Synergistic Nonreciprocity of Linear and Nonlinear Optical Diffraction

Linear optical diffraction of light is a basic natural phenomenon subject to a long history study and it obeys the well-known reciprocity in transport. In this work we report observation of synergistic nonreciprocal linear and nonlinear diffraction of a Ti:sapphire femtosecond laser beam against a periodic poled lithium niobate (PPLN) thin plate nonlinear grating with a front surface corrugated with a shallow grating of a depth only 67 nm and a smooth back surface. A high peak power pump laser beam shining upon the geometrically asymmetric nonlinear grating from either the front surface and back surface will both cause significant second-order nonlinear (2nd-NL) Raman-Nath diffraction and Cerenkov radiation, in addition to apparent linear optical diffraction and modest third-order nonlinear (3rd-NL) spectral broadening. These linear, 2nd-NL, and 3rd-NL optical processes all exhibit significant differences between the front-back forward and back-front backward transport processes, in that the linear and 2nd -NL nonlinear diffraction have distinctly different patterns, while the 3rd-NL spectral broadening extent is different. The apparent synergistic nonreciprocal linear and nonlinear diffraction can be ascribed to the different coupling strength between the linear, 2nd -NL, and 3rd-NL processes of pump laser within the PPLN nonlinear grating in the forward and backward pump. The observation of both strong nonreciprocal linear and nonlinear optical diffraction would enrich the concept and physics of fundamental optical reciprocity and nonreciprocity laws.