Robust topologically protected transport in photonic crystals at telecommunication wavelengths Mikhail I. Shalaev, Wiktor Walasik, Alexander Tsukernik, Yun Xu and Natalia M. Litchinitser Nature nanotechnology.14,31-34(2019) Previous Research The first photonic analogue of a quantum Hall topological insulator, realized in the microwave regime using gyromagnetic materials, and a strong magnetic field to break the time-reversal symmetry Wang, Z., Chong, Y., Joannopoulos, J. D. & Soljačić Nature 461, 772–775 (2009) Previous Research non-magnetic quantum Hall topological insulators mimicking time-reversalsymmetry breaking demonstrated at near-infrared frequencies Honeycomb lattice helical waveguides along z-direction Band structures for the case of non-helical/helical waveguides Rechtsman, M. C. et al. Nature 496,196–200 (2013) Previous Research photonic analogues of the spin and valley Hall effects that require spatial symmetry breaking Bianisotropic metawaveguide (BMW) The bianisotropy is generated by finite vacuum gap between rods and one of the metal plates Hexagonal lattice Ma, T., Khanikaev, A. B., Mousavi, S. H. & Shvets,G. Phys. Rev. Lett. 114, 127401 (2015) Previous Research SOC: spinorbit coupling QSH QVH QH gyromagnetic material Ma, T. & Shvets, G. Phys. Rev. B 95, 165102 (2017) Breaking of spatial(rotational) symmetry in QVH-PTI Design of The Device • Realization of optical topological insulator that demonstrates valley Hall effect at nearinfrared frequencies • Demonstration that transmittances are the same for light propagation along a straight topological interface and one with four sharp turns Silicon SiO2 a0 = 423nm h = 0.639a0 d1 = 0.4a0 d2 = 0.6a0 honeycomb lattice with two inverted equilateral triangular air holes per unit cell Optimization of The Design Results in the opening of an indirect band gap when asymmetry is introduced. The size is limited by the frequency of the first band at the K point and the second band at the M point Smaller triangle(d=0.5a) with reduced etched area introduces lower propagation losses per unit length than the large one The electron-beam-resist (EB-resist) contrast limits the minimum allowed distance between the triangles to around 40 nm. Band Structures Blue: C6 symmetry with Dirac cones at K and K’ Red: C3 symmetry. A band gap opens. The Berry curvature Ω is calculated with numerical plane wave expansion(PWE) method. The Chern number is calculated as below: C𝑘 . 2𝜋 𝐻𝐵𝑍𝑘 /𝑘′ 1 = Ω 𝑘 𝑑2𝑘 = ± 1/2 / 𝑘′ where the integration is carried over half of the first Brillouin zone around K or K’ points Simulation of Topologically Protected Edge Waves ωa0/2π c = 0.263 Observation of TPEWs Numerical calculation Experiment Results optical parametric amplifier Beam splitter for the sample with no edge, the low transmittance region(1420–1510 nm) indicates a band gap. for the samples with an interface, high transmittance in the band gap region proves the existence of edge states the transmittances of the samples with straight and trapezoidal interfaces presented very similar spectral dependencies. This confirms that edge state is immune to scattering at sharp turns. Conclusions • Demonstration of a silicon based VPC at telecommunications wavelengths • Observation of robust transport of light along a topological interface with four sharp turns and a straight interface. • Confirmed the suppression of backscattering and robust topological transport