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The question is whether it is physically possible to build a laser using only the photoelectric effect in the vacuum, with no lasing material.

I know that there is a whole history of laser development in which people found all sorts of materials that lase if you pump them hard enough, but my question is simpler and more fundamental. Is it really necessary to have a lasing material in order to have a laser?

There already exists a patented optical amplifier using the photoelectric effect in the vacuum, and so I think it should be possible to build a laser, which is basically an optical amplifier between two mirrors. The advantage of such a photoelectric-effect laser is that the applied voltage would allow you to smoothly vary the color of the light produced, instead of relying on a pre-existing electronic-state transition in your gas molecules or a fixed energy band gap in your semiconductor material.

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    $\begingroup$ Can you provide a link to this "amplifier using the photoelectric effect in the vacuum"? Otherwise, how do we know what you're talking about? $\endgroup$ – The Photon Jan 13 '15 at 18:50
  • $\begingroup$ Welcome to the EE.SE! In addition to agreeing with @ThePhoton, I would also recommend re-formatting your question to make it more clear and obvious what your actual question is. $\endgroup$ – Adam Head Jan 13 '15 at 18:59
  • $\begingroup$ google.com/patents/US8294983 $\endgroup$ – W. Stuart Edwards Jan 13 '15 at 19:02
  • $\begingroup$ I can restate it three ways. Is the photoelectric effect sufficient for a laser? In other words, why place a lasing material in a laser instead of using the photoelectric effect? Or, is there a fundamental physical reason why the lasing material is necessary? $\endgroup$ – W. Stuart Edwards Jan 13 '15 at 19:09
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    $\begingroup$ I don't know what you talk about energy conserved, is doesn't seem to me connected with the lasing. For obtaining the laser beam we need many atoms raised to a certain excited energy level. This is why we need the material, and why we do pumping. Now, the trigger that starts the lasing is the emission of one or more photons spontaneously. In travelling through the material they make the other atoms emit also, and all this gross emission is in phase. I am no specialist in lasers, but this is the main picture. Now, what can be the similarity between the photoelectric effect and the above? $\endgroup$ – Sofia Jan 13 '15 at 21:37
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There exists the free electron laser

A free-electron laser (FEL), is a kind of laser whose lasing medium consists of very-high-speed electrons moving freely through a magnetic structure, hence the term free electron. The free-electron laser is tunable and has the widest frequency range of any laser type, currently ranging in wavelength from microwaves, through terahertz radiation and infrared, to the visible spectrum, ultraviolet, and X-ray.

The magnetic structure generates the coherence in the photons produced.

Maybe with nanotechnology the design can be miniaturized by using a photoelectric source ( which I suppose will in any case be the source of the electrons in the FEL beam)

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I think that your question is basically the generation of laser via electron impact. The basic principle behind laser radiation which makes it coherent is stimulated emission of photons in population reversed media. If you have free electrons (whether thermionic or via photo electric effect) and you accelerate it to certain voltage then make it fall on a material, it will certainly generate a radiation. This is a basic principle behind the x ray tubes. However the radiation will not be coherent because each atom will generate photons according it its own phase and the coherence (which is gained by stimulated emission) will be lost. The duration of this radiation is a convolution of the lifetime of the transition level and the duration of electron pulse.

It may also be noted here that the spectrum will not be narrow band (like laser) in this case and only the characteristic x rays generated in this process have narrow band spectrum and this is again incoherent radiation.

The broad spectrum is due to the fact that electron do not loose its energy in one impact but it rather looses its energy continuously in several impacts. The radiation generated in this process is both Bremsstrahlung and line radiations.

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