[mitthrowaway2]

The Wikipedia article lists several advantages relative to reflective solar sails, but there's also a major disadvantage not mentioned. A reflected photon imparts twice its momentum to the solar sail, whereas a diffracted photon imparts only a small portion of its momentum. The overall thrust would therefore be much lower than a reflective solar sail. The fact that "photons can be reused... by passing through a second diffraction grating for more thrust" is not an advantage but rather a testament to the lower efficiency of the diffractive sail.

Perhaps that's offset by the other advantages, but this is not clear.

[monocasa]

I'm not sure that's necessarily the case.

A reflected photon can only impart half of its momentum before it's barreling off away from you.

Several layers of diffractive sails would seem to be able to (at least theoretically) exceed that.

[AnimalMuppet]

A reflected photon imparts twice its momentum.

[monocasa]

Momentum was the wrong word, but it feels like layers of diffraction could red shift the light more, absorbing more energy out of each photon.

[jiggawatts]

… which just heats up the sail.

To move, you want to absorb momentum, not energy.

[woleium]

Correct as per Newtonian physics, bit shit gets weird when you go quantum :)

[monocasa]

At the scale of individual particles, heat is movement.

[ianburrell]

Heat is random movement. Infrared photons go in random directions, not the direction wanting to go. Some go in the wrong direction.

Reflection means solar sail goes in direction you want to go.

[mitthrowaway2]

The light reflected from a mirror is already red-shifted due to the velocity of the mirror.

[monocasa]

Which is why I said red shift more.