Z₂ Photonic Topological Insulators in the Visible Wavelength Range for Robust Nanoscale Photonics

Abstract

Topological photonics provides an ideal platform for demonstrating novel band topology concepts, which are also promising for robust waveguiding, communication and computation applications. However, many challenges such as extremely large device footprint and functionality at short wavelengths remain to be solved which are required to make practical and useful devices that can also couple to electronic excitations in many important organic and inorganic semiconductors. In this letter, we report an experimental realization of Z2 photonic topological insulators with their topological edge state energies spanning across the visible wavelength range including in the sub-500 nm regime. The photonic structures are based on deformed hexagonal lattices with preserved six-fold rotational symmetry patterned on suspended SiNx membranes. The experimentally measured energy-momentum dispersion of the topological lattices directly show topological band inversion by the swapping of the brightness of the bulk energy bands, and also 2 the helical edge states when the measurement is taken near the topological interface. The robust topological transport of the helical edge modes in real space is demonstrated by successfully guiding circularly polarized light beams unidirectionally through sharp kinks without major signal loss. This work paves the way for small footprint photonic topological devices working in the short wavelength range that can also be utilized to couple to excitons for unconventional light-matter interactions at the nanoscale. Keywords: photonic topological insulator, photonic crystal, visible-wavelength topological photonicsArising from the demonstrated generality of band topology concepts borrowed from solidstate electronic band structures, photonic topological insulators (PTIs) exhibit topologically protected edge states that lead to unique optical transport properties such as immunity to defects and lossless propagation through sharp turns, hence providing a promising platform to build robust photonic waveguides, communication lines and circuits. 1-3 Furthermore, given the flexibility of system design and fabrication with various optical materials, PTIs serve as testbeds in demonstrating concepts that are otherwise difficult to achieve in condensed matter systems, such as Floquet topological insulators 4-7 , higher order PTIs 8-14 , and PT-symmetric topological structures [15][16][17] . PTIs have now been demonstrated in various geometries, including gratings and arrays in one dimension 18-23 , photonic and plasmonic lattices and fiber arrays in two-dimensions 24-39 , as well as three-dimensional PTIs 40-42 , and with different mechanisms such as SSH insulators 19-23 , Chern 24-27 and valley Chern insulators [28][29][30][31][32] , and 2D and 3D Z2 topological insulators [34][35][36][37][38][39] . PTIs

3D Printed Polymer Photonic Topological Insulators and their Robustness to Fabrication Disorder

“…We found complete photonic band gaps for structures with refractive index as low as n=1.55 with a0/R=2.4 for the topological lattice (Figure 1a,b) and a0/R=4.4 for its trivial counterpart (Figure S1), when the rod radius is set to r=0.12a0. In contrast, previously demonstrated photonic topological insulators have been fabricated from materials with much higher index, such as silicon [27] (n=3.8) and silicon nitride (n=2.0) [40] in the visible and alumina at microwave frequencies (n=3.1). [41] Our design thus greatly expands the pallet of available materials for producing photonic topological insulators, including polymers at optical, infrared, and microwave frequencies, and oxides at optical frequencies.…”

Rapid Design, Fabrication, and Optimization of 3D Printed Photonic Topological Insulators

“…We found complete photonic band gaps for structures with refractive index as low as n=1.55 with a0/R=2.4 for the topological lattice (Figure 1a,b) and a0/R=4.4 for its trivial counterpart (Figure S1). In contrast, previously demonstrated photonic topological insulators have been fabricated from materials with much higher index, such as silicon nitride in the visible [21] (n=2.0) and alumina at microwave frequencies (n=3.1). [22] Our design thus greatly expands the pallet of available materials for producing photonic topological insulators, including polymers at optical, infrared, and microwave frequencies, and oxides at optical frequencies.…”

Near- and Far-Field Excitation of Topological Plasmonic Metasurfaces

“…A number of experimental works have investigated the directionality of these edge states under far-field excitation in a range of frequency regimes [17,21,22]. In each of these experiments, purely unidirectional modes are observed.…”

“…This result is true for any bosonic breathing honeycomb lattice [16], since it is rooted in time-reversal symmetry and the absence of Kramer's degeneracy for bosons as opposed to fermions. There have been a range of experimental investigations on the breathing honeycomb photonic crystal [17][18][19][20], including the observation of edge modes in the visible regime [21,22]. Despite this, comprehensive theoretical studies of the directional excitation of pseudospin edge modes in the breathing honeycomb lattice based on an analysis of their inhomogeneous electromagnetic angular momentum are scarce; in particular, modelling with a circularly polarised incident beam, as used in recent experiments [17,21,22] are needed.…”