Mode spacing "anomaly" in InGaN blue lasers

An important experimental observation in InGaN laser diodes (LDs), which is not yet fully understood, is that the measured mode spacing of the lasing spectra could be one order of magnitude larger than that "calculated'' from the known cavity length. The aim of this letter is to shed light on the na ture of the mode spacing "anomaly'' in InGaN LDs. We have derived a formula which accurately determines the mode spacing in InGaN LDs. Our analysis ha s shown that the discrepancy between the "expected'' and observed mode spac ing is due to the effect of carrier-induced reduction of the refractive ind ex under lasing conditions and this discrepancy decreases and naturally dis appears as the threshold carrier density required for lasing decreases. Sin ce the carrier-induced reduction of the refractive index is expected only f rom an electron-hole plasma state, our results naturally imply that electro n-hole plasma recombination provides the optical gain in InGaN LDs, like in all other conventional III-V semiconductor lasers. The implications of our results on the design of nitride optoelectronic devices are also discussed . (C) 1999 American Institute of Physics. [S0003-6951(99)03008-9].