Low-temperature grown near surface semiconductor saturable absorber mirror: Design, growth conditions, characterization, and mode-locked operation

Abstract

We have developed a mode-locked diode-pumped Yb:KY(WO4)2 laser generating nearly bandwidth limited pulses as short as 101 fs. At 1.1 W of absorbed power and for 3% transmission output coupler, the laser delivers 150 mW in pulses of 110 fs duration, which corresponds to the efficiency of 14%. This has been achieved using semiconductor saturable absorber mirror (SESAM) grown by molecular beam epitaxy. The low-temperature (LT) absorbers were crystallized under the carefully optimized growth conditions. The resonantlike type structures ensured relatively high enhancement factor and in consequence high absorption modulation. The main device parameters such as group delay dispersion (GDD) and enhancement factor were chosen to be wavelength independent. The optimization of the growth conditions resulted in a reduction in the nonsaturable absorption in as-grown LT-InGaAs absorbing layer and ensured the fast carrier trapping and recombination. We assume that the nonsaturable losses of the annealed LT layers result from the absorption connected with defects generated in the crystal during LT growth. Moreover, the annealing deteriorates the interface sharpness and the crystal quality of LT pseudomorphic, nonstoichiometric InGaAs layer. On the other hand, higher growth temperature and lower ratio of group V to group III beam equivalent pressure (V/III ratio) ensure lower defect densities and high crystal quality but suffer from the absorption related to AsGa0 conduction band transitions. The careful balancing of these contradictory tendencies allowed for optimization of the absorber properties. The InGaAs quantum well absorbing layer was grown at the temperature as high as 420 °C, under the V/III ratio as low as 10. No postgrowth annealing was performed. The recovery time of the SESAM structure characterized by the pump-probe measurements was equal to 9.6 ps. The nonsaturable losses of 1.94% decreased the modulation depth to 1.48% but still self-starting and stable mode locking was observed. The saturation fluence of 27 μJ/cm2 did not suffer significantly from a little bit too high nonsaturable losses.