Light Engineering in Nanometer Space

Significant advances have been made in photonic integrated circuit technology, similar to the development of electronic integrated circuits. However, the miniaturization of cavity resonators, which are the essential components of photonic circuits, still requires considerable improvement. Over the past decades, various optical cavities have been utilized to implement next‐generation light sources in photonic circuits with low energy, high data traffic, and integrable physical sizes. Nevertheless, it has been difficult to reduce the size of most commercialized cavities beyond the diffraction limit while maintaining high performance. Herein, recent advancements in subwavelength metallic cavities that can improve performance, even with the use of lossy plasmonic modes, are reviewed. The discussion is divided in three parts according to light engineering methods: subwavelength metal‐clad cavities engineered using intermediate dielectric cladding; implementation of plasmonic cavities and waveguides using plasmonic crystals; and development of deep‐subwavelength plasmonic waveguides and cavities using geometric engineering. A direction for further developments in photonic integrated circuit technology is also discussed, along with its practical application. The recent advancements in miniaturized cavity resonators employing three light‐engineering methods are discussed: 1) cladding engineering in subwavelength metal‐clad cavities for room‐temperature lasing and efficient waveguide coupling; 2) low‐loss plasmonic cavities and waveguides engineered by plasmonic crystals; and 3) geometric engineering for photon squeezing into a deep‐subwavelength space.