A sawtooth anisotropic metamaterial (AMM) slab is designed to achieve ultra-broadband infrared absorption. The structure consists of alternating layers of gold and germanium plates, with a sawtooth profile that gradually increases in width from top to bottom. This design enables efficient absorption of light across a wide frequency range, with an absorptivity of over 95% and a full width at half maximum (FWHM) of 86% at normal incidence. The absorption is broadband and angle-insensitive, with high performance even at incident angles up to 60 degrees. Shorter wavelengths are absorbed at the narrower parts of the sawteeth, while longer wavelengths are trapped at the wider parts. This behavior is attributed to slowlight modes in the anisotropic metamaterial waveguide. The AMM slab is modeled as a three-layered air/AMM/air waveguide, where the effective permittivities are determined by the filling ratio of the metal and dielectric layers. The dispersion relationship of the waveguide is analyzed, revealing that slowlight modes are supported, allowing for the design of broadband absorbers. The absorption spectrum is tunable by adjusting the filling ratio and core width of the waveguide. The structure is scalable and can be applied to microwave and terahertz frequencies, as well as extended to 3D for polarization-independent performance. The absorber's performance is validated through simulations using the Rigorous Coupled-wave Analysis (RCWA) method, showing high absorption efficiency and angular independence. The study demonstrates that the ultra-broadband absorption is achieved through the slowlight waveguide mechanism, with the absorption spectrum being significantly broader than traditional single-band absorbers. The design has potential applications in photovoltaic devices and thermal emitters, offering a promising solution for efficient light harvesting and energy conversion.A sawtooth anisotropic metamaterial (AMM) slab is designed to achieve ultra-broadband infrared absorption. The structure consists of alternating layers of gold and germanium plates, with a sawtooth profile that gradually increases in width from top to bottom. This design enables efficient absorption of light across a wide frequency range, with an absorptivity of over 95% and a full width at half maximum (FWHM) of 86% at normal incidence. The absorption is broadband and angle-insensitive, with high performance even at incident angles up to 60 degrees. Shorter wavelengths are absorbed at the narrower parts of the sawteeth, while longer wavelengths are trapped at the wider parts. This behavior is attributed to slowlight modes in the anisotropic metamaterial waveguide. The AMM slab is modeled as a three-layered air/AMM/air waveguide, where the effective permittivities are determined by the filling ratio of the metal and dielectric layers. The dispersion relationship of the waveguide is analyzed, revealing that slowlight modes are supported, allowing for the design of broadband absorbers. The absorption spectrum is tunable by adjusting the filling ratio and core width of the waveguide. The structure is scalable and can be applied to microwave and terahertz frequencies, as well as extended to 3D for polarization-independent performance. The absorber's performance is validated through simulations using the Rigorous Coupled-wave Analysis (RCWA) method, showing high absorption efficiency and angular independence. The study demonstrates that the ultra-broadband absorption is achieved through the slowlight waveguide mechanism, with the absorption spectrum being significantly broader than traditional single-band absorbers. The design has potential applications in photovoltaic devices and thermal emitters, offering a promising solution for efficient light harvesting and energy conversion.