A novel directional light-emitting diode based on evanescent wave coupling

We report a novel approach towards realizing a high directionality in the emission of a light-emitting diode (LED) by employing the evanescent wave coupling effect in a small ridge or truncated cone structure. We fabricated an InGaAs/GaAs LED in which a stripe-shaped InGaAs/GaAs quantum well with a stripe width of about 100 nm is embedded at the center of a narrow GaAs ridge (of width 520 nm) by micro processing and epitaxial regrowth techniques. Strong directionality characterized by a half-intensity angle of 43° is observed in planes perpendicular to the ridge axis. The directionality is found to be almost independent of LED operating conditions, an important characteristic which cannot be achieved by conventional resonant-effect-based directionality control techniques and a prerequisite for practical applications of directional LEDs. The experimentally measured emission pattern showed good agreement with that simulated by using the finite-difference time-domain (FDTD) method.