Nanoscale pixels to advance augmented reality eyewear

Smart glasses, ie, glasses that display information directly in the field of vision, are considered a key technology of the future — but until now, their use has often failed due to cumbersome technology. However, efficient light-emitting pixels are ruled out by classical optics if their size is reduced to the wavelength of the emitted light.

Now, physicists at Julius-Maximilians-Universität Würzburg (JMU) have used optical antennas to create an ultra-small pixel. The research group, led by Professors Jens Pflaum and Bert Hecht, was responsible for the work. Their research findings have been published in the journal Science Advances.

“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 by 300 nanometres. This pixel is just as bright as a conventional OLED pixel with normal dimensions of 5 by 5 micrometres,” Hecht said.

To put this into perspective, a nanometre is one millionth of a millimetre. 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 millimetre. This, for example, enables integration of the display into the arms of a pair of glasses from where the light generated would be projected onto the lenses.

An OLED consists of several ultra-thin organic layers embedded between two electrodes. When current flows through this stack, electrons and holes recombine and electrically excite the organic molecules in the active layer, which then release this energy in the form of light quanta. Since each pixel glows on its own, no backlighting is necessary, which enables particularly deep blacks, vivid colours and efficient energy management for portable devices in the field of augmented and virtual reality (AR and VR).

A key problem in further miniaturising pixels is the uneven distribution of currents in these 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 the antenna,” said Jens Pflaum, describing the physical background. This antenna, made of gold, would have the shape of a cuboid with edge lengths of 300 by 300 by 50 nanometres.

“The resulting electric fields would generate such strong forces that the gold atoms becoming mobile would gradually grow into the optically active material,” Pflaum said. These ultra-thin structures, also known as “filaments”, would then continue to grow until the pixel is destroyed by a short circuit.

The new structure contains a specially manufactured insulation layer on top of the optical antenna, which leaves only a circular opening with a diameter of 200 nanometres in the centre of the antenna. This arrangement blocks currents that would be injected from the edges and corners — thus enabling reliable, long-lasting operation of the nano light-emitting diode. Under these conditions, filaments can no longer form.

The physicists now want to increase the efficiency from the present level of 1% and expand the colour gamut to the RGB spectral range. Then there will be virtually nothing standing in the way of a new generation of miniature displays. 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.

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