Enhancing thermal dissipation in amorphous silicon metasurfaces via laser-induced crystallization

Metasurfaces, artificial nanopatterned structures composed of arrays of subwavelength dielectric materials or metallic scatters, have demonstrated remarkable capabilities in manipulating light properties. Compared to metallic metasurfaces, dielectric metasurfaces are more favored due to their advantages of high refractive index and low loss. Among therm, amorphous silicon is a traditional dielectric material that has been widely used as the raw material for metasurface fabrication. The optical properties of metasurfaces primarily depends on the size, shape, and dielectric function of the nanostructures. Many researchers have successfully developed a variety of functionalized amorphous silicon metasurfaces by regulating their structural parameters. However, the process of manipulating incident light using metasurfaces is also accompanied by heat generation and transfer. Due to the poor thermal conductivity of amorphous silicon, heat is more likely to accumulate, leading to an increase in the operating temperature of the metasurface, which can significantly affect its reliability and service life. Thus, enhancing the heat dissipation capability of amorphous silicon metasurfaces is crucial for expanding their application range under high laser flux environments. Crystallization, a typical treatment process, has been widely employed to the modification of amorphous materials. Recently, laser-induced crystallization exhibits greater advantages in micro-nano device processing due to its precision and efficiency. During the laser crystallization process, the incident laser energy is partially absorbed near the surface and results in a temperature rise of the metasurface. Due to the metastable nature of amorphous silicon, low laser power density can be used to control the temperature rise below the melting point, enabling crystallization and improving material properties.

Prof. Yanan Yue and Dr. Xiaona Huang from Wuhan University, cooperating with Prof. Xiaoguang Zhao from Tsinghua University, proposed a strategy to enhance the thermal transport properties of amorphous silicon metasurface via laser-induced crystallization. Experimental results show that when the amorphous silicon metasurface is irradiated with a continuous laser at low power densities (areas I to III), crystallized grains formed on the surface of the amorphous silicon cylinders, without any significant change to the cylindrical structure. However, as the incident laser power density increases, heat accumulates and eventually causing damage to the structure of metasurface. Therefore, the trouble of thermal damage caused by laser absorption could be a significant consideration in metasurface design. Then, the research team used Raman spectroscopy to monitor the crystallization process of the amorphous silicon metasurfaces in real time. Experimental results showed that the crystallization thresholds of the metasurface were 1.1×10⁵ and 9.78×10⁴ W/cm², corresponding to the laser spot radii of 1.30 and 0.54 μm, respectively.

The research team further analyzes the thermal and electromagnetic responses of the metasurfaces through simulations. The calculation results show that the maximum temperature rise of the polysilicon metasurface is 52% lower than that of the amorphous silicon metasurface at the same incident laser power density. Besides, electromagnetic simulation results show that both amorphous silicon and polysilicon metasurface exhibit excellent light manipulation capabilities, maintaining high amplitude coefficients while achieving full 2π phase coverage.

This work investigates the thermal response of all-silicon metasurfaces under high-density laser excitation and proposes a method to enhance the thermal conductance properties of amorphous silicon metasurfaces through laser-induced crystallization. The researchers used a continuous laser with a wavelength of 532 nm to crystallize amorphous silicon and determined the crystallization thresholds of the amorphous silicon metasurface for different laser spot radii. Thermal simulation results show that the maximum temperature rise of polysilicon metasurface is 52% lower than that of amorphous silicon metasurface with the same incident laser power density. Besides, electromagnetic simulation results demonstrate that both polysilicon and amorphous silicon metasurfaces maintains high amplitudes of transmission coefficients while achieving a full 2π phase coverage. This study opens new avenues for enhancing heating dissipation of amorphous silicon metasurfaces and lays the foundation for their application in high laser flux conditions.