Electromagnetic Nonreciprocity

Abstract

$N\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}p\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}y$ has emerged as a fundamental concept in modern science and technology. Recent advances in magnet-free nonreciprocal systems have spurred a resurgence of interest in this subject, but have also led to questions and confusion. The authors review nonreciprocity from a global, first-principles perspective, with the aim of establishing a solid foundation for the topic, to resolve previous issues and promote forthcoming research in the field.