Optical methods for characterization of deep implants in semiconductors are evaluated, notably optical absorption below band-gap energies, and photoluminescence. Results from such measurements on 2-3-MeV O implants in GaAs are presented, and discussed with emphasis on possible physical interpretations of defect properties. The spectral shape of optical absorption from doses ≤1014 cm-2 indicates that a description of initial damage as amorphous regions is not appropriate. It seems possible to qualitatively explain this spectral behavior before anneal as mainly due to the electronic states caused by disrupted bonds along the ion track. Photoluminescence (PL) spectra of implanted material (before as well as after anneal) can give specific information on the various defect complexes created by the treatment. Furthermore, the intensity reduction of PL due to residual recombination centers is by far the most sensitive way of detecting residual damage in the material, and thus offers several applications for evaluation of, e.g., annealing efficiency or, for low doses, annealing during implantation. Such measurements also show that there is an apparent orientation dependence on the level of damage introduced, so that 〈110〉 is most favorable, while 〈100〉 and especially 〈111〉 orientation of the GaAs surface resulted in higher damage. A characteristic annealing temperature of 460±30 °C for PL efficiency was found in n-type as well as p-type material, indicating annealing of important nonradiative recombination centers at this temperature. The ultimate result of annealing is critically dependent on the encapsulation technique, and we found that neither Al2O 3 nor Si3N4 prepared by sputtering at 400 °C are satisfactory as capping material on n-GaAs, although they caused a slight reduction on the rate of Ga outdiffusion during anneal. Results from PL-profiling experiments are also discussed.