21 Sep 2007
Optical components could benefit from a blue LED which offers enhanced coupling efficiency, stronger light emission, and a more uniform optical power output.
Researchers from the Republic of China say that fabricating a Fresnel microlens onto the sapphire substrate of a blue LED enhances the optical power by around 1.68 times compared to the same device without the lens at a 20mA injection current. (Applied Physics Letters 91 051111).
"The LED chip can emit 80 mcd at 450 nm under a current of 20 mA," Ming-Kwei Lee, researcher at the National Sun Yat-Sen University, Republic of China, told optics.org. "The optical power output of conventional LEDs exhibit a Gaussian distribution. Our LED gives a flat distribution which means a more efficient coupling is obtained."
Incorporating a textured surface into LEDs made from gallium nitride has already been found to enhance its external quantum efficiency, but it is difficult to obtain directional light emission. By fabricating a Fresnel microlens array instead, the researchers have obtained a more directional light enhancement. "The nearly flat surface of the Fresnel microlens also enhances the coupling efficiency with other optical components such as optical fibers without complicated alignment," added Lee.
The key to obtaining increased power output from the sapphire substrate is due to the difference in refractive indices of the materials. "There is a smaller difference in the refractive index between sapphire (1.76) and air, compared to between GaN (2.5) and air," explained Lee. "This means fabricating the microlens on the sapphire side provides a higher external quantum efficiency and greater emission area."
The researchers used a focused ion beam (FIB) milling technique to fabricate the Fresnel microlens structure onto the sapphire. The technique involves a one-step machining process by computer program.
"The sophisticated etching skill we have achieved cannot be obtained using other technologies," commented Lee. "The method uses accelerated gallium ions to etch the target. The path and energy of the charged gallium ions can be well controlled by the electric field."
The researchers will next investigate how the overall power output is affected by introducing a Fresnel lens to different faces of the LED. "We will fabricate a Fresnel lens onto the GaN face and onto the sides of a blue GaN to check the overall output power," concluded Lee. "We will also explore suitable packaging for this type of LED."