Research

Topological photonics

Photonic topological waveguides attracted much interest recently for they promise immunity to backscattering unlike in ordinary photonic circuitry, where fabrication imperfections, disorder or arbitrary bends could severely reduce signal transmission. However, existing implementations have closed boundary, are complex, bandwidth limited, and require fine tuned materials. We have demonstrated a new simple approach based on ultrathin complementary metasurfaces free of these drawbacks. This bring the technology closer to practical settings including for use in flexible electronics and active CMOS devices.

I addition, we have demonstrated photonic higher-order topological insulators using metallic metasurfaces as well as silicon-on-insulator photonic crystals based on band inversion due to altered intra/inter-cell coupling strengths. By doing so, we have shown that not only surface and edge states, but also zero-dimensional corner states can be topologically protected.

Aperiodicity & real-space topology

Amorphous photonic topological insulators are superior to their periodic counterparts in that their topological modes are protected against long-range disorder. We have realized these in a reciprocal system using complementary metasurfaces by leveraging EM duality, and observed robust edge modes at the interface of such metasurfaces as well as along grain boundaries in their interior. Our platform can accommodate large degree of randomness, unlike crystalline symmetry-based platforms, making it promising for self-assembly production of optical topological metamaterials in lieu of arduous top-down nanofabrication technology.

In addition, we studied topological defects in real space, which are elementary lattice imperfections that cannot be removed by local perturbations. We have shown that photonic topological metasurface with such point and line defects in analogy to declinations and grain boundaries, respectively, in amorphous graphene, support cavity and guided modes at their interior. These modes are robust and may serve as alternative to 0D and 1D states in higher-order photonic topological insulators. Moreover, these point defects exhibit unique local-spin field distributions at subwavelength scale.

Valley-contrast physics

Valley-contrast physics has gained considerable attention for use in photonics, particularly for designing quantum valley Hall effect-type photonic topological insulators. These enable valley-polarized modes that are reflection-free in the absence of inter-valley scattering disorder, such as zigzag bends. We have demonstrated new paradigm to support such modes at telecommunication band using silicon-on-insulator triangular holey photonic crystal with C6v symmetry. This is done by rearranging one row of unit cells like in common line defect waveguides such that opposite orbital angular momenta states occur across the waveguide region.

Parity-Time-Duality symmetry

Reflection-less TEM waveguides can be achieved by exploiting the intrinsic time-reversal symmetry of the electromagnetic fields of TEM modes: counterpropagating modes have orthogonal polarizations that are time reversal counterparts of one another. We have designed a slot line waveguide that is enclosed by perfect-magnetic-conductors such that only fundamental quasi-TEM modes exist while coupling to extraneous modes such as plane waves is prevented. Perfect transmission efficiency is observed using ideal boundaries and practical PCB implementation shows clear enhancement over traditional designs.