Growth and nonlinear optical properties of GaAs absorber layers for AlGaAs/CaF2 semiconductor saturable absorber mirrors

Absorber layers of semiconductor saturable absorber mirrors are required to show high absorption modulation with recovery times on the order of 100 fs and low nonsaturable losses. While to provide such fast recovery times, de fect states for carrier trapping need to be incorporated into the absorber material, defects and surface roughness can cause additional losses in nons aturable absorption. A specially designed GaAs/fluoride multilayer stack wa s grown to study the relation between growth conditions, surface roughness, nonsaturable losses, and absorption modulation. The growth of the multilay er stack included three epitaxial growth regimes: (a) homoepitaxial growth of GaAs on GaAs (111) B, (b) heteroepitaxy of CaF2 on GaAs, and (c) heteroe pitaxy of GaAs on CaF2. While the homoepitaxial and first CaF2 layer growth proceeded two dimensional, island nucleation was obtained for the GaAs abs orber and top CaF2 layer. The CaF2 surface was exposed to an electron beam of different doses to increase free surface energy for subsequent GaAs over growth and the surface roughness of the absorber layer was found to decreas e with increasing electron dose. Nonlinear properties and light scattering were measured and con-elated to the growth parameters. Linear reflectivity and absorption modulation were close to the theoretical values of the desig ned multilayer stack fur the region exposed to the highest electron dose. W ith a recovery time of about 500 fs, the grown GaAs absorber layer is an ex cellent choice for an all-optical switching application in broadband AlCaAs /CaF2 semiconductor saturable absorber mirrors. (C) 2000 American Vacuum So ciety.