A Stable Broadband Antireflective Structure on MgAl₂O₄ Fabricated by Spatially Shaped Ultrafast Laser Pulses

Subwavelength structures on MgAl 2 O 4 hold significant technological importance in mid‐infrared (mid‐IR) optics, particularly for windows used in military mid‐IR devices that must operate under harsh conditions. Herein, a novel and efficient strategy utilizing spatially shaped femtosecond laser pulse delay for fabricating antireflective surfaces on hard, difficult‐to‐machine spinel with stable and broadband transmittance in mid‐IR waves is reported. With this method, a new micro–nanohybrid structure composed of a parabolic microhole associated with nanopores (MHNP) is achieved by optimizing Bessel femtosecond pulse delay since the subsequent pulse can irradiate at different stages of materials plasma diffusion, thermal expansion, and phase transformation. The mechanism for more stable transmittance enhancement of MHNP in broadband is explored by the finite‐difference time‐domain simulation, indicating that such localized field selective enhancement in structures is caused by the competition between interface equivalent refractive index effect and the light interference for the parabolic MHNP. As the height of the parabolic microhole and nanopore nears 1 μm and 400 nm, respectively, the resulting transmittance of MHNP with three different periods increases by 3.5–5.5% for 2.85–5.5 μm wavelength, and the maximum transmittance reaches a value of 94.4%.