Spiral phase contrast imaging in nonlinear optics: seeing phase objects using invisible illumination

Spiral phase contrast (SPC) imaging offers a vital and convenient tool for edge detection in the image processing. Despite significant experimental and theoretical progress in this area, the SPC imaging with invisible light is still lacking. In contrast to the general SPC scheme, here we construct the nonlinear spatial filter by equivalently imprinting the vortex phase plate onto the Potassium titanyl phosphate crystal via the process of second harmonic generation (SHG). The phase or intensity objects are displayed by a spatial light modulator (SLM) and illuminated with 1064nm infrared light. Then the combination of our nonlinear filter with SHG in the Fourier domain enables a concise yet high-efficient SPC imaging, leading to visible edge enhancement with invisible illumination. By programming a running dog cartoon with SLM, we also demonstrate the capacity of our scheme to detect edges and contours in real time. Our present scheme could find direct applications in the infrared monitoring.