NTT, in collaboration with Tokyo Institute of Technology, has achieved a significant breakthrough in photonics by demonstrating the first photonic topological phase transition induced by a material phase transition. This was accomplished using hybrid nanostructures consisting of a phase-change material, Ge2Sb2Te5 (GST), and a semiconductor photonic crystal. By leveraging GST’s phase-change properties, the team was able to induce a photonic topological phase transition, where the material transitions from a crystalline to an amorphous phase, thereby altering the topological properties of light propagation within the nanostructure. This achievement lays the groundwork for developing reconfigurable functional photonic integrated circuits and could lead to advanced photonic information processing technologies.
Topological phases are a concept originally derived from mathematics, involving discrete numbers known as topological invariants, such as the Chern number. These properties were first recognized in solid-state materials, earning the 1996 Nobel Prize in Physics. More recently, similar topological properties have been found in photonic crystals, which are artificial dielectric structures that modulate light waves. In a photonic topological insulator, light propagation can be controlled in specific ways, such as creating waveguides at boundaries where the Chern number changes. However, the challenge has been that these topological properties are typically fixed during fabrication and cannot be changed afterward. The NTT-Tokyo Tech team’s innovation solves this problem by demonstrating a material-driven approach to reconfigure topological phases after fabrication.
A key technical achievement in this research was the use of GST, a well-known phase-change material commonly used in rewritable optical media such as DVDs. GST can switch between crystalline and amorphous phases with precise control, and its refractive index changes drastically during this transition. The team utilized this characteristic to induce a topological phase transition in a silicon-based photonic crystal. The challenge was to enable band inversion, a critical process where the photonic band structure is altered, reversing the positions of the p- and d-bands. This inversion is required for the transition from a normal to a topological phase, which is marked by a non-zero Chern number. By carefully designing a patterned GST film on the silicon crystal, the researchers successfully demonstrated that band inversion could be achieved, thereby allowing for the transition between the normal and topological phases.
• The team used Ge2Sb2Te5 (GST), a phase-change material, to induce the topological phase transition.
• GST’s phase changes between crystalline and amorphous states, causing a significant shift in refractive index, enabling band inversion.
• Band inversion is critical to changing the Chern number, enabling on-demand creation of topological waveguides at phase boundaries.
• The use of GST and photonic crystals opens possibilities for reconfigurable photonic circuits that do not require physical re-fabrication.
• NTT-Tokyo Tech team employed advanced nanofabrication techniques, including two-step electron-beam lithography, to construct the hybrid photonic crystal with different patterns for the silicon and GST layers.
The experimental verification of the photonic topological phase transition involved measuring the photonic band structure using angle-resolved reflection spectroscopy. The team observed that when GST transitioned from its crystalline to amorphous phase, the brightness of the upper and lower photonic bands inverted, proving that band inversion—and thus a topological phase transition—had occurred. This is the first instance where a material phase transition has been linked to a photonic topological phase transition, offering a pathway for dynamically controlling light propagation in integrated photonic circuits.
This discovery is expected to have wide-ranging implications, especially for reconfigurable photonic circuits. Since GST can undergo phase changes in both directions through optical or electrical pulses, the transition can be localized to specific areas of a prefabricated circuit. This ability to reconfigure waveguides dynamically is promising for the development of advanced optical information systems, such as those used in AI, telecommunications, and quantum computing. Furthermore, this research highlights the potential for controlling photonic topological properties as a new degree of freedom in photonics.
NTT and Tokyo Tech are planning to extend this research by exploring optical pulse control of the topological boundary waveguides and its applications in reconfigurable photonic circuits. This work opens new avenues for controlling topological phases and could lead to breakthroughs in photonic information processing, as topological photonics continues to be an active field of study with novel discoveries on the horizon.
“This research demonstrates the potential for controlling the photonic topological phase through material phase transitions, which may lead to significant advancements in photonic circuit technologies,” said a representative from NTT Corporation.
