Can we change a photon's frequency in mid-air? Can we have a light source emitting photons in the infrared range and after, lets say, 5 meters, these photons become a photon in the x-ray range?
The only way I know we can change a photon's frequency after its creation is through the redshift effect.
But can we do this somehow?
EDIT: Please forgive me for using the "mid-air" expression. Vacuum was what I meant or better yet: "colloquially to mean while in flight".
EDIT 2: Let me rephrase this question in another way.
I have a light bulb above the ceiling. If I look at that light bulb I see no light, unless I am at 5 meters from it. Meaning that in a distance < 5m the light emitted is infrared, in a distance >5m the light emitted is within the visible range.
This would be interesting to help in tumor ablation inside the human body using high radiation doses but very focused and that would only turn lethal in a specific part inside the body. For example in a brain tumor in the middle of the brain, I would like to burn it with light, but without burning the surrounding tissues, like the skin, skull and the brain around the tumor.
For this photons would have to be in a frequency that would not interact with the human body, until they reached the tumor, where their frequency would change somehow and make them interact with the tumor cells, killing/burning them.
The advantage of this procedure would be high precision, the ability to reach regions of the brain that couldn't be reached before, and the ability to avoid opening the human skull.
EDIT 3: Another interesting application of this would be to hide ceiling lamps above the ceiling, having the light crossing the ceiling bricks in a low frequency (like radio waves do) and then below the ceiling brick they would turn into visible light and lit the room. So at the first glance, we would have a lit room with no lamps in it lol.
Is this possible?
EDIT 4: With sound waves we can overlap them (focusing them) and create more energetic waves. Since light also behaves like a wave, could we focus photons to one single point, adding up their energies and for example with two low energy photons create a more energetic one?
 A: To my knowledge, other than red/blue shifting light, the frequency cannot be changed after it's emission. However you may be able to somehow set up some other material which would emit x-rays when infrared light is absorbed. Due to the nature of light and how it acts like a particle when interacting with particles so the energy gained doesn't accumulate when more light is added this will be difficult if it is possible at all.
A: Point the light source up or down in a gravitational field and let Einstein take care of you.
Of course, if are forced to use the Earth as the source of your gravitational field you are going to need a pretty sensitive tool to measure the change in frequency. Luckily for us there is a set of of high precision sources in orbit and you can buy the corresponding high-sensitivity receivers Commercial Off-The Shelf for very reasonable prices.
A: In reply to various points you make in the question
Edit 4 - with sound - the energy of wave that you are talking about that can be increased by focusing is equivalent to he intensity of light that we can increase by focusing with a lens. To make an X-ray we need to change the frequency of light - focusing sound waves does not change their frequency - does this make sense?
Re Edit 2 and Edit 3: I am afraid that unless you have something like frequency doubling described in another answer the frequency of light cannot just change. The examples you describe are particularly interesting, but sadly not feasible. The closest that people can get to your idea in Edit 2 of attacking cancer very specifically is with proton therapy (or even anti proton therapy) - the way that protons damage tissue means that they deposit more energy at the end of their path than at the beginning - to find out more about this you could look up Bragg peak.
Sorry I don't think this is the answer you wanted to hear (or read).
A: Not "in thin air." The problem (well, a problem) is that if you take an infrared photon and have it become an X-ray photon, your new photon has greatly increased energy, which is obviously not something that just happens.
However, there are effects that can sort of do this. The key is that if you want to make one 400nm photon you need two 800nm ones to preserve energy. Now, in general, this wouldn't just happen, since most interaction of light and matter is "linear." In nonlinear optics, atomic media make use of different transitions to produce new frequencies not present in the stimulating light. The simplest cases involve putting two frequencies in and getting either their sum or their difference. Consider, for instance, frequency doubling.
A: I think we might be able to do this.  Supposing by the Bohr effect, a particular frequency is aboserbed and the electron is pushed up to the level, n=4.  Now this energy will be released as it moves to ground 0.  but if it is made to go down to say level 2 only and then to level 1 in stages, then at ground 2, it will theoretically emit a different light and hence a different frequency.  This is because the energy released is less.  So frequency must change to change the reduced energy with planck's constant.
I think this makes sense.  It might be good to find out if this is possible and if it happens.
Such a nice and well thought question.
