Introduction
Topological photonics is an emerging field that applies concepts from topological matter in physics to manipulate light propagation in carefully designed photonic structures. By engineering nanophotonic structures with topologically non-trivial properties, researchers have demonstrated exotic optical phenomena such as robust propagation of light along edges and localization of light at corners. These topological optical effects provide opportunities to create novel photonic devices with unique functions and inherent resilience to defects and fabrication disorder.
In this tutorial article, we summarize and explain the key ideas from a recent research paper published in Nature Communications on using programmable integrated photonics to realize different topological photonic effects. We start with an overview of topological concepts in photonics and highlight why a programmable photonics approach is significant. We then explain the photonic structures created in this work along with the observed topological phenomena. Finally, we discuss the outlook and potential applications enabled by this programmable topological photonics approach [1].
Topological Photonics Background
Topological photonics borrow concepts from the field of topological insulators in condensed matter physics, where exotic electronic band structures give rise to robust metallic states on the edges of certain materials. By carefully designing nanophotonic structures, researchers can engineer “topological photonic insulators” that confine and guide light in unusual ways along boundaries and defects.
Two hallmark signatures of topological effects in photonics are:
Topological edge states: Light propagation confined and guided along the edges in a robust, backscattering-immune fashion.
Higher-order topological states: Light localized at corners or other defects in the lattice in a robust manner.
These striking optical phenomena derive from the non-trivial windings in phase/polarization around the Brillouin zone in judiciously designed photonic lattices. As a result, topological photonic platforms have potential uses in building novel on-chip light sources, waveguides, and sensors that function reliably even with fabrication imperfections.
However, most topological photonics demonstrations have relied on customized platforms that implement one specific photonic lattice design. The ability to reconfigure the topology and associated optical phenomena has been lacking. This is where the concept of programmable photonics becomes highly enabling.
Programmable Nanophotonic Circuits
Programmable photonic circuits consist of a mesh of waveguides and tunable couplers that can be dynamically reconfigured by an external computer. In integrated silicon photonics for example, a mesh of Mach-Zehnder interferometers with phase shifters at each branch can realize arbitrary linear transformations on optical signals propagated through the mesh.
By controlling the individual phase shifters, researchers can remake the photonic circuit and change its function in real-time without fabricating a new chip. Complex optical circuits for communications, computing, and quantum technologies have already been demonstrated using this approach.
Programmable topological photonics now opens up new possibilities to realize different topological models using the same reconfigurable nanophotonic hardware. In this work, the authors demonstrate topological edge states and higher-order corner states using tunable silicon photonic circuits for the first time.
Topological Edge States in a Photonic SSH Lattice
The first topological model implemented on the programmable silicon photonics mesh is the Su-Schrieffer-Heeger (SSH) lattice. The SSH lattice consists of a one-dimensional chain of dimers with alternating weak and strong bonds (Fig. 1a). This structure gives rise to a topological phase supporting robust edge states localized on the ends of the chain.
To realize this model, the researchers configured a hexagonal mesh of tunable photonic couplers to create a chain of seven coupled optical resonators (microring cavities) as depicted in Fig. 1b. By tuning the couplers labeled “strong” and “weak”, they modulated the coupling rate between the resonators to mimic the SSH lattice. Probing the array with a tunable laser, they mapped out the topological edge state pinned to one end of the chain (Fig. 1c).
Varying the strong/weak coupling ratio directly controls the localization and robustness of this topological edge state, which was verified in experiments. Notably, the same reconfigurable hardware can explore SSH models with different tunable parameters.
Higher-Order Topological States in a Breathing Kagome Lattice
Next, the researchers utilized the programmable nanophotonic mesh to create a two-dimensional “breathing” Kagome lattice structure exhibiting a distinct topological phenomenon – corner localization.
The breathing Kagome lattice consists of an array of coupled resonators arranged in a pattern of upward and downward pointing triangles with alternating couplings (Fig. 2a). This structure is predicted to support higher-order topological corner states that confine light at the corners.
Through photonic circuit reconfiguration, the team simulated the breathing Kagome pattern with strong and weak coupling between the two triangle orientations (Fig. 2b). Scanning the lattice resonance reveals three degenerate corner states with light strongly localized at the corners, which is the unique signature of this higher-order topology (Fig. 2c).
Notably, modulating the coupling ratios controls corner state degeneracy and light localization strength, while the corner state frequency stays pinned. These higher-order topological properties were directly observed in simulation by reprogramming the virtual photonic mesh.
Discussion and Outlook
This work demonstrates how programmable nanophotonic platforms can provide researchers with an agile playground for studying diverse topological effects using the very same hardware. Instead of being limited to one topological design, reconfigurable photonic circuits present the flexibility to investigate different classes of topological models (1D, 2D, higher-order), topological phase transitions, as well as introduce controlled perturbations.
Looking forward, programmable chips that support topological photonic circuits could accelerate both fundamental science and applications. On the fundamental side, quantum topological phenomena and non-Hermitian skin effect could be explored by introducing tunable gain/loss regions. On the applied side, reconfigurable topological lasers, filters, and sensors could be implemented using the same multipurpose photonic hardware.
By bringing reprogrammability into topological photonics, this new approach opens the door to an exciting array of scientific and engineering opportunities. The ability to rapidly prototype different topological designs ‘on-the-fly’ promises to become a versatile platform for future advances.
Reference
[1]On, M.B., Ashtiani, F., Sanchez-Jacome, D. et al. Programmable integrated photonics for topological Hamiltonians. Nat Commun 15, 629 (2024). https://doi.org/10.1038/s41467-024-44939-3
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