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

Spiral phase contrast (SPC) imaging offers a vital, convenient tool for edge detection in image processing. Despite significant experimental and theoretical progress in this area, SPC imaging with invisible light is still lacking. In contrast to the general SPC scheme, here we construct a nonlinear spatial filter by equivalently imprinting the vortex phase plate onto the potassium titanyl phosphate crystal using second harmonic generation (SHG). The phase or intensity objects are displayed by a spatial light modulator (SLM) and illuminated with 1064 nm infrared light. Then the combination of our nonlinear filter with SHG in the Fourier domain enables concise, yet highly efficient SPC imaging, leading to a 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 infrared monitoring.