Light and matter can strongly mix together to form hybrid particles called polaritons. In recent years, polaritons in the so-called ultrastrong coupling (USC) regime have attracted much attention from both fundamental and applied points of view. A variety of nonintuitive phenomena and novel ground states with exotic properties have been predicted for systems in the USC regime, some of which have been experimentally realized. In this chapter, we review the current state of this exciting, rapidly developing field, focusing on USC phenomena in engineered semiconductor systems. We start by giving a brief historical survey of the field and describe the motivations to pursue USC studies. We then provide a detailed mathematical description of the existing theoretical models for USC physics, mentioning some of the controversies related to the approximations and assumptions that break down in the USC regime. Furthermore, we describe some of the groundbreaking experiments that have been conducted recently in diverse semiconductor-based platforms such as intraband transitions, plasmon–phonon polaritons, exciton polaritons, magnon polaritons, and magnon–magnon coupled systems, highlighting the new physics revealed. Finally, we discuss some of the technological applications that are expected to be enabled by USC, especially in connection with modern information technologies such as quantum computing and quantum information processing.

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Nicolas Marquez Peraca, Andrey Baydin, Weilu Gao, Motoaki Bamba, Junichiro Kono (Role: Contributor, Ultrastrong light–matter coupling in semiconductors)

Semiconductor quantum science and technology

Elsevier, Academic Press; Semiconductors and Semimetals 105: 89–151 (2020)