Abstract
Photonic crystal cavities enable strong light–matter interactions, with numerous applications, such as ultra-small and energy-efficient semiconductor lasers, enhanced nonlinearities and single-photon sources. This paper reviews the properties of the modes of photonic crystal cavities, with a special focus on line-defect cavities. In particular, it is shown how the fundamental resonant mode in line-defect cavities gradually turns from Fabry–Perot-like to distributed-feedback-like with increasing cavity size. This peculiar behavior is directly traced back to the properties of the guided Bloch modes. Photonic crystal cavities based on Fano interference are also covered. This type of cavity is realized through coupling of a line-defect waveguide with an adjacent nanocavity, with applications to Fano lasers and optical switches. Finally, emerging cavities for extreme dielectric confinement are covered. These cavities promise extremely strong light–matter interactions by realizing deep sub-wavelength mode size while keeping a high quality factor.
Highlights
A photonic crystal (PhC) [1] is the optical analogue of a solid-state crystal
Electrons in a crystal travel as waves through the periodic potential induced by the crystal atoms and exhibit energy band structures [2]
The frequency and wavevector of photons are linked in a photonic band structure
Summary
A photonic crystal (PhC) [1] is the optical analogue of a solid-state crystal. Electrons in a crystal travel as waves through the periodic potential induced by the crystal atoms and exhibit energy band structures [2]. Nanomaterials 2021, 11, 3030 a large photonic band gap for TE polarization (i.e., with the electric field predominantly in x direction) [1] and is often employed in the realization of PhC lasers In this case, the slab includes the active medium, made of strained quantum wells [3,4] or quantum dots [5,6], which provide high gain for TE-polarized light. A cavity may be realized in a semiconductor nanobeam [7] (Figure 1b) In this case, the photonic band gap confines the light in the longitudinal (i.e., z) direction, and confinement in the other directions is by total internal reflection.
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