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."

Comments

Heinlein

[Inominate]

Heinlein was here?

Re:Heinlein

[IvyKing]

This is more like a proposal from John Kraus (W8JK) of Ohio State - where he did propose making antennas for optical frequencies. In this case, Kraus was extending the rectantenna design for microwave power transmission to optical wavelengths.

Heinlein's idea was more along a generic resonator - in some ways he predicted LED's some 25 years before they were first made (Man who sold the moon ca 1940, LED's ca 1965). It is depressing to think that about the same amount of time has elapsed since the last moon landing and now as between when "The Man who sold the Moon" was written and the last moon landing.

Re:Heinlein

[Inominate]

Heinlein's idea is to handle light waves in the same way as radio waves. Having an antenna able to do so seems like the first step to me.

Interesting

[Undefined Parameter]

If the whole "petroleum companies keep their monopoly by buying viable competing technologies" thing is true (and I'm not saying that it is / is not), it's fairly clever that the inventor is disguising it as a new form of antenna. The only problem would be that it'd have to be LoS, which means that closing the curtains to watch TV with the antenna laying on top of the set would no longer be a possibility.

However, as a method of attaining electrical energy from light, it looks to be rather interesting.

~UP

Re:Interesting

[Anonymous Coward]

>>If the whole "petroleum companies keep their monopoly by buying viable competing technologies"
>>thing is true (and I'm not saying that it is / is not), it's fairly clever that the inventor is
>>disguising it as a new form of antenna.

But, it IS a new form of antenna. Radio receivers work by being the size of the wavelength that the EM is.* Radio wavelengths are big- AM waves can reach well beyond a kilometer, while FM waves are far, far shorter.

These nanotube antennaes can pick up wavelengths so tiny, that they can actually reach into the visible spectrum. Visible light has a wavelength of only a few hundred nanometers, so you need an antenna of equal scale to pick it up. It's amazing that it's actually been done. If smaller versions of these antennae are possible, I wonder if we can reach into ultraviolet or X-RAY antennae? Gamma Ray?

Who knows, maybe HDTV-deluxe will be beamed down from satellites that use visible light for data. Except on cloudy days.

I wonder how the FCC will handle this. :)

*(It may be half or twice the size, I do not remember.)

Hmm...

[GreyOrange]

I wonder what the sensitivity of these devices are. I also wonder if the people(aliens) on stars far away might of found a way to communicate more effeciently with light then with radiowaves. Wasn't there an article earlier saying we should scan the skies for lasers? I would love to see what kind of data could be collected from this device when we finaly find a way to interface with it. Also purhaps it could have other uses, since it would produce electrical currents that alternate at very high frequencies. I can't think of anything that could effeicnly transmit AC of that type of frequency, but purhaps it might be used with some other device. Its just cool to think of the things we might someday do with large arrays of light attennaes.

ET phone ... us?

[rudog]

In recent news it was found that "Alien Beings" have been trying to communicate with Earth for centuries via modulated starlight.

Just kidding. Honestly though, this could be looked at as another magic frequency thing. Except like Broadband in scale.

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.

Re:ET phone ... us?

[Christopher Thomas]

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.

Re:ET phone ... us?

[sidles]

As Darth Vader would say: "Impressive ... most impressive."

The author's one-bit-per-photon-absorbed rule of thumb is pretty darn accurate; a more detailed calculation suggest that the maximal channel capacity occurs at around 0.49 bits per photon.

It's always fun to see some creative tension between the physicists and the engineers. The physicists have the first word and the engineers have the last word!

Re:ET phone ... us?

[Anonymous Coward]

So, just to paraphrase what you've said:
There are a few tiny bugs to be worked out but after that, smooth sailing all the way and we can now welcome our new PetaWatt overlords?

TDz.

Re:ET phone ... us?

[barawn]

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).

Yah, but that gives a bandwidth of the frequency of the light. Given the fact that it's optical, that's gi-normous - 1E15 bps, or 1000 Tb/s (or 1 Pb/s, but terabits are at least thinkable currently).

Couldn't you just modulate the signal much slower and transmit at a lower bandwidth to lower the power requirements? If you reduced the power by a factor of 10 (so 1 photon/10 sampling periods) and lowered the amplitude modulation by a factor of 100, you still should be able to resolve the modulation.

The original poster still has many other problems, of course - not the least of which is the fact that optical is probably the worst frequency to transmit at, because just about everything absorbs it. Hence the reason we evolved to see it.

Re:ET phone ... us?

[Anonymous Coward]

>>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.
It's not magical. The human ear does the same thing all the time. The ear is filled with thousands of microscopic hairs, and each one is a different size. The different sizes respond to different frequencies, and put together it becomes what you hear. These nanotubes would work in the same way, only for the visible part of the EM spectrum.

Really cool stuff.

Can we just switch policy?

[exp(pi*sqrt(163))]

And post stories only when people discover things you can't do with nanotubes?

article text

[OneOver137]

An Antenna for Visible Light An antenna for visible light, analogous to antennas for radio waves, can be made with carbon nanotubes. In a radio antenna, whose size is equal to the wavelength of the incoming wave or a fair fraction of it, the wave excites electrons into meaningful currents . Such a response, amplified and tuned, is the backbone of radio and TV broadcasting. At optical wavelengths, where the wavelength is hundreds of nm, this is harder to do. Nevertheless, a rudimentary antenna effect for visible light has now been observed by scientists at Boston College using an array of carbon nanotubes, in which infalling light excites miniature electrical currents. According to Yang Wang (wangyq@bc.edu,617-552-3436) one would like to measure these electrical excitations directly, but this requires nano-diodes capable of processing electrical pulses oscillating at optical frequencies (1015 Hz), and these are not yet available. The next best thing is to observe the secondary radiation emitted by the faint excitations. The nanotubes used in the experiment are in effect little metallic antennas about 50 nm wide and hundreds of nm long (see figure). Not only can the nanotubes respond in the manner of dipole radio antennas to incoming light, but they also exhibit a polarization effect; when the incoming light is polarized at right angles to the orientation of the nanotubes, the response disappears. Possible applications for visible-light antennas? Optical television: a TV signal, superimposed on a laser beam sent down an optical fiber, is demodulated at the customer end by an array of nanotubes (each functionalized by a fast diode). Or efficient solar energy conversion: incoming light is turned into charge which is stored in a capacitor.

And of course...

[Lord Bitman]

lidar

Ultra high frequency rectifiers.

[Christopher Thomas]

It's interesting that this should come up, as last spring or so, I was sitting on the presentation of a paper about doing this with far-infrared. Conventional lithographic techniques were used to make waveguides and rectifiers. Photons entering the wave guide caused currents when they struck the walls, which were picked up and rectified by interesting devices that worked by exploiting ballistic electron transport (looked like a wedge inside a T-joint; electrons flowing in one direction were preferentially scattered).

Frequency limit of this technique was related to the sizes of their structures, but I didn't get the impression that it would work at optical wavelengths. Still very nifty, though.

[The paper was presented at CCECE 2004, but I'm having difficulty finding a citation.]

Wang should partner with Alvin Marks

[Anonymous Coward]

Article about Lepcon and Lumeloid, Marks' super-efficient solar cells. They use sub-micron antennae to convert light to electricity. Lepcon uses metal (aluminum) antennae, and Lumeloid uses organic (polythiophene?) antennae, instead of carbon nanotubes.

Patents by Alvin Marks

The carbon nanotube guys didn't produce DC electricity because they don't have a super-fast rectifier. Alvin Marks has patented a design for one. Dunno if it's actually been tested, though.

Hmmm, it looks like the femto-diode patent has expired (search for 4,720,642).

Re:Wang should partner with Alvin Marks

[cryptochrome]

Can't nanotubes be constructed in such a way as to be metal or semiconductor? If so, couldn't a little chemistry theoretically tweak these into comprising both a diode and the antenna?

Of course regular photovoltaics are just diodes anyway.

I was really interested in lumeloid (as well as another energy storage technology Marks was working on called Quensor which claimed to be rechargable with near-gasoline like energy density) but it's become evident that the research has been largely dropped. It's kind of a shame, but the market for very-high-efficiency solar cells (and very high capacity batteries) isn't going to go away anytime soon, so someone is bound to make a breakthrough someday.

MOD DOWN!

[Anonymous Coward]

Karma whore AND too dumb to format correctly.

New Prosthetic Eyes?

[4of12]

Converting visible light or beyond into electrical signals - I'm not sure what carbon nanotube antennae offer over established solid state devices based on Si or the III-V compounds, but perhaps they might be more useful in creating biologically compatible prosthetic eyes.