Can the laser light, in principle, take any wavelength in the EM spectrum? I don't think there is what prevent this in principle, right?
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In principle, yes. A coherent photon state can exist at basically any wavelength. In practice, finding a suitable lasing medium or cavity material will pose a difficulty. These limitations are somewhat circumvented by a free-electron laser, which works by sendind a beam of electrons along a "wiggling" path. Such devices can produce laser light anywhere between the infrared and soft X-ray bands.
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1$\begingroup$ @paulina is the acronym LASER reserved for ONLY visible light laser? Why am I saying that? Well, because the device that produced coherent microwave laser is called MASER instead and the hypothetical device that produces coherent gamma rays is called GRASER. Am I right? $\endgroup$– JackCommented Jun 21 at 4:02
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6$\begingroup$ @Jack Infrared and UV 'LASERs' are very very well established in industry, despite being outside 'visible light'. You could perhaps argue that while UV and IR are 'light', radio waves and gamma rays are typically not considered light. $\endgroup$ Commented Jun 21 at 6:09
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4$\begingroup$ @Jack Language is an interesting phenomenon. L.A.S.E.R. began as an acronym. It has now become a word. If using the acronym, it would be only for visible light. If using the word, it references the coherent beam itself, with the spectrum and emitter type being mentioned separately. $\endgroup$– David SCommented Jun 21 at 14:55
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$\begingroup$ @DavidS technically speaking, that would also mean that the electron "laser" I mention is not a true laser at all - there is no stimulated emission happening, just plain old radiation of accelerated charges. $\endgroup$– paulinaCommented Jun 21 at 15:22
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It depends on what you mean by "laser". In the classical sense, you have to boost a collection of atoms into a metastable excited (electronic) state, and then stimulate that collection to de-excite coherently by shooting a photon with the right wavelength through it. This means that if the excited state is an unoccupied electron orbital of energy ~a few eV above its unexcited state, then a bunch of visible light photons can excite or "pump" the medium and can then act as the trigger and the laser will produce a monochromatic visible light beam.
For radio frequencies (say, in the 1 megahertz to 100 megahertz range) atoms do not have empty orbitals of the right energy range, so there are no available lasing media that you could pump and then de-excite to produce a coherent beam of radio-frequency photons- so, no RF lasers.
For RF in the microwave range, ammonia molecules can be used as a lasing medium to produce a maser.
For higher frequencies it is in principle possible to excite a population of atoms into a metastable energy state in the ~tens of kiloelectron volts range using X-rays to pump electrons from the deeper-lying orbitals, and then using an x-ray to trigger the stimulated emission. This was attempted during the R&D phase of the so-called Star Wars program, where the objective was to blow up incoming nuclear warhead-tipped ballistic missiles with extremely powerful x-ray beams.
This required an atomic bomb to furnish the x-ray pump (!!!) and some lasing medium (a rod of metal) to absorb those x-rays and produce stimulated emission, all before the bomb blast destroyed the entire shebang. This concept was tried in an underground atomic bomb test but the results (which initially indicated actual X-ray lasing effect) were later shown to be wishful thinking and the efforts to use atomic bomb blasts to pump an x-ray laser were abandoned.
I'll stop here and have a look at what's been written about gamma-ray lasing, and perhaps I'll come back with an edit on that topic.
answered Jun 21 at 2:21
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$\begingroup$ With enough abstraction, a laser oscillator and, say, a Colpitts radio frequency oscillator, are pretty much the same thing. $\endgroup$ Commented Jun 21 at 2:53
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3$\begingroup$ @john doty, with enough abstraction, I will agree with you! $\endgroup$ Commented Jun 21 at 4:30
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Not sure whether you are after a laser at weird wavelengths, or 'laser light' at weird wavelenghts. The latter may be more pragmatic — we can use light to do things irrespective of where it came from. So what makes the laser light special? Mostly coherence, over time and over space. Normal light from a candle is like many light sources all emitting randomly. Laser light is like all of them working together. That means laser light looks much more like a sine wave at any place you observe it (temporal coherence), and this wave has a very large wavefront, so it looks similar from many places (spatial coherence).
It is fairly easy to get good spatial coherence — simply pass your light through a pinhole, and then collimate. You will lose a lot of power, but it will be spatially coherent. Temporal coherence can be achieved using wavelength filters, but you will lose even more power. So you can, in principle, make even candle light look like a laser light, but it will be very low power. The special thing about lasers is that you get powerful coherent light from them, without needing to use so much energy.
This problem — of creating powerful coherent light — does not necessarily need lasers. At radio frequency, a simple antenna array can do the same job.
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$\begingroup$ The antenna is the structure that couples energy between the transmitter and everything else. It's the transmitter that's like a laser. $\endgroup$ Commented Jun 21 at 21:57
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$\begingroup$ @John Doty. When I said 'antenna array' I meant the whole device. The point was that it is the properties of light, irrespective of source, that are special for 'laser light'. Diving deeper into the weeds of the device that makes them does not help to convey the message. It does the opposite $\endgroup$– CryoCommented Jun 21 at 22:31
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1$\begingroup$ @Cryo I think the point that's worth highlighting is that at those lower frequencies we can create those coherent states just by wiggling the electrons in a metal in rather pedestrian ways (semiconductors, usually, but you can do it mechanically if you go low enough). If anything there's something special about the IR-and-up frequencies where we have to use weird indirect methods. $\endgroup$– hobbsCommented Jun 22 at 17:19
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$\begingroup$ @hobbs Pumped stimulated emission is a rather simple process that makes the effective resistivity of a medium negative. Put that in a resonator, and you have an oscillator. Very direct. At lower frequencies we we usually use more complicated negative resistance machinery. $\endgroup$ Commented Jun 23 at 12:02
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Wavelength is not an intrinsic property of a photon. Rather, it describes the interaction a photon has with an observer in a particular frame of reference. Given any photon and wavelength, there is some frame of reference where that photon has that wavelength. And given any emission wavelength, target wavelength, and observer, a laser emitter that in emits the emission wavelength in its frame of reference could, in principle, be made to go at sufficient velocity relative to the observer such that the red or blue shift causes the observer to observe the target wavelength.
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$\begingroup$ Interesting observation, though there are limitations. In particular, only plane-wave interactions will allow such abuse, any interactions that rely on non-uniform distribution of light will be subject to non-trivial distortions. $\endgroup$– CryoCommented Jun 21 at 16:38