An index of refraction of 3 is relatively high (like silicon carbide). Glass and a lot of optically transparent materials are only ~1.5. But, if you look at IR

An index of refraction of 3 is relatively high (like silicon carbide). Glass and a lot of optically transparent materials are only ~1.5. But, if you look at IR transparent materials like silicon, gallium arsenide, or germanium, you can get up to and over 4. More to the point, diode pumped solid state lasers are a common thing these days so photons interacting just isn't news... (edit) however if (and I can't tell without reading the original paper) it is actually the effect of photon mass (eg due to light density) on the photons itself, then it's a significant technical achievement! Just not how I'd have phrased it to a technical audience.