The ISS

In vacuum, what is the highest conversion efficiency we can get
and at what frequencies for each of the following:

1) RF to DC
2) DC to RF
3) 1) in series with 2), i.e. DC -> RF -> DC

For each item above please provide as much of the following as
possible:
a) best conversion efficiency
b) frequency at which this efficiency is achieved
c) name of technology/device which does this
d) supplier(s)
e) references/datasheets

I find it difficult to believe conversion efficiencies greater than
50% for optical wavelengths.

I would expect the answer to this question to be somewhere in the
microwave region.

Thanks,

Charles F. Radley