Mode reduction, Q loss, gain saturation, and bandfilling modification of the light versus current characteristics of thin (∼0.9 μm) quantum well heterostructure lasers

Data are presented demonstrating the effect that cavity length, and thus Q, has upon quantum well heterostructure edge-emitting laser diodes that are reduced to microcavity thickness. The lasers, with reduced mode density and enhanced spontaneous emission, are defined vertically by a Ag top-contact mirror and a closely spaced (∼0.9 μm) high-contrast AlAs native oxide-GaAs distributed Bragg reflecting bottom mirror. For shorter and shorter diode lengths (700→70 μm, and still lesser mode density) the light versus current (L–I) characteristic below threshold is at first steeper and steeper (amplified stimulated emission), until, at a diode length of ∼100 μm, the loss in Q and insufficient gain are manifest as a downward bend in the L–I curve and a shift to higher threshold current where bandfilling to a higher state (shorter wavelength) contributes more gain.