22 votes
Accepted

Is it possible to create a mirror that redshifts light?

Yes, this is possible using nonlinear optics. This kind of frequency shift can be done using acousto-optic modulators and electro-optic modulators, and it is normally done in a transmission geometry. ...
Emilio Pisanty's user avatar
9 votes
Accepted

SPDC and single photon production

There are indeed some subtle but important differences between an SPDC-based heralded source and a true single photon source. In order to understand these differences, consider what a true single-...
Girish Kulkarni's user avatar
9 votes
Accepted

What colour is an electron?

I think to first order what you are looking for is the Klein Nishina cross-section. What is important here is that light can inelastically scatter from electrons, but can never be absorbed by a free ...
KF Gauss's user avatar
  • 7,743
9 votes
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Why are pulsed lasers are the main tool used in nonlinear optics applications, instead of CW sources?

The simplest answer is that for nonlinear optics, we want the intensity of the light to be high. Using a pulsed laser allows us to concentrate the energy into smaller temporal bins, which increases ...
Emilio Pisanty's user avatar
8 votes

Is it possible to create a mirror that redshifts light?

As Jon Custer said in a comment, the light returning from a receding mirror will be redshifted. This is easiest to see if you consider the problem in the rest frame of the mirror. A symmetry argument ...
benrg's user avatar
  • 26k
5 votes

Relation between intensity of light and refractive index

As mentioned in the other answers, if the medium is linear then the refractive index is independent of the intensity of light, and the intensity can be related to the electric field amplitude through $...
Emilio Pisanty's user avatar
5 votes
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Difference between photon upconversion and two photon absorption?

These two processes are fundamentally different processes. The Photon upconversion Wikipedia article has some problems with it, notably by including the image (first imaged below) that is of a two-...
Punk_Physicist's user avatar
5 votes

Is it possible to have solid light?

No because solid is a state of matter. Light cannot be considered matter since it is made up of particles which have no mass and I'm pretty sure occupy no space (i.e. photons have no volume). Edit: ...
M Barbosa's user avatar
  • 509
5 votes
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How to find Intensity from field of train of pulses?

You're conflating two different views on the description of the attosecond pulse train; in particular, you're flitting back and forth between the time-domain and frequency-domain descriptions, and it'...
Emilio Pisanty's user avatar
5 votes
Accepted

How is it possible to ionize air with wavelengths longer than the UV dictated by the ionization potential?

Your conversions are correct - if you want to ionize air molecules with a single photon, then you'll need photons with wavelength shorter than 80nm or so. However, for the videos you give as examples, ...
Emilio Pisanty's user avatar
4 votes
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How does frequency mixing work in a non-linear crystal?

Frequency mixing is the mechanism. Mental pictures of the form "two photons are absorbed and one bigger photon is emitted" are useful but they're ultimately just not that accurate, unless you're ...
Emilio Pisanty's user avatar
4 votes
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Finding four-wave-mixing terms in photon-atom interaction (by adding for fields to EIT)

I hope it's okay if instead of trying to find errors in your approach, I just describe how I'd solve the problem in a similar way. By the way, I think you're actually going in the right direction but ...
aekmr's user avatar
  • 300
4 votes

Why is two-photon absorption a third-order non-linear process?

This is actually rather confusing. So, good question! This describes single-photon interaction with a material: $$ \mathbf{P}=\chi^{(1)}\mathbf{E} $$ Or rewritten to assume time-harmonic fields, $$ ...
Gilbert's user avatar
  • 11.8k
4 votes
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Purity of a quantum state in the Heisenberg picture

For a closed system, whose time-evolution is unitary and given by the Schrodinger equation, any quantity $S$ quantifying a state's "mixed-ness" (or entanglement entropy, depending on your perspective) ...
tparker's user avatar
  • 47k
4 votes
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Is second harmonic generation a special case of high harmonic generation?

By HHG I mean a process in which many photons are combined into one photon via virtual levels under conservation of energy. This is pretty close to a completely misleading mischaracterization of the ...
Emilio Pisanty's user avatar
4 votes

Frequency Chirp, Instantaneous Frequency and Photons

I wonder whether this is a case where you have a physical intuition about photons which isn't quite working. If we say that 'a photon' has frequency $\omega$ (and energy $\hbar \omega$) then we are ...
Andrew Steane's user avatar
4 votes

Spin conservation in spontaneous parametric down-conversion (SPDC)

It isn't. SPDC, like all second-order nonlinear optical phenomena, happens in an anisotropic medium, which means that the optical interaction (when seen exclusively as an interaction between optical ...
Emilio Pisanty's user avatar
4 votes

Furry's theorem and spontaneous parametric down-conversion

The theorem you've linked to is true - in vacuum. SPDC happens in a non-linear medium which breaks the symmetries that underlie the hypotheses of that theorem. As such, there is no contradiction ...
Emilio Pisanty's user avatar
4 votes
Accepted

How does optical "self-torque" make sense in terms of Maxwell's equations?

(Full disclosure: I'm an author in the paper in question.) The name self-torque was chosen because this is a time derivative of an angular momentum, $$\xi = \frac{\mathrm{d} ⟨\ell⟩}{\mathrm d t},$$ ...
Emilio Pisanty's user avatar
4 votes

Can there be two different physical units (or dimensions) for a same physical quantity?

The 1st order susceptibly is "dipole moment per electric field", the second order moment is "quadruple moment per squared electric field", so that's how you get power of "...
JEB's user avatar
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3 votes
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Nonlinear PT-symmetric model and integrability

OP is studying a coupled system of the form $$ \begin{split} i \dot{u}_1 & ~=~ - i\beta u_1 + \chi |u_1|^2 u_1 + \gamma u_2, \qquad u_1,u_2~\in~\mathbb{C}, \cr i \dot{u}_2 & ~=~ + i\beta^{\...
Qmechanic's user avatar
  • 200k
3 votes

Negative refractive index of gold

The condition for propagating surface plasmon waves is not that the refractive indices n of the metal and the surrounding medium have opposite signs! The condition is that the real parts of the ...
freecharly's user avatar
  • 15.8k
3 votes
Accepted

Optical Frequency Divider?

Optical parametric down conversion; this can be accomplished with a variety of non-linear crystals, such as BBO. Efficiency depends upon matching of energy and momentum conditions with the proper ...
Peter Diehr's user avatar
  • 7,145
3 votes
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Self-phase modulation induced spectral broadening

As you point out, self-phase modulation can be thought of as an added chirp, but the crucial point is that this is a local chirp that changes from the front to the back of the pulse, in essence ...
Emilio Pisanty's user avatar
3 votes

Why is two-photon absorption a third-order non-linear process?

The relation between the number of photons - or, equivalently, order of the electronic transitions - involved in a TPA process (two) and the order of the corresponding nonlinear susceptibility (third) ...
Marco Piccardo's user avatar
3 votes
Accepted

What are the material restrictions for a $\chi^{3}$ non-linear optical material?

Long answer: Basically, $\chi^{(3)}$ is a rank four tensor with 81 components. You then apply the Neuman principle saying that symmetries of the crystal that supports $\chi^{(3)}$ should also be the ...
Cryo's user avatar
  • 3,018
3 votes

Numerical simulation for spontaneous parametric down conversion

So the most basic way to understand parametric down conversion is that of a two mode squeezed vacuum states. This is typically what you see in introductory textbooks in quantum optics where the ...
ChrisM's user avatar
  • 1,650

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