Enhanced Emission from a Light-Emitting Diode Modified by a Photonic Crystal
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Enhanced Emission from a Light-Emitting Diode Modified by a Photonic Crystal Alexei A. Erchak, Daniel J. Ripin, Shanhui Fan, John D. Joannopoulos, Erich P. Ippen, Gale S. Petrich, and Leslie A. Kolodziejski. Center for Materials Science and Engineering and Research Laboratory of Electronics Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, U.S.A. ABSTRACT Enhanced light output is observed from a light-emitting diode (LED) structure containing a two-dimensional (2D) photonic crystal. The capture of emitted light into planar waveguide modes reduces the extraction efficiency of LEDs. Here, 2D photonic crystals are utilized to: 1) enhance the extraction of light in the vertical direction from an LED and 2) enhance optical pumping by directly coupling light from a pump laser into the LED structure. Spatially and spectrally-resolved photoluminescence (PL) is used to characterize the enhancements. A 100-fold enhancement in extraction for a single wavelength into the vertical direction is obtained without the photonic crystal penetrating the active material.
INTRODUCTION Semiconductor LEDs have the potential to be low-cost and long lifetime solid-state lighting sources for applications as varied as room lighting and flat-panel displays [1]. LEDs are also used in short-range telecommunication systems and may be desirable for optical interconnects in computers [2,3]. Unfortunately, most of the light emitted from a semiconductor LED is lost to planar guided modes propagating within the high dielectric material resulting in a low extraction efficiency. Several improvements have been made to enhance the extraction of light from a LED. Surface texturing randomly scatters light out of the high-dielectric semiconductor material, improving the extraction efficiency [4]. By placing the emitter inside a microcavity, the number of optical modes may be limited to a set of resonant modes that efficiently couple light out of the device [5]. Geometrical strategies, such as the truncated inverted pyramid configuration, improve the LED extraction efficiency [6]. In addition, placing the active region on a transparent substrate results in higher extraction efficiencies [7]. In this work, the LED extraction efficiency is enhanced by using a 2D photonic crystal. In addition to improving the extraction of light, the photonic crystal provides several useful features for LEDs. The photonic crystal may lead to highly-directional light output from the LED for improved coupling to an optical fiber. The 2D photonic crystal may be integrated with other microphotonic devices in a planar geometry. Perhaps most important, the photonic crystal design may be scaled to any wavelength of operation to improve the extraction from an LED fabricated from any material system.
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THEORY A semiconductor LED consists of an active material, such as a quantum well, that emits light on a substrate. Light emitted from the quantum well, embedded within the active region, couples into two distinct types of optical modes that are s
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