Photonic crystal slow light waveguides in a kagome lattice

Slow light topological photonics with counter-propagating waves and its active control on a chip

Topological slow light exhibits potential to achieve stopped light by virtue of its widely known robust and non-reciprocal behaviours. Conventional approach for achieving topological slow light often involves flat-band engineering without disentangling the underlying physical mechanism. Here, we unveil the presence of counter-propagating waves within valley kink states as the distinctive hallmark of the slow light topological photonic waveguides. These counter-propagating waves, supported by topological vortices along glide-symmetric interface, provide significant flexibility for controlling the slowness of light. We tune the group velocity of light by changing the spatial separation between vortices adjacent to the glide-symmetric interface. We also dynamically control the group delay by introducing a non-Hermitian defect using photoexcitation to adjust the relative strength of the counter-propagating waves. This study introduces active slow light topological photonic device on a silicon chip, opening new horizons for topological photon transport through defects, topological light-matter interactions, nonlinear topological photonics, and topological quantum photonics.

Reversible Conversion of Odd/Even One-Way Modes in Magneto-Optical Photonic Crystal Double-Channel Waveguides

We have studied the transmission properties of odd/even one-way modes and their reversible conversion in a double-channel waveguide consisting of two magneto-optical photonic crystals (MOPCs) sandwiched with Al₂O₃ PC. There exist two pairs of even and odd modes, i.e., M1(even)/M2(odd) or M3(odd)/M4(even) modes, for the double-channel waveguides with one- or two-stranded coupling layer of Al₂O₃ rods, respectively. Among them, the M1, M2, and M3 modes are caused by the weak coupling strength of two sub-waveguides, while the M4 mode results from the strong coupling effect and supports dispersionless slow-light propagation. Furthermore, we realize the reversible conversion between odd and even modes (i.e., between M1 and M2 modes, or M3 and M4 modes) in the one- or two-stranded structure, respectively, by adjusting the length and position of the perfect electric conductor (PEC) defect properly to cause the desired significant phase delay along the upper and lower equivalent transmission paths. Additionally, we find that the robustness of the M1 even mode is poor because of extra excitations of counter-propagation modes near the right Brillouin boundary, while the other three modes have extremely strong robustness against PEC defects and their one-way transmittances are nearly 100%. These results hold promise for many fields, such as slow-light modulation and the design of topological devices.

Slow wave and truly rainbow trapping in a one-way terahertz waveguide

Slowing down or even trapping electromagnetic (EM) waves attract researchers' attention for its potential applications in energy storage, optical signal processing and nonlinearity enhancement. However, conventional trapping, in fact, is not truly trapping because of the existence of strong coupling effects and reflections. In this paper, a novel metal-semiconductor-semiconductor-metal (MSSM) heterostructure is presented, and novel truly rainbow trapping of terahertz waves is demonstrated based on a tapered MSSM structure. More importantly, functional devices such as optical buffer, optical switch and optical filter are achieved in one single structure based on the truly rainbow trapping theory. Owing to the property of one-way propagation, these new types of optical devices can be high performance and are expected to be used in integrated optical circuits.

Zero GVD slow-light originating from a strong coupling of one-way modes in double-channel magneto-optical photonic crystal waveguides

We have studied the coupling effect of topological photonic states in a double-channel magneto-optical photonic crystal waveguide by introducing a two-stranded ordinary Al₂O₃ photonic crystal as the coupling layer. There exist both M1 (odd) and M2 (even) one-way modes simultaneously in the bandgap. Interestingly, M1 mode is always a fast-light mode with large group velocity (vg) and large group velocity dispersion (GVD) regardless what the radius (RA) of Al₂O₃ rods is. However, when RA is appropriate, M2 mode becomes a very slow-light mode exhibiting near-zero vg and zero GVD simultaneously. The physical reason of such slow-light is attributed to the strong coupling effect between the one-way edge modes in both sub-waveguides. Furthermore, the simulation results show that the robustness of both the fast- and slow-light modes are extremely strong against perfect electric conductor defect and the one-way transmittance is close to 100%. Besides, the PEC defect can cause significant phase delay. These results hold promise for many fields such as signal processing, optical modulation, and the design of various topological devices.

Role of Entropy in Colloidal Self-Assembly

Entropy plays a key role in the self-assembly of colloidal particles. Specifically, in the case of hard particles, which do not interact or overlap with each other during the process of self-assembly, the free energy is minimized due to an increase in the entropy of the system. Understanding the contribution of entropy and engineering it is increasingly becoming central to modern colloidal self-assembly research, because the entropy serves as a guide to design a wide variety of self-assembled structures for many technological and biomedical applications. In this work, we highlight the importance of entropy in different theoretical and experimental self-assembly studies. We discuss the role of shape entropy and depletion interactions in colloidal self-assembly. We also highlight the effect of entropy in the formation of open and closed crystalline structures, as well as describe recent advances in engineering entropy to achieve targeted self-assembled structures.

Slow light with high normalized delay-bandwidth product in organic photonic crystal coupled-cavity waveguide

In this theoretical study, a photonic crystal coupled-cavity waveguide created on polymer substrate was demonstrated to provide high-performance slow light with low group-velocity dispersion and large normalized delay-bandwidth product. Combined with structural-parameter optimization, the normalized delay-bandwidth product was enhanced to a large value of 0.809, and the group-velocity dispersion was on the order of 10⁴(ps²/km). Furthermore, the optimized coupled-cavity waveguide had tunability capabilities by changing the external pump laser power. Importantly, while adjusting the slow light, the normalized delay-bandwidth product values remained above 0.8, which was necessary to maintain the performance of optical buffering devices.

Ultra-slow light in one-dimensional Cantor photonic crystals

Ultra-slow light and complete transmission properties in one-dimensional Cantor photonic crystals are presented. In contrast to traditional dielectric photonic crystals, the proposed structure has large group delay, slower group velocity, and a high quality factor within the same layers and materials. This study shows that larger than 1 μs group delay and slower than 1 m/s group velocity are achieved in the fifth-order Cantor photonic crystal with 52.75 μm length. This ultra-slow-light structure is very promising for application in advanced slow-light devices. A high quality factor of 10⁹ and multiband filters with complete transmission can also be obtained by using this approach.

Highly controlled Bloch wave propagation in surfaces with broken symmetry

We propose and demonstrate reduced symmetry photonic surfaces providing highly controlled Bloch wave propagation. The backward and dual directional propagations have been observed in the proposed low-symmetric periodic structures without variation in the unit-cell filling factor. Frequency-domain analyses present group indices up to negative/positive -237/+96 as strong indicators of the observed directional controlled surface waves driven by the orientation angle in the range of 20°-90°. Further verification of the index-based propagation direction has been achieved through detailed time-domain analyses and microwave experiments. Smart management of the propagation direction in low-symmetric surfaces has great potential for next-generation photonic applications.