Phase change materials (PCMs) have demonstrated significant potential in the application of non-volatile integrated photonic devices, garnering considerable attention in photonic integrated circuits. Ge₂Sb₂Te₅ (GST), as a classic phase-change material, exhibits rapid and reversible transitions between amorphous and crystalline states under optical or electrical pulses, accompanied by a substantial difference in the complex refractive index between the two states. In conventional photonic devices, PCMs are typically coated directly on the waveguide, covering the top and both sidewalls. This approach offers excellent modulation performance, but it also increases the transmission loss in the amorphous state. To address this issue, various optimized structures have been proposed by researchers. However, the influence of different attachment methods of PCMs on the optical transmission loss and effective refractive index of waveguides has not been fully investigated. In this study, simulations and analyses are conducted on the light transmittance of waveguides coated with GST in different configurations, as well as on the modulation effects of phase transitions on the waveguide’s transmission loss and effective refractive index. An optimized method for attaching GST to the waveguides is proposed and experimentally verified.

A ridge waveguide is used to conduct the simulation, which has a width of 450 nm, a thickness of 220 nm, and a ridge height of 90 nm. The waveguide cross-section and optical field distribution at 1550 nm are shown in Fig. 1(a). When the upper cladding material is air, the optical field in the waveguide is well confined to the center, exhibiting low optical transmission loss. Four different coverage configurations of GST on the waveguide are compared: full coverage (all), top coverage only (top), side coverage only (side), and trench filling (trench), with their waveguide cross-sections illustrated in Fig. 1(b). The optical field transmission in the waveguide is simulated using the finite-difference time-domain (FDTD) method, with the test structure shown in Fig. 1(c). This simulation yields the transmission rate of the waveguide in both the crystalline and amorphous states of GST. Additionally, the spectral response of a racetrack micro-ring resonator is used to analyze the modulation differences in the effective refractive index and transmission loss when GST is attached to a straight waveguide and a bent waveguide. Finally, to reduce the fabrication process requirements, GST is applied only to the top surface of the waveguide. An optimized method is proposed and experimentally verified. Specifically, the etching groove of the waveguide is filled with silicon dioxide, and the phase change material is deposited on the plane formed by the silicon dioxide and the top surface of the waveguide.

The transmission rates of waveguides under different GST coverage configurations in both crystalline and amorphous states are shown in Fig. 3. The simulation results demonstrate that covering only the top surface of the waveguide with GST yields lower transmission loss than the conventional full-coverage approach while retaining strong modulation effects during phase transitions between the amorphous and crystalline states. GST full coverage on a bent waveguide introduces higher transmission loss compared to a straight waveguide, yet fails to improve the effective refractive index modulation during phase transitions. The optimized method is then proposed: filling the waveguide trench with SiO₂ and depositing GST on the plane formed by SiO₂ and the top of the waveguide. As illustrated in Fig. 7, this method reduces the difference in the effect of covering the phase-change material on straight and bent waveguides, while also decreasing the waveguide transmission loss in the amorphous state compared to the full-coverage approach. The optimized method also features lower fabrication process requirements. Experimental spectral tests confirm the efficacy of this method in minimizing transmission loss while achieving over-30-dB intensity modulation during GST transitions from amorphous to crystalline states.

We investigate the effect of different coverage configurations of GST on the optical transmission loss in silicon ridge waveguides. The study reveals that covering only the top surface of the waveguide outperforms the traditional full-coverage approach in reducing transmission loss while maintaining excellent modulation performance during phase transitions. By analyzing the resonant spectral shift and peak intensity changes in a racetrack micro-ring resonator, we compare the modulation effects of the GST material on the effective refractive index and transmission loss between straight and bent waveguides. Based on simulation results, we propose and experimentally validate an optimized method for covering phase-change materials on waveguides. This method involves filling the waveguide trench with SiO₂ and then depositing the phase-change material on the plane formed by the SiO₂ and the top surface of the waveguide, which reduces the complexity of device fabrication and improves the uniformity of phase-change material coverage. Experimental results demonstrate that, compared to the full-coverage approach, this method effectively reduces transmission loss after GST material attachment. Additionally, the transmission spectra exhibit strong intensity modulation when the GST material transitions from the amorphous to the crystalline state. This method provides a new approach for studying low-loss modulation devices based on phase-change materials.