Metals can have their topmost, oxidized layers scoured away, leaving shiny insides. Principle of it seems sound, especially if it reacts to the wavelength of light to change configuration to be more reflective. I mean, nothing's gonna be perfectly reflective, but with good heatsinks, it just has to prevent a penetration. Superconductor armor is alsoan idea for this, as the heat of the laser would be spread over the entirety of the connected armor plating, and dissipate fairly quickly into atmosphere. Add heatsink backing and, well...good luck with that.
Ablative armor is also an option.
But yes, theoretical material. Not on par with FTP material theoretical, though, I would think.
And yes, changing the light wavelength is a thing, but you need static electron lasers for that. Also, changing the focal length of the fish isn't necessarily easy; getting the bean focused at the right point could be difficult to get a good penetration. Beam pulses are also unlikely to hit exactly the same spot as previous in a combat situation unless specifically set up to do so like the 'splodey lasers, and pulse length(and therefore, to some extent, power) and rate are inversely correlated.
The gas cloud released would either do it's job of stopping that one pulse, and then you'd be aiming at a different point on the armor, or it would help reduce or stop the next pulse because you fired it quickly enough after the first that there wasn't time for the gas to dissipate.