Nonlinear absorption of highly stacked InAs quantum dot layers on an InP(311) substrate

Semiconductor quantum dots have unique characteristics which advantage especially for saturable absorbers. We characterized nonlinear optical absorption of highly-stacked InAs quantum dot layers on an InP (311)B substrate in 1.5-μm band. High-density (5 x 1012 cm-2) quantum dots by stacked more than 150 layers were recently fabricated with a strain-controlled technique for 1.5-μm communication devices. The transmission increase for the vertical incidence was observed as much as 1%, and transmittance decreased at higher intensity of incident pulse. The temporal behavior of the transmission increase showed two decay components of a few picoseconds and several hundred picoseconds decay. The nonlinear absorption was explained by the saturable absorption of the quantum dots layer and the two photon absorption of the InP substrate. The features of the saturable absorber are suitable for application to a mode-locker for generating short optical pulses in the 1.5-μm band. Analysis showed that we can tune the saturable absorption characteristics by adjusting the numbers of quantum dot layers, thickness of residual InP substrate and antireflection coating on the surfaces. We obtained planar type saturable absorbers at a typical saturation intensity of 35MW/cm2 with 3.5% transmittance change for a traveling type and 8% change for reflective type, respectively.