VNU Journal of Science Mathematics – Physics, Vol 37, No 4 (2021) 51 58 51 Original Article Effective Couplings between Two Photons and Axion in the 3 3 1 Model Hoang Ngoc Long1, Vo Van Vien2,*, Vu Hoa Binh1,3 1Institute of Physics, Vietnam Academy of Science and Technology, 10 Dao Tan, Ba Dinh, Hanoi, Vietnam 2Tay Nguyen University, 567 Le Duan, Buon Ma Thuot, Dak Lak, Vietnam 3Graduate University of Science and Technology, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay[.]