light absorbing diodes characteristics

Question

Recently, there was a publication regarding some new materials that would let thru light in one direction and either absorb it or reflect it in the other direction; the publication does not clarify on which one. It does not get as well into other important details as the bandwidth where the diode is effective.

This is interesting because if this really does what they say it does, it is potentially a way to drive a spaceship without any propellant or propulsion energy expenditure.

Assuming the effective bandwidth is not in some dark region of the cosmic spectra, you could conceive a radiation sail with this material, and reach significant speeds over a few years of acceleration. But for doing any concrete estimations, we need hard numbers on the transmittivity and dielectric eigenvalues of these things or something that would allow to calculate the expected impulse. Do anyone knows how to find more info on this material? that MIT article does provide a reference to the paper, but i don't have access to that journal gateway, and the abstract is really scant on the interesting details (probably because they are orienting its application for optical computing!)

Answer 1

Optical diodes (aka isolators) are nothing new

http://en.wikipedia.org/wiki/Optical_isolator

Recent work like the one you cite is not about demonstrating that optical diodes are possible, it is about making optical diodes that are compatible with silicon-based integrated circuits. Here is more about that.

A solar sail does not need to be a special material, because sunlight only comes from one direction. Therefore I assume you are imagining propelling a spacecraft through deep space off the cosmic microwave background radiation. That radiation is about 3K, so if the spaceship is also at 3K, the second law of thermodynamics forbids its acceleration, no matter what it is made of, unless it is burning power. Even if it is made of optical diodes, it will also have its own blackbody radiation that will exactly cancel out the effects of the CMB. The passive acceleration you're hoping for is impossible.

On the other hand, if the ship is hotter than 3K, it can certainly be propelled. But you don't need any special kind of material to make that happen, you just need one side to have higher emissivity than the other side. (cf. pioneer anomoly.) This can be easily done in practice, and should be simple to model in theory.