The widely adopted butterfly-shaped electrode configuration demonstrates high thermal efficiency but shows incompatibility with in-situ testing systems. Additionally, existing modulation methods are constrained by limited phase modulation range and lack multistate control capabilities, failing to meet the demands of high-precision photonic computing. To address these issues, we propose a novel U-shaped heating structure that combines the thermal efficiency advantages of butterfly-shaped structures with in-situ testing compatibility. This advancement provides high-precision, non-volatile weight-control hardware units for optical matrix operations and neuromorphic computing, significantly improving the practical utility of reconfigurable photonic devices. The proposed structure lays the critical technical foundation for the development of next-generation intelligent photonic chips.

Building upon the principles of Mach?Zehnder interferometer (MZI) phase modulation and thermal field regulation, we conduct comparative simulations of thermal distribution characteristics across butterfly-shaped, I-shaped, and other electrode configurations (Fig. 1). Based on this, we propose a novel U-shaped heating structure designed to enhance electrothermal modulation efficiency and meet in-situ operational requirements. Subsequently, electrical potential simulations are conducted to analyze the effective driving voltage distribution within the core functional regions of this optimized structure (Fig. 2). After the simulation-guided design optimization, the device fabrication proceeds through two distinct micro/nanofabrication phases [Fig. 3(a)]. The first phase focuses on fabricating a rib waveguide on an SOI wafer, with a 220 nm thick silicon layer on top of a 3 μm thick buried oxide and 675 μm thick silicon substrate. The MZI pattern is defined by e-beam lithography (EBL) and etched using an inductively coupled plasma etching system (ICP). The waveguide core, measuring 500 nm in width and 120 nm in depth, is flanked by symmetrically fabricated 5 μm trenches on both sides. This optimized geometry ensures single-mode propagation characteristics at a 1550 nm wavelength. The second phase uses a lift-off process to deposit 30-nm-thick Sb₂Se₃ phase-change material layers on one MZI arm. Subsequently, 450 nm indium tin oxide (ITO) and 20 nm/1 μm Cr/Au electrodes are deposited similarly, ultimately forming the U-shaped heating structure. The final device and phase-change material regions is characterized using optical microscopy [Fig. 3(b)] for structural verification and scanning electron microscopy [SEM, Fig. 3(c)] for interfacial morphology analysis. Finally, we implement an in-situ testing system [Fig. 4(a)], enabling real-time calibration of phase-state transitions by monitoring optical transmission variations during pulsed electrical stimulation cycles.

Through precise pulse number control, we successfully achieve 98 non-volatile distinguishable states, corresponding to a dynamic output characteristic window slightly exceeding 2π [Fig. 4(b)], fulfilling the phase modulation requirements for electrically-induced phase-change photonic devices. Furthermore, the localized effective driving voltage in the micro-heating region accounts for 14.7% of the total potential field distribution derived from device-scale modeling. Excellent consistency is observed among three resistance values: theoretical Ohm’s calculation (95.065 Ω), simulation with measured parameters (95.572 Ω), and experimental measurement (~94 Ω), validating the model’s reliability. These results demonstrate that stable crystalline phase transitions in the U-shaped structure can be induced with pulse parameters of (2.94 V, 500 μs), indicating the structure’s capability for phase-change material control at low driving voltages (<3 V). The stable crystallization energy consumption is calculated as 34.18 fJ/nm3. In contrast, I-shaped structures require (13.4 V, 1 ms) pulses with a corresponding energy consumption of 130.40 fJ/nm3, confirming the superior energy efficiency of the proposed U-shaped design. After achieving 2π phase modulation, we explore amorphization conditions using the same pulse-increment method. Despite varying voltage (20?40 V) and pulse width (500 ns?500 μs), no significant amorphization response is observed until ITO layer fracture occurred. This failure likely stems from thermal stress induced by the large thermal expansion coefficient mismatch between Sb₂Se₃ and ITO. Microscopy reveals phase-change material diffusion into non-heated regions, suggesting Sb₂Se₃ reaches its melting temperature. However, the thick ITO layer hinders rapid quenching, preventing stable amorphous phase formation. To achieve controllable amorphization, we propose two improvements in the future: 1) integrating metal heat sinks in heating regions to enhance thermal conductivity and quenching rates; 2) replacing conventional ITO heating layers with graphene or other materials exhibiting superior thermal conductivity.

In this paper, we establish Multiphysics-coupled simulations to compare I-shaped, butterfly-shaped, and U-shaped heaters, proposing a U-shaped heating structure that combines the high thermal efficiency of butterfly structures with test system compatibility. The experimental results show that the structure can realize 98 stable states, with an optical phase modulation dynamic range of 2π. It fully meets the electrothermal control requirements of phase-modulated photonic devices and provides an innovative paradigm for structural optimization of phase-change photonic devices. For the amorphous control of phase-change materials, future research will focus on micro/nano cooling structure to accelerate the quenching of molten PCM, or the use of high thermal conductivity materials to improve heat transfer efficiency. These methods can work together to enhance the control and stability of amorphous phases, providing a reliable hardware foundation for reconfigurable photonic devices.