Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

THz AC question

  1. Mar 18, 2009 #1

    I'm working on a design idea and have run into a problem. Despite multiple google searches from different angles, I can't seem to find what I'm looking for.

    Question 1: What is the highest frequency of alternating current ever produced here on earth? (if you know the voltage, that's cool too)

    Question 2: How was this produced? I'm sure it was solid state, but the highest frequency multiplier I could find available had an input frequency of 40 GHz, and could multiply 3X/ (120 GHz output).

    Question 3: Is there a theoretical limit to this frequency? Maybe the highest a fiber optic cable can hold? Is there a better material?

    Thank you... I have more related questions, but I'd probably better start with these.
  2. jcsd
  3. Mar 18, 2009 #2


    User Avatar
    Science Advisor
    Gold Member

    First of all: all types of light can be described as "alternating current" if you want; although the concept becomes less and less useful as you go up in frequency.
    By just switching between different technologies we can cover the range from DC up to whatever the frequency of the most energetic radiation we can produce is (remember that energy and frequency is related via W=h*f, with h being Planck's constant).
    The frequency of gamma radiation is of the order of 10^19 Hz and is produced in nuclear reactions and I suspect this is about the highest frequency we can reach.

    Note that there is no sharp line between the frequencies we can reach with solid state technology and optical techniques; they tend to overlap quite a bit around 1 THz or so. Frequencies above about 1 THz are generally referred to as far-infrared radiation (can be produced by lasers) and are "optical" and lower frequencies are known as sub-mm radiation and are "microwaves" (used in e.g. radar, can be generated by e.g. backward wave oscillators or Hall bars).
    The range between about 500 GHz to 1.5 THz can be reached form both directions. In this range a mixture of optical and microwave techniques are used; e.g. waveguides and lenses are often used in the same experiment.
  4. Mar 18, 2009 #3
    Like said before, gamma rays are a form of electromagnetic wave of a very high frequency. I believe the highest gamma rays ever observed was due to very energetic cosmic rays where a single proton was detected carrying energy of a few joules in the atmosphere. If you convert that to the frequency of released photons, it's incredibly high.

    Frequency multipliers are very common in frequencies up to a few THz.

    http://www.virginiadiodes.com/multipliers.php [Broken]

    Frequency doublers are also used in optics. A green laser pointer you can get cheaply now, is actually made of an infrared laser diode that pumps a special crystal frequency doubler that doubles its frequency to 532 nm in green color range. Blue laser used in blue-ray DVD players works the same way.
    Last edited by a moderator: May 4, 2017
  5. Mar 18, 2009 #4
    Thank you for the reply, and good point about the AC! Perhaps I should have been more specific:

    What is the highest frequency of alternating electrical current I can produce when I start with the 60 Hz current coming out of the wall in my house? I can use any number of methods, but I have to end up with alternating electrical current.

    It's funny you mention THz range, because that's the area I was looking for; specifically 1 to about 300 THz.

    The reason I wanted electrical current is because I need an alternating magnetic field... Unless this can be done with a Microwave?

    Thanks again for responding.
  6. Mar 18, 2009 #5
    Just got your reply after I posted waht. Thank you for this extra info!
  7. Mar 19, 2009 #6


    User Avatar
    Science Advisor
    Gold Member

    It would be rather silly to "start" with 60 Hz and use multipliers to reach RF frequencies.
    It is much better to build some kind of oscillator and generate the signal that way.

    That said, there is no easy way to generate signals in the THz range. In the lab this range is usually covered by pulsed far-infrared lasers, but that is hardly equipment suitable for DIY (it is both expensive and dangerous).

    The klystron in a MW owen operates at 2.45 GHz, i.e. it is nowhere near the THz band.
  8. Mar 19, 2009 #7
    Indeed, THz sources are difficult to generate. One promising method is heterodyning two infrared lasers in a photomixer to generate the difference frequency. By keeping the frequency of one laser fixed, and slightly varying frequency of a second laser, a wide tuning range can be achieved from 100 GHz to a few THz in one sweep.

    There are tubes that operate well into 500 GHz range. One is called a "clinotron " and it's a derivative of a BWO (backward wave oscillator) which is a derivative of a klystron.

    Other sources of millimeter wave are gunn, impatt, tunnel, and SRD diodes.

    But still, the cheapest way to go up that high is to build a long frequency multiplier chain that is locked to a GPS stable reference clock.
  9. Mar 19, 2009 #8


    User Avatar
    Science Advisor
    Gold Member

    You mean magnetron right?
  10. Mar 20, 2009 #9


    User Avatar
    Science Advisor
    Gold Member

  11. Mar 20, 2009 #10
    Thanks again everyone for the great answers...

    I'm about to post another question, but I think I should start a new thread so as to stay on topic.

    But still, I really appreciate everyone chiming in. :biggrin:
  12. Mar 20, 2009 #11
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook