Research On Topological Devices Based On Edge States In Silicon-based Valley Photonic Crystals

With the rapid development of information technology,the era of big data has arrived,and it has become particularly important to realize an integrated,high-speed,and large-capacity optical information processing platform.In the past decade,topological photonics has been widely studied due to the robust,single-mode,and unidirectional transmission of optical transmission under topological protection,and is considered to be one of the most promising platforms to realize integrated optical communication.The concept of topology was first applied in condensed matter physics,dating back to 1980,when physicists discovered the quantum Hall effect and realized that electron energy bands can be characterized by integer topological invariants.Later topology was introduced into photonics,first realized as quantum Hall systems,which can construct edge states that propagate unidirectionally and have intrinsic backscatter-resistant transport.Subsequently,several types of classical optical topological effects were also discovered,such as the quantum Hall effect,the quantum spin Hall effect,and the quantum valley Hall effect.Among them,the quantum valley Hall topological insulator has gradually become the most promising topological photonic platform due to its advantages of easy on-chip implementation and low loss.In this paper,the control of the edge state mode field in valley photonic crystals is mainly studied,including the manipulation of the energy distribution of the longitudinal mode field,the coupling of topological modes and the control of topological phase.The goal of the research is to use the topology mode with topology protection and other characteristics,expand its application in on-chip optical regulation,and achieve integrated,high-speed,large-capacity all-optical interconnection.To achieve this goal,this paper first studies how to control the longitudinal distribution of topological modes,and proposes an asymmetric topological waveguide.Based on the asymmetric topology waveguide,a compact topology power divider with a power division ratio of 33:67 was designed,and its high-speed data transmission characteristics were tested.Secondly,based on the topological waveguide of valley photonic crystal,the characteristics of its phase modulation are theoretically studied,and the experiment proves that its phase shifting efficiency is 1.57 times higher than that of ordinary waveguides.Based on the fast transmission characteristics of topological phase shifters and topological waveguides,an ultra-compact topological optical switch is proposed,and the switching characteristics are experimentally demonstrated.The research work and innovation points of this paper can be summarized as follows:I.Mode coupling based on asymmetric topological waveguideswe demonstrate a new method to manipulate the mode field distributions in topological waveguides by introducing asymmetric edge states,thus the mode coupling with an arbitrary coupling ratio can be achieved between topological waveguides.As an example,we designed and fabricated an on-chip topological power splitter(TPS)with a33:67 splitting ratio to verify the feasibility of the proposed scheme.In addition,although all previous studies on topological photonics claimed a unique property of robust transmission,to the best of our knowledge,no transmission experiment using topological devices at the telecom wavelengths has been demonstrated to date.In this paper,we carry out a high-speed transmission experiment employing the proposed TPS in the telecom band for the first time.A 128-Gb/s four-level pulse amplitude modulation(PAM-4)signal is transmitted over an on-chip TPS with a low power penalty of < 1 d B.The results verify the feasibility of utilizing topological modes as robust information carriers,which paves the way for future development of a large number of functional topological devices.Key innovations include:(1)A new mode regulation method is proposed by changing the parameters of the valley photonic crystal above and below the edge state of the valley photonic crystal;(2)Using the plane wave expansion method,the coupling mechanism of topological modes is clarified;(3)Using topological mode coupling and asymmetric topological waveguide,a compact 33:67 topological power divider is designed.(4)Based on the designed topological power divider,it is the first experimental demonstration of high-speed transmission of on-chip topological devices in the telecom frequency band.II.Ultra-compact topological photonic switch based on valley-vortex-enhanced high-efficiency phase shiftFirst,the thermos-optic phase-shifting principle of topological edge state modes is investigated,and an ultra-compact 1 × 2 thermo-optic topological switch based on valley photonic crystals operating at telecommunication wavelengths is proposed.Benefiting from the phase vortex formed by the VPC structure,the optical path of the topological edge mode in the propagation direction is approximately two-fold that of the conventional optical mode in a strip waveguide.The experimental results show a 1.57-fold improvement in π-phase tuning efficiency.Based on the high phase-shifting efficiency and sharp-turn transmission of topological waveguides,an ultra-compact thermo-optical topological switch is proposed.The device consists of a heater and a heated metal,and the output of light can be switched between different ports by applying a π phase shift to one arm of the topological Mach-Zern interferometer.Key innovations include:(1)We find that the optical path of the topological edge mode in the propagation direction is approximately two-fold that of the conventional optical mode in a strip waveguide by taking the advantage of the phase vortex formed by the VPC structure.The π phase-shifting efficiency is about 1.57 times that of the normal mode.(2)In the experiment,an ultra-compact topological switch is demonstrated with an area of 25.66 μm × 28.3 μm by leveraging the reduced waveguide length and the sharp turn feature of the topological waveguide to shrink the footprint.To the best of our knowledge,this topological photonic switch is the smallest switch compared with any dielectric or semiconductor 1×2/2×2 broadband optical switches,including thermo-optic and electro-optic ones.(3)A high-speed transmission experiment is carried out by using the proposed TOTS.This is the first bit error ratio testing to demonstrate the robust transmission of high speed data in a tunable topological device,to the best of our knowledge.A 132-Gb/s four-level pulse amplitude modulation(PAM-4)signal is transmitted over an on-chip TOTS with a low power penalty of < 0.5 d B.