The author is currently working on a textbook about electromagnetic waves for graduate optics students. They were researching zero refractive index materials and decided to write a popularized account of their properties. Over the past two decades, there have been significant discoveries in optics that have challenged our understanding of what light can do. One area where the rules have changed is the refractive index of light. Materials with a negative index of refraction were once believed to be impossible, but they have been successfully fabricated and have led to new optical phenomena. Epsilon near zero (ENZ) materials, which have a refractive index close to zero, have also been explored and have intriguing applications.
https://skullsinthestars.com/2023/12/09/what-is-a-zero-refractive-index-material/