Very strong nonlinear optical absorption in green GaInN/GaN multiple quantum well structures

摘要

The efficiency of green GalnN/GaN light emitting diodes (LEDs) is known to drop under high current densities. To ex-plore the reason, 535 nm emitting GaInN/GaN multiple quan-tum well (MQW) and GaN epilayers were characterized using Z-scan techniques under comparable excitation conditions with a continuous wave laser. Two wavelengths, 514 nm and 488 nm, were selected right below and above the apparent op-tical absorption edge. At 514 nm, a very large nonlinear ab-sorption coefficient β= 2.6 cm/W was obtained. This leads to a 20% absorption enhancement at a photon flux of 21 kW/cm~2. We attribute this to nonlinear free-carrier absorp-tion. On the other hand, at 488 nm, a nonlinear saturable ab-sorption with β = -1.7 cm/W was observed. This induced transparency indicates photon bleaching in the MQW. Appar-ently, free carrier dynamics strongly affects optical nonl inear-ity, while nonlinear absorption provides a small contribution to the limitations of current green LEDs.