Good question; had to go to the supplementary materials of the paper in Science [1]. It is called a Bragg Mirror [2], which basically seems like a special coati

Good question; had to go to the supplementary materials of the paper in Science [1]. It is called a Bragg Mirror [2], which basically seems like a special coating consisting of layers of alternating refractive index. I don't know the details but this isn't _necessarily_ more exotic than, say, the antireflection coatings on eyeglasses or photographic optics. The supplementary material states that the mirror is 99.998% reflective in the wavelength under study. [1] https://www.science.org/doi/10.1126/science.abm2543 https://www.science.org/doi/10.1126/science.abm2543 [2] https://en.wikipedia.org/wiki/Distributed_Bragg_reflector https://en.wikipedia.org/wiki/Distributed_Bragg_reflector