We had radio amplifiers that could transmit powerful signals and those were pushed to microwaves. We had lasers that were originally natural transitions in solids and gasses but now tunable lasers go from deep in the infrared to near UV at least, and with wigglers, undulators and free electron lasers we can go from UV to X-rays.

Above X-rays is a semi-infinite range of photon energies that have yet to qualify, so gamma rays would not count as a "hole".

I am certain I'm missing other technologies as well.

What I'd like to know is which wavelength range was the last "hole" in the spectrum where there was no technology to make amplified narrow-band power?

Question: What was the last "hole" in the electromagnetic spectrum where amplified, tunable, narrow-band signals couldn't be produced artificially?

I'm guessing it's somewhere between a millimeter and a micron in wavelength, but I could be way off.

note: I say "tunable" to indicate that you can center your narrow band anywhere within a wide range so that there is continuous coverage. Radio, optical lasers, synchrotron radiation-based sources are all tunable, while the old gas and solid state lasers had fixed frequencies and big gaps in between.

  • $\begingroup$ What makes you say that there was a last hole and not a continuous fading of the limits? $\endgroup$
    – Mauricio
    Nov 2 at 16:17
  • $\begingroup$ @Mauricio based on a conversation in Electronics SE chat and Wikipedia's Terahertz radiation: Terahertz gap you are right that at the current moment the edges of this hole are fuzzy - approaching this gap from both above and below the power drops and the reliability drops. But I haven't asked for the exact limits. It appears that "the THz gap" is indeed the answer. $\endgroup$
    – uhoh
    Nov 2 at 21:57
  • $\begingroup$ @Mauricio That Wikipedia article says that "(i)t is defined as 0.1 to 10 THz (wavelengths of 3 mm to 30 µm) although the upper boundary is somewhat arbitrary and is considered by some sources as 30 THz (a wavelength of 10 µm)" It appears to be that tunable lasers with frequency multiplication (visible to UV conversion) plus syncrhotrons closed the vacuum UV gap earlier. $\endgroup$
    – uhoh
    Nov 2 at 21:59


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