The paper presents an ultra-broadband thin-film infrared absorber made of a saw-toothed anisotropic metamaterial (AMM). The absorber exhibits an absorbency of over 95% at normal incidence across a wide frequency range, with a full absorption width at half maximum (FWHM) of about 86%. This property is maintained over a wide range of incident angles. The mechanism behind this performance is explained by the slow-light modes in the anisotropic metamaterial waveguide. The AMM consists of alternating layers of gold and germanium, with the sawtooth shape designed to capture light at different wavelengths at positions with different tooth widths. The study demonstrates the potential of this design in applications such as photovoltaic devices and thermal emitters. The slow-light modes in the air/AMM/air waveguide are key to achieving broadband absorption, and the absorption spectrum can be tuned by adjusting the filling ratio of the metal. The work is scalable and can be extended to microwave and terahertz frequencies.The paper presents an ultra-broadband thin-film infrared absorber made of a saw-toothed anisotropic metamaterial (AMM). The absorber exhibits an absorbency of over 95% at normal incidence across a wide frequency range, with a full absorption width at half maximum (FWHM) of about 86%. This property is maintained over a wide range of incident angles. The mechanism behind this performance is explained by the slow-light modes in the anisotropic metamaterial waveguide. The AMM consists of alternating layers of gold and germanium, with the sawtooth shape designed to capture light at different wavelengths at positions with different tooth widths. The study demonstrates the potential of this design in applications such as photovoltaic devices and thermal emitters. The slow-light modes in the air/AMM/air waveguide are key to achieving broadband absorption, and the absorption spectrum can be tuned by adjusting the filling ratio of the metal. The work is scalable and can be extended to microwave and terahertz frequencies.