Optical absorption of silicon nitride membranes at 1064 nm and at 1550 nm

Because of a low mechanical loss, thin films made of silicon nitride (${\mathrm{Si}}_{3}{\mathrm{N}}_{4}$) are interesting for fundamental research and development in the field of gravitational-wave detection. ${\mathrm{Si}}_{3}{\mathrm{N}}_{4}$ membranes allow for the characterization of quantum radiation pressure noise (RPN), which will be a limiting noise source in gravitational-wave detectors of the second and third generations. Furthermore, ${\mathrm{Si}}_{3}{\mathrm{N}}_{4}$ is an interesting material for possible thermal noise reduction in highly reflective mirror coatings. For both applications, the optical absorption of ${\mathrm{Si}}_{3}{\mathrm{N}}_{4}$ needs to be low. This paper presents absorption measurements on low-stress ${\mathrm{Si}}_{3}{\mathrm{N}}_{4}$ membranes showing an absorption a factor of 7 lower at 1550 nm than at 1064 nm resulting in an estimated 2 times higher sensitivity in RPN experiments at the higher wavelength and making ${\mathrm{Si}}_{3}{\mathrm{N}}_{4}$ an interesting material for highly reflective multimaterial mirror coatings at 1550 nm.