Angular- and polarization-insensitive free-standing graphene oxide films for broadband light absorbing applications

Free-standing graphene oxide (FSGO)-based films with angular and polarization insensitivities are demonstrated for broadband absorption applications. The modified Hummers’ method was used to prepare the GO powder. An easily scalable process (evaporation-induced self-assembly) was used to synthesize the FSGO films. The FSGO broadband absorbers with omni-direction and polarization-independent nature was achieved by varying the GO concentrations. The FSGO films were prepared at different GO concentrations to achieve different thicknesses (2–15 μm). The FSGO film with the highest GO concentration (∼10 mg/ml) exhibited high absorptance (α = 0.91) in the broadband region (i.e., 250–2500 nm). The transition from narrow band absorber to a broadband absorber is achieved by varying the GO concentration. The optimized FSGO film (8 mg/ml) shows α of 0.91 with angular and polarization-insensitivity in the visible region. At concentrations <8 mg/ml, the FSGO films are translucent and do not exhibit broadband absorption behavior. Spectroscopic ellipsometry measurements indicate the low refractive index nature of the FSGO films (n = 1.7–1.2 at 400–800 nm). The low refractive index combined with the unique surface morphology of the FSGO films resulted in broadband absorption. Optical simulations were performed to validate the absorption mechanism of the FSGO films.