Implementing structural slow light on short length scales: the photonic speed bump

Abstract

One--dimensional (1D) infinite periodic systems exhibit vanishing group velocity and diverging density of states (DOS) near band edges. However, in practice, systems have finite sizes and inevitably this prompts the question of whether helpful physical quantities related to infinite systems, such as the group velocity that is deduced from the band structure, remain relevant in finite systems. For instance, one may wonder how the DOS divergence can be approached with finite systems. Intuitively, one may expect that the implementation of larger and larger DOS, or equivalently smaller and smaller group velocities, would critically increase the system length. Based on general 1D--wave--physics arguments, we demonstrate that the large slow--light DOS enhancement of periodic systems can be observed with very short systems, whose lengths scale with the logarithm of the inverse of the group velocities. The understanding obtained for 1D systems leads us to propose a novel sort of microstructure to enhance light-matter interaction, a sort of photonic speed bump that abruptly changes the speed of light by a few orders of magnitude without any reflection. We show that the DOS enhancements of speed bumps result from a classical electromagnetic resonance characterized by a single resonance mode and also that the nature and the properties of the resonance are markedly different from those of classical defect--mode photonic--crystal cavities.

Light Interaction with Photonic and Plasmonic Resonances

“…The pole‐response approach has led to the development of a freeware to compute and normalize QNMs in the general case of resonators made of dispersive and anisotropic materials. It has already been successfully applied to a variety of photonic‐crystal microresonators and plasmonic nanoresonators, eventually placed on a substrate or coupled to a periodic waveguide …”

“…The present period is marked by a deployment of QNM concepts in various applications, quantum plasmonics, spectral filtering with diffraction gratings, energy loss spectroscopy in plasmonic nanostructures, second‐harmonic generation in metal nanoparticles, coupled cavity‐waveguide systems, single‐photon antennas, ultrafast‐dynamics nanooptics, scattering‐matrix reconstruction in complex systems, spontaneous emission at exceptional points, wave transport in disordered media, spatial coherence in complex media, mode hybridization and exceptional points in complex photonic structures, random lasing, light localization and cooperative phenomena in cold atomic clouds, spatially nonlocal response in plasmonic nanoresonators, and thermal emission . We discuss some of these numerous applications of QNM concepts in this Review and Figure summarizes a few.…”

“…QNM theory can be used to study the leakage of photonic‐crystal cavities into periodic waveguide mode and cavity resonances. Reproduced with permission . Copyright 2014 and 2017, Optical Society of America.…”

Slow Light in Subwavelength Grating Waveguides

“…We note that the singularity of the group index is a physical concept specific to infinite crystals that cannot be replicated in real, finite devices. Nevertheless, engineering the input and output of the slow light devices can significantly increase the achievable group index in real devices [8], [17].…”

“…Although in simulation a completely flat band, infinite group index and infinite local density of states (LDOS) is easily obtainable due to the infinite extent of the simulated geometry, the finite nature of real devices prevent the group velocity to completely vanish [17]. Multiple methods have been proposed to circumvent the limitation imposed by the finite geometry [8], [9], [17], [22]. As mentioned in the previous section, we opted to implement long physical tapers to convert the routing strip waveguide mode to the slow light SWG Bloch mode.…”