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/CaF2 SESAM with a high reflection bandwidth of more than 300nm for ultrashort pulse generation with GaAs used as saturable absorber. We will demonstrate a pulse spectrum supporting sub-G-fs pulses obtained fi um an AlCaAs/CaF2 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 CaF2. Defects in the GaAs absorber are necessary to achieve fast response times. However, they can generate surface roughness causing nonsaturable losses by light scalti ring. A GaAs saturable absorber was growth on a two pair AlGaAs/CaF2 Bragg mirror with CaF2 spacer layer without significant degradation of the saturable absorber parameters relevant for passive e mode-locked solid-state lasers.