Three-Dimensional Waveguide Topological Photonic Structures in Glass Fabricated by Femtosecond Laser Direct Writing(Invited)
Significance Topological photonics represents a forefront research field that integrates topological concepts from condensed matter physics,particularly topological insulators,into optical systems.These insulators,originally derived from the quantum Hall effect,denote materials where electron conduction is confined to their surfaces,exhibiting immunity to dissipation and backscattering even amidst defects and disorder.Leveraging the ease of control in photonic systems,topological photonics structures provide a convenient experimental platform for investigating related topological phenomena from condensed matter physics,such as the integer and spin quantum Hall effects.Furthermore,topological photonics fosters exploration into pioneering topological concepts,including higher-order topological insulators,anomalous Floquet topological insulators,Anderson topological insulators,and fractal topologies.Photonics has introduced innovative physical phenomena to topology,such as non-Hermitian and nonlinear topological photonics.Integrating topological theory into optics has introduced a new dimension of freedom to optical systems,offering novel techniques for stable transmission and manipulation of light fields and quantum states in integrated photonic devices.This encompasses robust state transformations under topological protection,topologically safeguarded quantum entanglement states,and advancements in topological quantum computation.Femtosecond laser direct writing(FLDW)technology provides exceptional 3D direct writing capabilities,enabling the fabrication of three-dimensional optical waveguides with arbitrary cross-sections and the construction of complex waveguide arrays.Waveguide devices made with FLDW have become crucial research platforms for optical communication,optical quantum computing,quantum algorithms,optical quantum storage,optical quantum simulation,topological photonics,and non-Hermitian systems.Since the paraxial equation of electromagnetic wave propagation in waveguides mirrors the single-particle Schrödinger equation,waveguide systems are highly suitable for studying various two-dimensional photon evolution and distribution problems.FLDW allows flexible regulation of the waveguide evolution path and effective refractive index,enabling precise control over light coupling between waveguide arrays to simulate particle propagation in periodic potential fields.Consequently,FLDW has demonstrated various topological photonic structures in recent years,including Floquet photonic topological insulators,anomalous Floquet photonic topological insulators,nonlinear topology,non-Abelian topology,higher-order topological insulators,and fractal topology.Moreover,FLDW can introduce scattering points and machine segmented or curved waveguides to manage waveguide losses,thereby facilitating precise control over non-Hermitian effects.This capability advances the study of topological properties in non-Hermitian systems and opens new avenues for applications.Progress In this review,we comprehensively summarize studies on three-dimensional waveguide topological photonic structures in glass fabricated by FLDW.It covers periodic lattices that preserve or break time-reversal symmetry,as well as non-Hermitian topological waveguide structures.Firstly,the mechanisms and types of waveguides fabricated by FLDW are introduced,along with methods to improve their performance(Fig.1).Subsequently,we systematically discuss research progress in optical waveguide topological photonics that break time-reversal symmetry.This includes the design principles of Hamiltonians to introduce artificial gauge fields,achieve equivalent magnetic flux,and thereby break time-reversal symmetry.Various FLDW constructions are highlighted in the review,such as Floquet photonic topological insulators,anomalous Floquet photonic topological insulators,Anderson photonic topological insulators,"chain-driven"honeycomb lattices,fractal photonic topological insulators(Fig.2),and Aharonov-Bohm cages(Fig.3)in glass.Furthermore,we explore optical waveguide topological insulators based on chiral symmetry,including the one-dimensional Su-Schrieffer-Heeger(SSH)model,the Aubry-André-Harper(AAH)model(Fig.4),and two-dimensional higher-order topological insulators(Fig.5).We also demonstrate their applications in quantum information processing(Fig.6).The introduction of gain-loss or nonreciprocal coupling into topological photonic structures through suitable design is proved crucial for both fundamental physics and applications.For instance,the review reveals how non-Hermitian lattice engineering can tune the topological properties of an open system(Fig.7).Additionally,the interaction between non-Hermitian modulation and topological phases can create novel non-Hermitian topological materials.The review also unveils the non-Hermitian skin effect,which leads to the breakdown of conventional bulk-boundary correspondence and the introduction of a generalized Brillouin zone(Fig.8).Furthermore,dynamically encircling exceptional points can achieve robust asymmetric mode transformation(Fig.9),offering a new method for robust state transformation and manipulation in integrated photonic devices(Fig.10).Conclusions and Prospects FLDW excels in 3D direct writing,swiftly and precisely fabricating intricate topological photonic structures.It enables the exploration of topological phenomena in both Hermitian and non-Hermitian systems.Glass,commonly used for topological optical waveguide arrays,presents challenges due to its material properties.Achieving electro-optic,acousto-optic,and magneto-optic modulation is rather difficult,hindering the creation of a tunable topological photonic platform.Additionally,the absence of path-dependent gain in glass results in considerable transmission loss in studies of non-Hermitian topology,thereby limiting its application in quantum information processing.To overcome these hurdles,FLDW is expected to craft topological photonic structures in materials like phase-change materials or laser glass.Current topological insulators based on optical waveguide arrays support only single-mode waveguides.Developing multi-mode waveguides or those supporting orbital angular momentum modes instead of single-mode would be pivotal for advancing topological insulators in optical information processing.In summary,FLDW has become indispensable for preparing topological photonic structures.As FLDW and material research continue to advance,they will drive further progress in topological photonics.
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