Nonlinear differential interference contrast imaging

Differential interference contrast (DIC) imaging is essential in both biological research and medical diagnostics. Despite considerable progress in theoretical and experimental frameworks, limited by the inefficient cameras, achieving direct DIC imaging with infrared (IR) illumination remains a formidable challenge. However, infrared DIC imaging is urgent for diverse fields. Here, we creatively leverage the walk-off effect, a limitation in nonlinear optics, to solve this obstacle and present the nonlinear DIC imaging. The critical component of our scheme is a nonlinear beam displacer (NBD) made up of two quadrature-cascaded type I nonlinear crystals. When the infrared beam carrying object information passes through the proposed NBD, it undergoes nonlinear coupling with the pump beam and then generates two orthogonally polarized visible beams with a slight spatial displacement dominated by the walk-off effect. Accordingly, by selecting polarization, the lateral shear interference for realizing DIC imaging can be realized, and thus the phase discontinuities of the object can be visualized with infrared illumination. Our finding brings DIC imaging technology into the realm of upconversion infrared imaging, paving the way for infrared phase microscopy imaging.