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I have heard the terms up and down conversion in photonics/photovoltaics articles. What do the terms mean?

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up vote 5 down vote accepted

I complete Gerard's answer: up-conversion is the reverse process, where two low frequency photons are converted to a single high-frequency photon.

So basically down- and up-conversion correspond to a frequency conversion of the photons through a nonlinear interaction. The up/down term correspond to the "direction" of the frequency change.

I have to add that the "parametric" term has nothing to do with entanglement, but with the fact that the χ⁽²⁾ nonlinearity used is analogue to a classical parametric oscillator http://en.wikipedia.org/wiki/Parametric_oscillator .

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"where two low frequency photons are converted to a single high-frequency photon" was a big leap in my understanding! I'm not sure I understand yet how that should happen, but I know it would be related to a concentration of energy (due to higher frequency). This one really helped. Thanks! –  Kit Nov 15 '10 at 14:42
    
In photovoltaics, parametric up- or down-conversion is never used. The intensity of sunlight -- about 1 photon per square-micron per nanosecond -- is far far too low for that kind of nonlinear optical effect to occur at a noticeable rate. Instead the photons are absorbed and re-emitted by a chemical that has at least three different energy levels. –  Steve B Nov 12 '11 at 18:25
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In the field of quantum optics "parametric down-conversion" is a process where an intense pump laser shines an optical non-linear crystal and strongly correlated or entangled photon pairs are generated.

In an optical non-linear crystal the polarization response of the material to the optical field is non-linear, which typically occurs at high intensity.

The photons of the pump laser have energy $\hbar \omega_p$ and momentum $\vec{k_p}$ which must be equal to the sum of the energy and sum of momentum for the produced photon pair: $\hbar \omega_p = \hbar \omega_1 + \hbar \omega_2$ and $\vec{k_p} =\vec{ k_1} + \vec{k_2}$. Conservation of the parameters energy and momentum lead to strong correlations or entanglement between the produced pairs. I believe the name parametric indicates the fact that certain parameters are entangled whereas other parameters are not.

The process is not very efficient but the experimental setup is not very difficult and acts as source of entangled photon-pairs in many experiments on the foundations of quantum mechanics addressing the EPR paradox.

Parametric up conversion probably is the reverse process where entangled photon pairs form one new photon - I am not sure however.

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+1 for the rigorous answer. I couldn't really understand this yet, but if not for this answer, @Grosshans may not have connected with a simpler explanation. Thanks Gerard! –  Kit Nov 15 '10 at 14:44
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