[sasmith]

Should I be considering this differently from solar? They're both harvesting ambient EM radiation; and the energy density of solar is much higher. I guess that radio goes through walls and never sleeps, so that's a plus. Anyway, I thought that the mentioning of a solar calculator was quite appropriate and warranted further discussion.

[sliverstorm]

It sounds like the device on the hard hat is more like a passive RFID chip; It's hard to be sure, but the article says all the dangerous equipment has transmitters of their own.

Of course, the effective difference between the passive RFID chip and a device that simply consumes ambient radio-spectrum EM radiation is small, mostly a paradigm change.

Your initial question begets an interesting question- since light is EM radiation, just like radio waves, shouldn't we be able to pick up light with an antenna? If we can figure that out, we can forget about solar cells with their sad efficiency levels.

edit: with some quick research, the answer is obvious; while it seems extremely weird to imagine, light can be absorbed by an antenna- and the first and foremost reason this isn't being done already is because the appropriate antenna would be ~700 nanometers long.

[kragen]

You can absorb EM radiation just fine with an antenna that's "too long". It's when it's too short that you run into problems.

500nm-wavelength light oscillates at about 600 terahertz, with a period of about 1.7 femtoseconds. If you want to rectify that and turn it into DC current so you can run current semiconductor devices, you need a diode that can switch on once and off once in that period of time. So your forward recovery time plus your reverse recovery time needs to total less than 1.7 femtoseconds. Among other things, I think this implies that the depletion region in the diode needs to be less than 0.9 femtoseconds in width --- at the electron drift velocity of the semiconductor, which I think is typically around 12 orders of magnitude less than c, although in silicon it can be as high as only three orders of magnitude less than c. Which means that your depletion region needs to be 3 orders of magnitude smaller than the wavelength. Unfortunately the wavelength we're talking about here, at around 1000nm, is only four orders of magnitude bigger than a smallish atom, at 0.1nm. So you're pushing up against the bounds of possibility here with an insulating depletion region of a few atoms in thickness.

Forward and reverse recovery times for silicon diodes vary widely. Typical values for discrete components are measured in the tens to hundreds of nanoseconds. Schottky diodes bring that down to tenths of nanoseconds. One nanosecond is one million femtoseconds, so that's still five orders of magnitude too slow.

Anyway, I don't know anything about this stuff, really.

[sliverstorm]

Seems like it'd be easier to just drive a 600 terahertz motor, if such a motor can be made, and use that to drive a good old fashioned 60hz generator.

The silicon would be a much nicer solution though. Thank you for the details on the diodes, I forgot about that part. You're probably right on the diodes being the hold-up.

[kragen]

Now that's an interesting idea. The light contains an oscillating magnetic field that you could use directly to spin a permanent magnet, if you had one that was small enough. (Any atomic nucleus would do, but you can't connect a shaft to it.) Maybe you could build a multipole nanomotor so that the rotor itself doesn't have to spin at 600THz; if you have 100 poles, which is not that far out of what people commonly do with macroscopic stepper motors, then you could get the rotation down to only 6THz. (But then you're only potentially absorbing light at the rim of this rotor.)

Gearing that down to 60Hz at the nanoscale — without losing most of the energy to friction — could still be a significant challenge. I don't know of any hundred-billion-to-one gearboxes.

The basic difficulty with the nanomotor approach, I think, is that electrons are lighter than nuclei, so it's easier to get them to oscillate over useful distances in any particular frequency range, and this is especially tricky in the terahertz to petahertz frequency range. A nucleus, under the influence of the same electrical field as an electron, will accelerate about three or four orders of magnitude more slowly.

Ultimately this should be a scale advantage for mechanical computation, since it means you can localize an atom to a much smaller region, given a certain momentum uncertainty, than an electron. The atom can't tunnel as far, so it can store a bit reliably in a much smaller region. I don't think we're there yet.

[sliverstorm]

Is it necessary to make it a nanomotor? Friction is not a concern once you have things suspended by magnets in a vacuum. If power is an issue for driving the larger rotor, instead of using just one antenna use an array. It's typically better to have one giant Engine than many small ones.

Forget gearboxes, a belt drive would be superior until you start cranking out huge power, and in that case you could try chains instead. Also, don't forget that a 60hz motor does not have to spin at 60rpm to generate 60hz.

[kragen]

The difficulty with making it larger than nanoscale is that the centripetal acceleration becomes very great, which makes holding a large rotor together tricky; you need very strong materials. Actually, I did the calculations, and for visible light, it isn't even feasible at the nanoscale.

The centripetal acceleration of the rim of a rotor of radius r is rω². Rotating at 600THz (i.e. 600 trillion rotations per second), ω = 600T2π/s ≈ 3.8 × 10¹⁵/s. If your radius is 1mm, then your acceleration is about 1.4 × 10²⁷ G. The smallest rotor you can make out of atoms is probably around 0.1nm, which reduces the acceleration to only about 1.4 × 10²⁰ G. If your rotor was, say, an orthohydrogen H₂ molecule, with a distance between the nuclei of about 62pm, and thus a radius of about 31pm, the acceleration is about 4.5 × 10¹⁹ G, which would be a weight of about 74 micrograms pulling on that single covalent bond. The nuclei would be whirling around the covalent electron cloud that bound them together at about 11.7 kilometers per second, and each of them would have about 1.13 × 10⁻¹⁷ J of kinetic energy. Unfortunately, hydrogen's ionization energy is about 2.2 × 10⁻¹⁸ J, so that's about five times as much energy as you'd need to rip the molecule apart. I think. It could work out in the infrared, maybe. You'd just need a way to get the molecule started spinning.

So I guess you'd have to make your rotor a lot smaller than a diatomic molecule, or a lot stronger than a mere covalent bond.

Generally a 60Hz generator must spin at 360rpm or slower to generate 60Hz.

[modeless]

Wouldn't 700 nm antennas would be easy to make? We've been fabricating features smaller than that on silicon wafers since 1994 (according to Wikipedia).