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MicroLED Arrays Spawn Diverse Applications

EuroPhotonics
Nov 2008
Christine Connolly

BIRMINGHAM, UK -- MicroLED arrays, a technology just now emerging from UK research, promises applications ranging from lab on a chip, high-throughput screening, maskless lithography, and printing and projection displays to advanced illumination in confocal microscopy. Work on the devices began in 2002 with a Scottish Enterprise Proof of Concept award to the Institute of Photonics (IoP) at the University of Strathclyde, which has continued as a pioneer in the work.

A £4 million Basic Technology Research Programme grant from the Engineering and Physical Sciences Research Council brought together contributions from Imperial College London, the University of Sheffield, Heriot Watt University and the department of chemistry at Strathclyde.

“This is a very strong partnership, with an ambitious and exciting research agenda,” said professor Martin Dawson of the IoP, coordinator of the project. “We have made major progress during the course of the grant.”

So far, the group’s scientists have developed working prototypes of microLED arrays in a variety of configurations, producing ultraviolet, blue and green versions, resulting in pattern-programmable light with the potential for mass production in a simple, integrated solid-state package with no moving parts. Researchers in the physics department at the Institute of Biomedical Engineering at Imperial College London have been applying the arrays to biomedical photonics systems.

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Figure 1. The programmable lighting pattern and rapid switching capability of a huge array of microscopic LEDs open up novel biomedical applications. Courtesy of the Institute of Photonics, University of Strathclyde.


A less costly approach

One application area is high-resolution 3-D microscopy, where techniques such as confocal imaging reduce the blurring effects caused by light from outside the focal plane and yield optically sectioned images. Conventional confocal microscopes require expensive systems to scan a focused laser spot across the specimen. The electronically addressable microLEDs illuminate the sample with rapid changes of patterned light and brightness at many kilohertz, enabling the construction of compact scanning microscopes that have no moving parts.

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Figure 2. These microscope images of fluorescent pollen grains show that confocal imaging with microstripe LED illumination (right) reduces the blurring from objects outside the focal plane of conventional wide-field imaging (left).


Another application is neuron stimulation, where researchers optically stimulate the electrical firing of neural cells genetically modified to be photosensitive. Previously, lasers, arc lamps and broad-area LEDs stimulated these neurons, either with flood illumination or at a single point. MicroLEDs, however, allow precise, synchronized 2-D illumination at multiple locations with subcellular resolution.

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Figure 3. This fluorescence image shows a neuron cell (green) illuminated by three pixels from a MicroLED (blue). The diameter of the blue illumination is ~20 μm. The inset shows the optical stimulation pulses (lower trace) and the recorded electrical response from the cell (upper trace).


The technology is applicable to fundamental neuroscience studies and to areas such as retinal prostheses. It integrates CMOS drivers and single-photon avalanche diodes in the same footprint as the LED emitters, while lens arrays fabricated in the sapphire substrate focus or collimate the output light. The experimenters also have fabricated diamond lens arrays and polymer lenses with photocurable UV transmission self-aligned over selected LEDs and have demonstrated quantum dot colour conversion.

Further work has blended light-emitting polymers into the UV transmissive materials to produce red, green, blue or white output from a UV-emitting LED. The technology is flexible, providing arrays of various sizes in disc, ring or striped formats, and with switching speeds as fast as 100 ns and LED sizes down to 4 μm. The IoP has patented a key manufacturing process and is pursuing commercial application opportunities. It is seeking to raise start-up funds from entrepreneurs and is proposing to offer development kits for industrial and academic laboratories to evaluate and demonstrate novel applications.


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