Lightweight and efficient beam-shaping metalens for converting Gaussian beams into 2D flat-top beams.

Flat-top laser beams have significant applications in fields such as laser processing due to their uniform intensity distribution and clear boundaries. Since lasers typically emit Gaussian beams in the fundamental mode, devices that convert Gaussian beams into flat-top beams are essential. However, traditional beam shapers composed of refractive or diffractive optical elements need improvements in volume, weight, and cost. In this study, we propose a beam-shaping metalens to address these issues. We present an extended Gerchberg-Saxton (GS) algorithm to design two beam-shaping metalenses that convert Gaussian beams into 2D circular and square flat-top beams, respectively. We develop an analytical rapid simulation (ARS) method for simulating metalenses and verify its accuracy. Additionally, we consider the microloading effect in etching and compensate for the impact caused by etching depth inconsistencies. We experimentally fabricated and characterized two beam-shaping metalenses, achieving a circular flat-top beam with a conversion efficiency of 91.8% and an RMS value of 16.7%, and a square flat-top beam with a conversion efficiency of 89.7% and an RMS value of 24.0%. This method for designing beam-shaping metalenses that convert Gaussian beams into flat-top beams of arbitrary shapes provides a novel solution for laser beam shaping.