Abstract
Semiconductor saturable absorber mirrors (SESAMs) are used for ultrashort pulse generation. They are required to support self-starting sub-10-fs pulse generation with Ti:sapphire lasers. So far conventional AlGaAs/AlAs SESAMs have been limited by their small reflection bandwidth of about 60 nm. Broadband SESAMs with high reflection bandwidth were needed. In this paper, we report for the first time on the successful growth and operation of a broadband AlGaAs/CaF 2 SESAM with a high reflection bandwidth of more than 300 nm for ultrashort pulse generation with GaAs used as saturable absorber. We will demonstrate a pulse spectrum supporting sub-6-fs pulses obtained from an AlGaAs/CaF 2 SESAM in a Ti:sapphire laser. Furthermore, we have studied the formation of twin defects depending on the growth mode for the GaAs absorber layer heteroepitaxially grown on CaF 2. Defects in the GaAs absorber are necessary to achieve fast response times. However, they can generate surface roughness causing nonsaturable losses by light scattering. A GaAs saturable absorber was grown on a two pair AlGaAs/CaF 2 Bragg mirror with CaF 2 spacer layer without significant degradation of the saturable absorber parameters relevant for passive mode-locked solid-state lasers.
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