This is a window that increases the frequency of light that travels through it.

The window would be made of a sheet of nonlinear crystal. An extremely bright pump laser would be fired into the sheet from the edge at a critical angle and bounce between the two faces of the sheet due to total internal reflection. The laser light would be trapped within the sheet and thus invisible to someone looking at the sheet.

Light that passes through the sheet, through the magic of nonlinear optics, would be upconverted via sum frequency generation. That is, the frequency of the incoming light would be added to the frequency of the pump laser, to make a new higher frequency. The pump laser light would be chosen based on the frequency desired to be made visible.

For infrared upconversion, it would be interesting to see people glow according to their body temperature.

The efficiency of the upconversion would be low because: a) the pump beam and incoming light are not colinear, and b) the incoming light is not coherent (and thus not phase-matched to the laser light). But if the pump beam were bright enough, the upconverted light might still be visible.

This could be used as a camera add-on to take nice infrared photos.

If filters were used on both sides of the window, it might also be possible to create a one-way window.
-- xaviergisz, Sep 08 2014

Nonlinear optics http://en.wikipedia...of_nonlinear_optics
[xaviergisz, Sep 08 2014]

non-collinear sum frequency generation http://www.intechop...se-front-excitation
see figure 10 [xaviergisz, Sep 08 2014]

But what would it do to the visible spectrum? I suspect the image would get awfully squiffy.
-- neutrinos_shadow, Sep 08 2014


It would upconvert most or all of the visible spectrum into the ultraviolet range I suppose.

I agree the image would probably be faint and/or squiffy.
-- xaviergisz, Sep 08 2014


[+]
-- FlyingToaster, Sep 08 2014


How does the system ensure that the departing shifted photon preserves the angle of incidence of the incoming photon ?
-- 8th of 7, Sep 08 2014


^ While it would make sense for it to be like a snooker shot - the outgoing photon's direction depending on the tangential angle the second incoming photon hits at, they do put NLO crystals in front of lasers and the upconverted output is straight, so guess not.

Maybe the outgoing photon is actually the same photon, which has retained its velocity and just gathered more energy from intersecting the excited molecule.
-- FlyingToaster, Sep 08 2014


// actually the same photon //

If it's been upshifted from IR to visible, it's got a different energy, and therefore cannot be the 'same' photon.

Shame on you for even suggesting such a thing. Go and re-read your Quantum Physics primer.
-- 8th of 7, Sep 08 2014


Physicists also claim that the reason light slows down when passing through a transparent medium (and therefore refracts) is that it's absorbed and re-emitted frequently by the atoms or molecules of the material. But light is usually largely retains its direction of travel when passing through a transparent medium. What gives?
-- notexactly, May 28 2019



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