A High-Efficiency Broadband Superconducting Nanowire Single-Photon Detector with a Composite Optical Structure**Supported by the National Basic Research Program of China under Grant Nos 2011CBA00100 and 2011CBA00200, the National Natural Science Foundation of China under Grant Nos 11227904 and 61101012, the National High-Technology Research-Development Program of China under Grant No 2011AA010204, and the Jiangsu Key Laboratory of Advanced Techniques for Manipulating Electromagnetic Waves. (March 2015)