A collaborative team from NTT Corporation and Tokyo Institute of Technology (led by Prof. Masaya Notomi) has successfully demonstrated a photonic topological phase transition through a material phase transition. This was achieved using innovative hybrid nanostructures that combine a phase-change material with a semiconductor nanostructure.
Topological properties in solid-state materials, recognized by the Nobel Prize in 1996, have been adapted for light waves in nanostructures, sparking extensive research into new photonic capabilities. Traditionally, these photonic topological properties are fixed and unchangeable post-fabrication. However, NTT and Tokyo Institute of Technology’s breakthrough shows that photonic topological phases can be reconfigured using material phase transitions. This advancement opens new research avenues by merging material and photonic topological phase transitions, offering significant potential for developing reconfigurable photonic integrated circuits and innovative photonic information processing technologies.Background
Topology, a mathematical concept, deals with properties that remain unchanged under continuous deformation and are described by topological invariants. An example is the genus, which counts the number of holes in a geometric structure. These invariants are unaffected by smooth transformations. The concept of topology was applied to solid-state physics, culminating in the 1996 Nobel Prize awarded to physicists who identified topological properties within the electronic wavefunctions of solid-state materials. Their work revealed topological characteristics linked to the Chen number, a topological invariant for electronic wavefunctions, leading to the discovery of topological insulators.
In recent years, similar topological properties have been observed in photonic crystals—artificial structures with periodic variations in refractive index. This has given rise to the field of topological photonics. Photonic crystals with a non-zero Chern number act as photonic topological insulators, where light propagation is restricted, but edge waveguides automatically form at the boundaries between regions with different Chern numbers. These edge waveguides exhibit unique properties such as unidirectional propagation and reduced backscattering, making them promising for future photonic integrated circuits.
Phase transitions, which involve abrupt changes in material properties (e.g., solid-liquid or liquid-vapor transitions), can theoretically be applied to photonic topological insulators. By inducing a phase transition between a photonic topological insulating phase and a normal phase, one could create optical waveguides with adjustable positions. However, traditional photonic topological properties were fixed post-fabrication, limiting the ability to alter these properties after nanostructure creation. For example, in a honeycomb-lattice photonic crystal, shifting triangular holes can transition the structure from a normal photonic insulator to a photonic topological insulator with a non-zero Chern number, as illustrated by the inversion of p- and d-band positions. This band inversion, crucial for topological phase transitions, has been challenging to achieve, and previous efforts to manipulate photonic topological properties have struggled to accomplish a true photonic topological phase transition.Key Points of This Work
Innovative Structure for Photonic Topological Phase Transition Using Phase-Change MaterialGe₂Sb₂Te₅ (GST) is a well-known phase-change material that undergoes a significant phase transition between its crystalline and amorphous states through temperature control or optical pulse irradiation. The refractive index of GST changes dramatically during this transition, and the altered phase remains stable for extended periods without additional power, making GST useful in various optical applications, including rewritable DVDs. Recent research has focused on leveraging GST for optical memories and circuits.In this study, the team from NTT and Tokyo Institute of Technology explored integrating a GST film with a silicon photonic crystal to utilize GST’s large refractive index change for inducing photonic topological phase transitions. While simply applying a homogeneous GST film proved insufficient for inducing band inversion or altering the Chern number, the researchers discovered that a patterned GST film could achieve this effect. By carefully designing the GST film’s pattern on the silicon photonic crystal, the phase transition from crystalline to amorphous states caused the photonic bands to invert, resulting in a transition from a normal phase to a topological phase. This novel approach, detailed in Figure 3, represents a breakthrough that has not been reported previously.
