Intense photoluminescence was measured in a strained 2-mu m-thick GaAs
film grown on a (100)-oriented CaF2 substrate. Circular polarization
of 77%+/-2% was obtained at 77 K under excitation with circularly pola
rized photons below 1.575 eV. For excitation above 1.610 eV the polari
zation is limited to 30%+/-2%. These results are clear indications of
strain induced splitting between the M(j)=\+/-3/2> and the M(j)=\+/-1/
2> hole bands. The deduced splitting was 62.5 +/- 2.5 meV, correspondi
ng to a stress of the order of -12 kbar. The polarization is maximum f
or reception energies very close to the excitation. Strained GaAs/CaF2
is thus a good candidate structure for efficient strongly polarized e
lectron sources.