[LargoLasskhyfv]

Not a physicist, but you may want to look for 'hollow core fiber / photonic crystal fiber'.

Where some are said to reach up to 99.x % the speed of light in a vacuum.

[datenwolf]

Laser physicist here, did some work with PCF: Expensive as fuck, highly polarization dependent, almost impossible to splice.

[LargoLasskhyfv]

Nonetheless it feels like at least every few months or even weeks a new announcement appears in pop-science sites like eurekalert and phys.org. Feels a little bit like the always around the corner next big battery tech.

Most fascinating thing i've read years ago they'd be the prime candidate for manufacturing in space, because real vacuum.