a resolution of 1920 x 1080 pixels would easily fit onto an area of just one square millimeter. This, for example, would enable the integration of the display into the arms of a pair of glasses from where the light generated would be projected onto the lenses.
A key problem the Würzburg-based researchers were facing in further miniaturizing their pixels was the uneven distribution of currents in such small dimensions. “As with a lightning rod, simply reducing the size of the established OLED concept would cause the currents to emit mainly from the corners of an antenna,” said Jens Pflaum. Such an antenna, made of gold, would have the shape of a cuboid with edge lengths of 300 nm x 300 nm x 50 nm.
“The resulting electric fields would generate such strong forces that the gold atoms becoming mobile would gradually grow into the optically active material,” said Pflaum. “These ultra-thin structures, also known as filaments, would then continue to grow until the pixel is destroyed by a short circuit.”
The structure now developed in Würzburg, contains a new insulation layer on top of the optical antenna, which leaves only a circular opening with a diameter of 200 nm in the center of the antenna. This arrangement blocks currents that would be injected from the edges and corners – enabling reliable, long-lasting operation of the nano light-emitting diode. Under these conditions, filaments can no longer form. “Even the first nanopixels were stable for two weeks under ambient conditions,” said Bert Hecht.
Looking ahead, the physicists want to further increase the efficiency from the present level of one percent and expand the color gamut to the RGB spectral range. Then there will be virtually nothing standing in the way of a new generation of miniature displays “made in Würzburg,” they said. With this technology, displays and projectors could become so small in the future that they can be integrated almost invisibly into devices worn on the body – from eyeglass frames to contact lenses, the announcement states.
Photons ‘prefer state shared by many others’
As far as particles of light are concerned, the collective is more important than the individual. When they get to choose between two states, they favor the state that many of their fellow particles have already adopted. However, this collectivist tendency does not begin until enough photons have assembled in the same place.
These findings, revealed by University of Bonn physicists in a recent study, could aid the development of ultra-powerful laser sources, among other things. They have now been published in Physical Review Letters.
Photons prefer a state already shared by many other photons. Click for info
Physics knows two fundamentally different kinds of particles, the fermion and the boson. Fermions are committed individualists: If they are confined in a tight space, they cannot assume the same state. Electrons surrounding a nucleus are one example of this. If two of them want to be in the same “cloud”, they have to have a different “spin”.
Bosons, by contrast, like to do things together and prefer to all share the same state. Photons are part of this group. If they are cooled down enough and confined in a tiny space, they merge into a kind of gigantic “super-photon.” But if the light particles are forced to take on one of two slightly different colors first, would that create two differently colored super-photons? Or would they default to the same color?
This was the question investigated by the working group led by Professor Martin Weitz from the Institute of Applied Physics at the University of Bonn. “We started by using a certain method to create cooled photons,” said Weitz, who is also a member of the University’s Matter Transdisciplinary Research Area (TRA) and its ML4Q—Matter and Light for Quantum Computing Cluster of Excellence.
“We then shut these particles of light in a space in which they had to adopt one of two marginally different energy levels—slightly different colors, in other words. Consider it as a restaurant with two long tables where diners can sit to eat.”
The researchers then looked at which “table” the photons chose and found that the first few of them were distributed fairly randomly between the two. “Although the lower energy level was marginally more popular, the difference was so fine as to be virtually immaterial,” said Weitz. “However, that only remained the case while the number of photons was low.”
As soon as the gathering numbered into the dozens, the new arrivals began to sort themselves, always being more likely to pick the table with more occupants. This trend continued to the extent that the emptier table was hardly ever selected again once it had gathered a few hundred photons. This collectivist behavior has already been demonstrated for gases containing various types of bosons. In gases, however, the particles always have a very wide variety of possibilities to choose from, rather than just two as in this case.
More powerful lasers
The Bonn group says that this principle could potentially be harnessed to design extremely powerful laser sources, because the energy in laser light can theoretically be increased by combining multiple sources of radiation. “However, this requires them all to be in phase, meaning that their waves must always be exactly in synch,” said Weitz. “If not, the peaks of the wave of the first laser beam may encounter the troughs of the second beam, and they would cancel each other out.”
Although aligning the light waves from two lasers so precisely is no easy task, it might be possible to exploit the photons’ penchant for collectivism to bring the beams together. “Our findings suggest this could work,” the researcher explained. “But there’s a long way to go until the technology is up and running.”
Source: optics.org
