Spiral phase contrast imaging in nonlinear optics: seeing phase objects using invisible illumination Qiu, Xiaodong ; Li, Fangshu ; Zhang, Wuhong ; Zhu, Zhihan ; Chen, Lixiang Abstract Publication: Optica Pub Date: February 2018 DOI: 10.1364/OPTICA.5.000208 Bibcode: 2018Optic...5..208Q