摘要
通过引入液晶,设计一种拓扑边界态可调的太赫兹谷光子晶体,集中研究边界上液晶折射率变化对传输特性的影响。未加液晶时,在0.90~1.03 THz范围内的太赫兹波都保持较高的透射率;填充液晶后,当折射率n=1.8时,透射率曲线上边沿向低频收缩;随着液晶折射率变小,通带范围增大,受拓扑保护的边界态发生显著变化。此外,还构造一种太赫兹波分复用器,实现对拓扑边界态的动态调控,展示其优异的分波特性和缺陷免疫性。研究结果对深刻理解拓扑边界态特性和太赫兹集成芯片的研发具有重要意义。
Objective Terahertz(THz)technology has broad application prospects in astronomy,security,biomedicine,broadband wireless communication,and other fields.However,the current THz system is bulky and has limited applications.THz integrated photonics is the key to further development and wide applications of THz technology,among which photonic topological insulator(PTI)is a good integration platform.Topological edge states(TESs)in the PTI bandgap have caught extensive attention.They can realize light transmission only along the interface and have no backscattering,with robustness to disorder and defects.Among PTIs,valley photonic crystal(VPC)constructed based on the photonic quantum valley Hall effect do not need to introduce magnetic fields or pseudospins,but only need to break the spatial inversion symmetry,and TESs will be formed on the edge of two photonic crystals with opposite valley Hall phases.However,once a traditional topological photonic device is designed,its functional characteristics are difficult to change.Manipulating the topological phase and realizing dynamic TES tuning will result in breakthroughs for designing THz photonic crystal chips,which becomes a research hotspot in this field.As a soft material with excellent properties,liquid crystals(LCs)are sensitive to external fields such as light,electricity,magnetism,and heat.Meanwhile,it is an ideal method to realize the dynamic control of THz topological devices by dynamically tuning the refractive index of LCs with an external electric field.Conventional LCbased topology devices adjust the TESs or topological angular states by changing the overall topological properties,and they are difficult to fabricate via experiments.Additionally,the study on THz tunable TESs based on local LCs has not been reported.Methods Different from the entire device filled with LCs,we only fill the hole of the topological interface with the LCs and design a tunable THz VPC.Firstly,a twodimensional photonic crystal is constructed to break the spatial symmetry by changing the duty cycle of the two air cavities to open the bandgap.Then a VPC is constructed,and the tunable TESs are studied.Meanwhile,we construct a Zshaped waveguide and add LCs after the first bend,further design a forked wavelength division multiplexer(WDM),and add LCs to the upper branch.Additionally,the TES characteristics with different THz frequencies are analyzed,with the effect of a defect on TES transmission studied finally.The refractive index change of LCs at the VPC interface can tune the TES transmission.This transition of TESs breaks conventional bulkboundary correspondence,which attributes the existence of TESs in VPCs to bulk topology while disregarding the role of the interface refractive index.Results and Discussions We start with the basic properties of THz VPCs(Fig.1)and pattern the VPCs on a silicon slab.Each unit cell of these VPCs comprises two inequivalent circle holes,R1=0.25a,R2=0.08a.The Dirac point originally located at K(K')is opened,creating a bandgap.Hz phase distributions of the upper and lower bands of VPC 2 and VPC 3 at the K(K')point have opposite directions.The Poynting vector also exhibits vortex properties of opposite chirality(black arrow)along with topological band inversion.VPC 2 and VPC 3 with bandgaps and band inversion will generate TESs at the interface of their composition.The influence of changing the LC refractive index of the interface on edge states in VPC is demonstrated.The projected energy band of the supercell with beard interfaces is calculated(Fig.2).The dispersion curve of the edge state shifts down,indicating that some operating frequencies no longer maintain TESs.As the LC refractive index increases,the curve shifts down further,but the TES always maintains a frequency range.When the LC refractive index is adjusted under different applied voltages,TESs can be tuned over a certain frequency range(between blue and black dashed lines).At the boundary,the electric field has a local enhancement effect with its direction along the x direction.Then,an LC tunable Zshaped topological waveguide is constructed(Fig.3).When the LC is not filled,the waveguide maintains a high transmittance in the range of 0.90‒1.03 THz.After the LC is filled and n=1.8,the upper edge of the transmittance curve shifts to low frequency.The passband range rises as the LC refractive index decreases.No matter what the refractive index of the LC is,the THz transmittance is the same at 0.950 THz,while at 1.005 THz,the THz wave has a very different field distribution.At 0.990 THz,the tunable transmission of the TESs in the Zshaped waveguide and the electric field diagrams is shown(Fig.4).Additionally,an LCtunable wavelength division multiplexer(WDM)is designed(Fig.5).At 0.990 THz and n=1.5,the THz wave passes through the upper branch,and under n=1.7 the THz wave passes through the lower branch.This is consistent with the THz transmission rule of the Zwaveguide above.When n increases from 1.50 to 1.68,the THz wave transmits mainly from port 2,and the transmittance is about 80%.As n increases from 1.68 to 1.80,the output of THz waves is mainly from port 3,and the transmittance is close to 100%.A point defect with no LC added is introduced to the interface and the electric field on the interface at 0.990 THz is shown(Fig.6).The transmittance spectrum is almost unaffected by the defect.The LC tunable topological photonic devices constructed by pure LCs or dielectric rods have high requirements for LC packaging and manipulation.The VPC structure in our study has sound backscattering immunity and stable mechanical strength.The enhanced THz nearfield at the boundary can interact with LCs in the air cavity.Additionally,it is convenient to control the orientation of LCs to change the refractive index by an external electric field,which ensures the TES tunability.We have simplified the LC integration and manipulation methods,which is conducive to followup experiments and further research.Conclusions THz integrated photonics is the key to further development and widespread applications of THz technology.VPCs are a good platform for realizing integrated devices and their dynamic control is in high demand.We propose a THz VPC with tunable TESs based on LCs,with a focus on the influence of LCs on the topological transmission characteristics.The topologyprotected edge state of the Zshaped waveguide can be dynamically tuned in the range of 0.98‒1.00 THz,while the topological transmission characteristics in the range of 0.90‒0.98 THz are unchanged,indicating that the device has sound robustness.Additionally,we construct a THz WDM which shows excellent multiplexing properties and defective immunity.In the future,the design can be further optimized to implement programmable broadband THz topology onchip devices.Therefore,our study plays a significant role in promoting the wider application of PTIs and THz technology,and the results are of significance for a deep understanding of TESs and the development of THz integrated chips.
作者
朱徐栋
陈召宪
孟霞
李澳
李炳祥
王磊
胡伟
陆延青
Zhu Xudong;Chen Zhaoxian;Meng Xia;Li Ao;Li Bingxiang;Wang Lei;Hu Wei;Lu Yanqing(College of Electronic and Optical Engineering&College of Flexible Electronics(Future Technology),Nanjing University of Posts and Telecommunications,Nanjing 210023,Jiangsu,China;National Laboratory of Solid State Microstructures,College of Engineering and Applied Sciences,Nanjing University,Nanjing 210093,Jiangsu,China)
出处
《光学学报》
EI
CAS
CSCD
北大核心
2024年第20期121-128,共8页
Acta Optica Sinica
基金
国家重点研发计划(2022YFA1405000)
江苏省自然科学基金(K20211277)
江苏省前沿引领技术基础研究专项(BK20212004)
江苏省研究生科研与实践创新计划(KYCX23_0971)
南京邮电大学校级自然科学基金(NY223087)。
关键词
集成光学
太赫兹
液晶
谷光子晶体
可调拓扑边界态
波分复用
integrated optics
terahertz
liquid crystal
valley photonic crystal
tunable topological edge state
wavelength division multiplexing
