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The Feynman Lectures of Physics states:

In fact, although we mentioned many frequencies, no phenomenon directly involving a frequency has yet been detected above approximately $10^{12}$ cycles per second. We only deduce the higher frequencies from the energy of the particles, by a rule which assumes that the particle-wave idea of quantum mechanics is valid.

The copyright on the lectures is from 1963. Have higher frequencies than 1THz been directly detected since then?

Doug Richardson
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    What would you say qualifies as a 'direct' detection? a waveform in an oscilloscope? – diffeomorphism Sep 25 '14 at 18:45
  • Honestly, I don't know. I'd use the same criteria Feynman did, if I knew what that was. – Doug Richardson Sep 25 '14 at 19:58
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    My guess is, that Feynman may have been wrong even in 1963, but I don't know the state of the art of non-linear optics at the time. Today one can surely say that time domain methods for generation and frequency counting have certainly been used for the 1e15Hz range (visible light to near UV), and if you take http://jilawww.colorado.edu/yelabs/sites/default/files/uploads/nature10711.pdf as an indicator, that has already been expanded into the 1e16Hz range. – CuriousOne Sep 26 '14 at 04:08
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    I agree with @CuriousOne, I think since we can transfer up to something like ~255 Tbps now, that would qualify as being able to receive higher frequencies. I also found a petahertz optical oscilloscope here/PDF. So I think the answer to your question is yes. – honeste_vivere Oct 27 '14 at 13:11
  • Thanks @honeste_vivere, I hadn't seen that. However, after skimming that doc, it isn't clear to me that they are directly counting the frequencies or measuring the energy of photons. – Doug Richardson Oct 27 '14 at 22:17
  • I also think I need to clarify "directly detecting". I take it to mean you count cycles, rather than infer them from some other known relationship (e.g., like the relationship between photon energy and frequency). For example, counting days by observing sunsets would be direct; calculating days from change in Earth's orbital position would be indirect. – Doug Richardson Oct 27 '14 at 22:29
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    Okay, though I think that counting days and measuring a change in the orbital position of Earth are just two different methods to measure the same thing (in your example). Part of the limitation, if we choose your constraint of counting cycles, will be the natural frequencies of materials. There will be a point where we can no longer "count cycles" in the strict sense because the materials used in the detector cannot respond fast enough. However, I do not think that using known quantum relationships or other properties of matter should be considered cheating. – honeste_vivere Oct 28 '14 at 11:21
  • I agree @honeste_vivere, using known relationships (at least within their experimentally determined bounds) is valid. However, this question is about understanding the distinction Feynman was making between direct and deduced measurements (which, as I mentioned before, I'm not entirely clear about) of frequency, in which he specifically states frequency/energy relationship of particles is what allows very high frequencies to be measured using the known relationship. – Doug Richardson Oct 29 '14 at 15:49

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This kind of depends on what you mean by "directly detected", but there is a strong case to be made that optical frequencies have indeed been directly detected.

This has a large overlap with my answer to Have we directly observed the electric component to EM waves?, so I'll leave the details there, but the short answer is that via the experiment known as attosecond streaking we are now able to directly detect the oscillating electric field of a pulse of near-IR radiation:

The experiment works by looking at ionization under a very short pulse of UV light which is co-propagating with the near-IR beam whose electric field is being imaged; the energy of the outgoing electrons then shifts up or down depending on the instantaneous value of the electric field of the near-IR field. You then scan the time delay between the two pulses by moving a mirror, and the wiggles in energy trace out the waveform of the pulse.

This gives you a direct detection of the waveform, and thus of the period and with it the frequency.

And while we're here, I wonder what Feynman would make of the frequency comb (examples of which have been reported at frequencies as high as $7 \times 10^{15}\:\rm Hz$). Does it count as a "direct detection"?

Emilio Pisanty
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  • I do wonder about this, have there been follow-up experiments and replications of this? Not sure if this is an appropriate comment, but it'd be really interesting to see more of this type of tech used. – Maximal Ideal Aug 03 '20 at 18:34
  • @Maximal Ideal streaking is now the standard way for studying attosecond physics. It's used pretty much across the board as a beam diagnostic and measurement scheme for ultrafast attosecond phenomena. – KF Gauss Aug 03 '20 at 18:49
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    @MaximalIdeal Attosecond streaking has now been replicated hundreds of times by a couple dozen groups. It is a solidly established technique and is used all the time (for certain things). For a (now somewhat dated) review, see e.g. this. – Emilio Pisanty Aug 03 '20 at 20:34