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Carbon Nanotube Antenna for Light
Suidae writes "Researchers at Boston College are reporting that carbon nanotubes can be used to build an antenna that receives optical wavelengths in much the same way a radio antenna receives longer wavelengths. The electrical effects can not yet be directly measured as diodes that operate at optical frequencies would be required, but secondary radiation from the excitation can be observed. Potential applications include fiber optic data transmission and photovoltaics."
Modulate a thousand frequencies of sunlight at the same time and pass them through your transmission medium of choice (space?) and don't stress about diffraction or diffusion as long as the light reaches the other side; because your receiver is an array of several tens of thousands of carbon nanotubes that auto-magically sort out the frequencies.
Ta-da! You just transmitted the entire Library of Congress in a matter of seconds.
The problem with using this for data transmission is that in order to measure amplitude accurately, you need several photons received in your measuring period. As frequency gets higher, the photons get more energetic and the sampling period (at the maximum rate of modulation) gets shorter. This results in power per unit data going up directly with frequency, and power per unit time going up as the square of frequency. The same relation turns out to hold even if you use other methods of signal processing (you could split the modulated light into its component frequencies and end up with a bunch of lower-bandwidth signals that way, for instance).
For signals modulated much more slowly than the frequency of the light itself, this penalty in power-per-bit may be acceptable if using light gives other advantages (like smaller dish size for a given divergence, or ability to pipe through fiber). However, at the maximum rate of modulation, both transmission power and power per unit area get prohibitive.
At one bit per sample (the most power-efficient encoding), you get a minimum power for an intelligeable signal of about 0.7 mW (1e15 samples of 4 photons at about 1 eV each). This is per nanotube antenna. This is unlikely to be survivable. For an 11 angstrom single-walled nanotube seen end-on, it corresponds to a power flux of about 7e14 W/m^2. At radiative equilibrium, this gives a surface temperature of around 300,000 K on your antenna array (room temperature is 300 K, nanotubes change phase somewhere between 3500 and 4000 K, and the surface of the sun is 5800 K). If you instead use the nanotubes side-on as antennae about the size of a photon's wavelength (around 1 micron), you get a power flux of about 7e8 W/m^2, giving an equilibrium temperature of around 8700 K (still hotter than the sun). This is misleading, though, as the signal would have to be coupled into a single nanotube antenna, with a much smaller surface area (giving a power flux on the order of 1000 times higher, and temperature 5-6 times higher).
Transmitting over interstellar distances is also very difficult, as you need to assume a collecting mirror size, and make sure that enough photons strike the collector to get an intelligeable signal. For a 10m telescope mirror, power needs to be about 9e-6 w/m^2. A broadcast signal at a range of, say, 10 light-years covers a surface area of about 1e35 m^2. This gives a broadcast power of about 9e29 W. By comparison, the sun puts out about 4e26 W. So broadcasting a beacon like that, even to a nearby star system, is impractical. Beaming it still covers a large area, due to divergence induced by aperture diffraction at the sending mirror. If we assume it's being broadcast from a 10 m telescope, divergence is about 1e-7 radian, for a spot diameter of 9e9 m. This gives a spot area of about 6e19 m^2, and a power of about 4e16 W (40 petawatts). A bit steep for a beacon, when you could save many orders of magnitude by either using radio, transmitting data more slowly, or both.
In summary, modulating data on an optical carrier has drawbacks, and doing it at optical data transfer frequencies almost certainly requires enough power to vapourize the detector. Still a nifty thought-experiment, though.