Investigation of the influence of high reflective coatings on NiTi SMA-bimorph for realization of deformable mirror

Abstract

In adaptive optics wavefront correction, the surface of the deformable mirror experiences tiny deformations to generate specific Zernike mode shapes. These mode shapes are used to compensate for distortions in the incoming wavefront caused by atmospheric turbulence or optical system imperfections. In this regard, there is a need to develop a highly reflective, deformable mirror structure capable of controlled micro-actuation for adaptive optics applications. Such mirrors can adjust the aberration caused by atmospheric turbulence, thereby improving the quality of the optical signal. In this work, the highly reflective deformable mirrors were fabricated by integrating NiTi shape memory alloy bimorph with reflective coatings. NiTi shape memory alloy (SMA) thin film was deposited onto a 200 µm Kapton substrate using electron beam evaporation, forming a flexible bimorph structure. This structure was deposited with aluminum (Al), silver (Ag), and copper (Cu) coatings individually via thermal evaporation to achieve Al-NiTi, Ag-NiTi, and Cu-NiTi-based deformable mirrors. Electro-thermomechanical simulations incorporating Joule heating were performed to predict actuation behavior, which was experimentally validated using a laser displacement sensor. The simulation results demonstrate an enhancement in the shape recovery ratio of the NiTi bimorph actuator from 77.7% to 93.7% with an increment in the convection coefficient from 100 to 200 W/m2*K for the same cycle time. Similarly, reducing the bimorph width from 20 mm to 10 mm improves recovery from 76.9% to 88.9% due to faster thermal reaction and lower structural inertia. Wettability results show a maximum improvement of 5.8% for the Al-NiTi mirror compared to the NiTi bimorph mirror in the martensite phase. The reflectivity of the NIR wavelengths was observed as 95 ± 1% for Al-NiTi, 96 ± 1% for Ag-NiTi and 98 ± 1% for Cu-NiTi-based deformable mirrors. These findings demonstrate the potential of NiTi-based deformable bimorphs with metallic coatings as reflective surfaces for wavefront correction in adaptive optics. © 2026