Optical gain and gain saturation of blue‐green InGaN quantum wells

Using varied stripe length method we systematically studied optical gain properties of blue‐green 3 nm InGaN QWs grown on c‐plane and (11−22) semipolar substrates. We determined that for such structures when the product of modal net gain at peak and stripe length exceeds factor 5 the gain saturation occurs due to depletion of pumped carriers. We then focused our attention on the gain in unsaturated conditions. We observed strong gain peak position blue shift with increase of pumping power for both substrate orientations due to quantum well state filling and for c‐plane due to piezoelectric field screening. Thus in order to increase lasing wavelength, minimizing optical losses, and maximizing modal gain are essential. We then found that for the semipolar QWs the gain at ∼500 nm was 2× higher with the stripe along [−1−123] direction despite the fact that at low pumping level the polarization switching of spontaneous emission resulted predominant E||[−1−123]. Finally we compared the semipolar and c‐plane QWs and found that the gain increase with pumping power of c‐plane QW is slower than that for semipolar QW in high gain direction while the transparency pumping power is lower for c‐plane.