NEED TO KNOW
A µLED coupling light is used in a 0.125-mm plastic optical fiber.
Tyndall National Institute (Cork, Ireland; www.tyndall.ie) wanted to shed some light on the difficulty associated with extracting light from a semiconductor. By the time the research was completed, the institute had created a new source for miniature lighting applications: microLEDs (µLEDs).
Based on free-standing gallium nitride, the tiny emitters are smaller and more efficient than current diodes, according to researchers. They also have potential uses as alternatives to lasers, which can produce heat that can create problems in applications such as examining tissue or fluid samples from a patient. Because the µLEDs produce less heat, they are less likely to cause a change in the state of the materials being analyzed. Potential applications include their use in fiber-optic probe tips and inside chips used to examine blood samples.
The problem with trying to extract light from a semiconductor into air is that a lot of light remains trapped due to total internal reflection, which leads to severe efficiency losses when converting electricity into light. “As we attempted to minimize these losses through semiconductor shaping, the realization arose that it was possible to get virtually all the light out,” says Bill Henry, Tyndall applications specialist.
To achieve the extraction, Tyndall built an enclosure around the light-emitting area. The enclosure needed to have a high height-over-diameter ratio so that it could, in a single attempt, deflect all of the light rays that were typically trapped. By choosing a parabolic shape for the reflector, researchers discovered that virtually all the light could be guided out of a semiconductor. “The ‘lensing’ effect of the parabolic shape allows the light to be produced in a quasicollimated beam, making the µLED ideal for applications where the light needs to be close to an application, or where stray light may be an issue,” says Henry.
The efficiency of the µLED and extraction technology results in minimal heat generation and allows for simpler device designs. In fact, the extraction efficiency of µLEDs is up to eight times more efficient than conventional LEDs, according to the researchers.
The µLEDs also offer high coupling efficiency for both glass and plastic optical fibers and are available in a range of colors. These features have helped identify potential uses for the technology in microfluidics, lab-on-a-chip applications, and handheld medical devices where power consumption is a key concern. “We are focusing on devices where the production of collimated light for a miniature application in an efficient manner is of utmost importance,” says Henry.
In addition to mounted displays and low-power visible indicators, Henry adds that the institute is also exploring applications for µLEDs in tethered and capsule endoscopy, optical coherent tomography, fiber-coupled light sources, and miniature spectroscopy, such as capillary electrophoresis and liquid chromatography.