What is the fine-grained topic of the following text: L. S. Amato, N. Ghofraniha, V. Folli, and C. Conti, "Formation and propagation of shock waves in nonlocal media," in Advanced Photonics Congress, OSA Technical Digest (online) (Optical Society of America, 2012), paper JM5A.44. Only return the topic.
Formation and propagation of shock waves in nonlocal media