(PDF) A Survey of CPU-GPU Heterogeneous Computing Techniques | Sparsh Mittal and Jeffrey Vetter - Academia.edu Accepted in ACM Computing Surveys 2015
This paper clearly shows how research on heterogeneous computing has increased greatly in last few years.
Table 3 of this paper summarizes which programming language has been used for GPU and CPU in heterogeneous computing research. Evidently, OpenCL is increasingly being used for programming CPU-GPU heterogeneous systems.