Funding for this research was provided by:
the Hong Kong Scholar Program (XJ2018052)
National Natural Science Foundation of China (61774021, 61504010)
the Fundamental Research Funds for the Central Universities (2018XKJC05)
the Fund of State Key Laboratory of Information Photonics and Optical Communications (Beijing University of Posts and Telecommunications), P. R. China (IPOC2017ZT02)
Article History
Received: 3 June 2019
Accepted: 7 July 2019
First Online: 23 July 2019
Competing Interests
: The authors declare that they have no competing interests.