Recycling Heat

[exchemist]

1 hour ago, sethoflagos said:

We're pretty much aligned here, and I too am pushing the limits of my understanding of EM field behaviour.

I was hoping that @swansont or someone else with expertise in this field would have picked up on my previous post and confirmed or otherwise the mental picture I have of this. But in the absence of such... 

I think it reasonable to picture the thermal energy of a metallic antenna as being largely contained in the motion of a 'gas' of the unbound electrons. If you can persuade a significant excess of these to move in a coordinated way in a particular direction then that yields an electrical current distinct from the thermal motion. However, I suspect that this requires the incoming radiation to be both directional and/or phase-coordinated. Thermal radiation from the sun satisfies the directionality requirement and I presume the higher frequency part of the spectrum may have sufficient 'kick' to push a few electrons across the junction gap of a diode which might explain the measurable current reported in that scenario. But ambient thermal radiation is omnidirectional and the individual photons are much weaker. Trying to extract energy from this scenario just sounds a little too Maxwell's Demonish for comfort.  May be this picture is all wrong, but it at least tends towards consistency with the Kirchoff's Law/2nd Law objections I raised earlier.  

I've just caught up with this discussion and I agree about the apparent Maxwell's Demonish nature of the explanation on Wiki. The problem I have with it is just as you say, that the arriving photons are not in phase so there won't be any coordinated flow of electrons induced. All you will get is an increase in their thermal, i.e. uncoordinated) kinetic energy. The diode aspect of the device would seem to have to work at the level of individual photon-induced excitations.

I am reminded of the "Brownian Ratchet": https://en.wikipedia.org/wiki/Brownian_ratchet

But I would be most interested if someone can point out what I am missing. 

 

 

[sethoflagos]

5 hours ago, exchemist said:

I am reminded of the "Brownian Ratchet": https://en.wikipedia.org/wiki/Brownian_ratchet

Excellent link!

Cherry-picking one particularly apt paragraph:

Quote

The Feynman ratchet model led to the similar concept of Brownian motors, nanomachines which can extract useful work not from thermal noise but from chemical potentials and other microscopic nonequilibrium sources, in compliance with the laws of thermodynamics.^[3][4] Diodes are an electrical analog of the ratchet and pawl, and for the same reason cannot produce useful work by rectifying Johnson noise in a circuit at uniform temperature.

 

[exchemist]

1 hour ago, sethoflagos said:

Excellent link!

Cherry-picking one particularly apt paragraph:

 

Yes that's the thing. A photovoltaic device induces a non-equilibrium population of electrons and holes in a semiconductor from excitation by absorption of photons, which creates an electrical potential and can do electrical work as the population is restored to equilibrium. At the moment I can't see how this antenna produces a non-equilibrium situation.

Edited January 19 by exchemist

[Ken Fabian]

All interesting, if disappointing. Does it mean this - "Demonstration of resonant tunneling effects in metal-double-insulator-metal (MI2M) diodes" (a demonstration of an optical rectenna in IR) only works because the source is in phase? Would not work otherwise?

Quote

The rectenna was illuminated with 10.6 μm linearly polarized radiation from a pulsed Synrad 48-1SWJ CO₂ laser. The laser source was pulse width modulated by Agilent 3220A function generator at 20 kHz. The noise level under dark conditions was determined by having the laser beam pass through a ThorLabs SH05 shutter. A half-wave plate (ThorLabs PRM1Z8) was used in the optical path to rotate the laser polarization with respect to the antenna axis. Rectified voltage and/or current responses were measured by a lock-in amplifier (SRS830) and the reference signal for the lock-in amplifier was generated by a mechanical chopper at 1.8 kHz.

I suppose that means it was in phase but I don't know. "Linearly polarized" ? "Modulated by a 3320A function generator"? Polarized to a specific axis?

It is all beyond my paygrade - but my surmise that it had to do with the heat emissions of the receiver overlapping with it's tuning seems wrong as well.

[Carrock]

On 1/16/2024 at 5:42 AM, sethoflagos said:

Revisiting Kirchoff's Law briefly, there are some useful bits of information to be had from Electromagnetic Reciprocity.

This caught my eye:

Quote

Forms of the reciprocity theorems are used in many electromagnetic applications, such as analyzing electrical networks and antenna systems.^[1] For example, reciprocity implies that antennas work equally well as transmitters or receivers, and specifically that an antenna's radiation and receiving patterns are identical.

 

As you've mentioned reciprocity a few times in this thread, just a clarification that reciprocity is not always valid.

From Electromagnetic Reciprocity

Quote

One case in which ε is not a symmetric matrix is for magneto-optic materials, in which case the usual statement of Lorentz reciprocity does not hold (see below for a generalization, however).

This is  commonly used in radar, when the transmitter and receiver are both permanently connected and are effectively part of the aerial; output power to the rest of the aerial is from the tx, while input power is to the rx, isolating tx and rx.

Using the ionosphere for radio communications similarly sometimes produces non reciprocal paths.

[sethoflagos]

4 hours ago, Carrock said:

As you've mentioned reciprocity a few times in this thread, just a clarification that reciprocity is not always valid.

I mentioned it once in passing. It isn't a cornerstone of my argument.

While grateful for your advice, I'm at a loss to see how it applies to the optical rectennas under discussion.