h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Entry: Nonlinear Optics - A773174
Author: Euan - U196968

Entry: Nonlinear Optics - A773174
Author: Euan - U196968

Erm, the pages on third order nonlinear effects haven't been written yet, but the rest should work. Just wondering if this is too techie, or whether it's OK

Euan

Hi there Euan!

Very brave of you to tackle such a y subject! I say 'brave' because it's quite hard to bring stuff of that level closer to the average h2g2 readership... perhaps won't attract many readers anyway, but there *are* a few around who know what a complex number or a power series are. There's a 'trick' to keep readers, by stuffing the math/ pieces under one header, adding a warning and making sure that the layperson can get the idea by skipping around it. This requires *examples*. It's them that a normal mortal might understand far easier

You might want to run it through a spellchecker because there are some typos ('suscept*a*bility', among others). Please avoid first person style ('I' and 'we'), the Editors frown upon it. h2g2 has a problem with correctly displaying greek letters, so it's better to stick to 'chi' etc.


Under 'Frequency Doubling' you could mention 2f-Nd:Yag lasers at 633nm (green) as examples. As far as I know, there's not yet a genuine laser material which would operate in the green part with significant efficiency, so frequency doubling an infrared laser is the choice. With a combination of red (ruby), blue (??) and green lasers you'd be able to build gigantic TV screens etc, so there is a desire and a market.

Under 'Sum and Difference' mixing: perhaps indicate that this is the same method used to put terrestrial TV and radio channels side by side on the air. Doing the same with optics can make better use of a fibre, for example.

Under 'Optical Parametric Generation': (I'm not sure here) -- perhaps that's the same principle as the one used in microwave generation by 'pumping' signals on frequencies A and B into a nonlinear circuit and obtaining the desired frequency C at the output.


... or so


That's about what I know about the subject matter. I'll see to it that Hell (U171578) finds his way here for his part. After all, I'm only a low frequency (GHz, not THz) fellow and he's the optics man




Bossel

Just a small point from me on the chi character. It looks as though you've used the 'lower-case' character χ

It will work much better if you use Χ instead. Try it!

Bels

seems fine to me.

Hi Bels.

In the literature it comes as lower case chi. I was just trying to make it consistent.

Euan

Hi Bels.

In the literature it comes as lower case chi. I was just trying to make it consistent.

Euan

Hello,

Here are some totally optional suggestions and some comments. The basic problem with this entry is that it is not accessible for lay people, so I took the liberty to re-write some parts of the intorduction. Oh, and don't get me wrong, I do not want you to take these suggestions as if all you said is wrong, and that you should replace it by my version. No. It's just a suggestion, to show in what direction you should re-write some bits to make it easier for lay people...

Here comes the first part:
_______________________________

In a classical sense Optics (A666128) is the science that deals with the paths of light. In a more modern sense optics will have to deal with the interactions between light and matter, because the paths are strongly dependent on this interaction. For a long time it was thought that the interactions between light and matter were strictly linear. That is, the amount of some interaction was directly proportional to the amount of light (or the light's intensity). For example: If one shines three photons through a lens, three photons will get diffracted. If one shines five photons through the lens, then five photons will get diffracted. If one was to observe that five photons get bent stronger than three photons, then some nonlinear effect is taking place.

Non-linear optics is a relatively new field. While the first non-linear optical effects have been observed in the 1960s, and many non-linear effects have been postulated even before the invention of in the early 1960s (A597215). After the broader availability of lasers nonlinear effects became more accessible for the ones
conducting experiments. Nonlinear optics is nowadays at a developed stage, and is used for many technical applications.


Linear optics.

To explain nonlinear optics (NLO) [we need to recap linear optics.] Linear optics [...] includes the well-known phenomona of absorption, emission and refraction. [Note: I am afraid that absorbtion and
emission are not a classical field of optics - they're tightly related to optics of course, but... Hmm, maybe it's just my pedantry. I'd include reflection and diffraction to the list, as these are
more well-known to the lay-reader, this would make absorbtion and emission less eye-catching.]

[Bit in 1st person can be deleted]

The formal physical description behind nonlinear and linear optics can get extremely complicated, as some bits of quantum mechanics are used. Instead of writing down the Schrödinger equation, one can imagine the processes by looking at the most common analogy used.

[In the next part you start to explain the oscillator... This might seem a little bit odd for the layperson, because it is hard to see the analogy between a oscillating spring and light. When I have to explain NLO to my students I often use what I call the marbles-in-water model, where photons are the marbles and the medium
is water...]

Linear optics is responsible for, or is the domain of, effects like refractive index, dispersion, birefringence, absorption and emission. - [Hmm, I would not state it like this, as nonlinear optics also happen, it's just that the higher-order susceptibilities are so small that it will only become significant at higher intensities]

________________________________________________
[I'll come up with some more comments soon]

Cheers, let me know what you think

HELL

Hi Hell.

Point(S) taken. I haven't tried this sort of thing for the lay person recently, so am a little rusty. Umm, I'll get along and batter out a better version shortly.

Cheers,

Euan

The next chapter can also be improved IMHO. Take a look at my suggestions:
____________________________


Nonlinear Optics

[In the marbles-in-water model I use the turbulence caused by the moving marbles... But what the heck]

In optics, a similar thing happens. The exact relationship between polarisation and electric field is a power series [Here, you could write down the power series:

P(E) = X^(1)E + X^(2)E^2 + X^(3)E^3 + X^(4)E^4 + ...

];

thus polarisation is an addition of a linear component, plus a term with field squared, plus a term with field cubed and so on to higher powers. Each term has its own [note] susceptibility factor, X^(1), X^(2), X^(3), ... Optional footnote: The term 'susceptibility' can be roughly translated to something like proneness - End of optional footnote. The susceptibilities from X^(2) onwards are known as the nonlinear susceptabilities. The symbol for susceptibility is the Greek character chi (which looks almost like an X), so for the squared term, which is known as second order nonlinearity, one talks about X^(2), for third order nonlinearity (field cubed) one talks about X^(3) and so on. [Note: In my literature (D.Mills, Nonlinear Optics, 2nd Edition 1999 Springer, New York) the susceptibilities are written in superscripts and in parenthesis, like X^(2) - you could use the pair to get them where they belong]

[Add a paragraph here]

In a strict sense X^(n) is a tensor, which is why symmetry will play an important role in the involved nonlinear processes. In rough terms, though, X^(n) will deliver a system of numbers which become very small the higher the order (or: the number in the parenthesis) is. For that reason, under normal light intensities one will not observe second- (or higher-) order nonlinearities, because the numbers in the second (and higher) order susceptibility(ies) are small (orders of magnitude) compared to the first order susceptibility. However, because the field goes in squared (cubed etc...), at some intensity the field will compensate for the small numbers, and nonlinearities will become detectable. Such intensities can be achieved using lasers. [now your original text: Each order of nonlinearity gives rise to different effects, and within each order there can be several slightly different effects.

_______________________________


Note: I would also not use the Greek character Chi but upper-case X, as some browsers have difficulties in displaying them correctly. You could mention the correct notation of the tensor using Greek characters in an explanation-footnote.

Thanks Hell. I'll get on it.

I must admit I've never come across the marbles in water explanation - we have always been taught it as like springs because the damped, driven harmonic oscillator equations transfer straight across (more or less) thus making the maths a bit simpler.

Yeah the maths is simpler, but the picture of an oscillator generating light often confuses the lay reader. I am not suggesting you to include that, it was just a comment.

HELL

Hi again. I've done some of what you suggested. A question. Do you want to be listed as one of the Researchers, given the amount of help you've given me?

Also, do you think that it's suitable for Peer Review?

Hi... Credits are always nice, but it's up to you.

I'm preparing the last bit to post here, then I think the Entry could be fine for PR.

See you tomorrow...

HELL

Hi,

Take a look at A791453

I've prepared some more stuff on NLO there.

Note: You must not take everything I said from there, as it is a bit too 'y' - IMO.

HELL

Hi HELL.

Umm, could you nose around the entry and its assorted links and see what you think. I have tried to take some of the techyness out, but we'll see. I've completed most of the entries, and the other effects that I haven't touched on could be added later (with the help of the subs/italics), like phase conjugation, for example.

Cheers,

Euan

I'll see to it ASAP.

Hell

I seem to have lost the thread I found last time I accessed this article and found one I didn't see before. So confusing!

I would like to suggest some changes to 'improve' the grammar, though I know this isn't fashionable these days!

e.g. were you to agree (and it is up to you), the first paragraph may be replaced by

"In a classical sense, optics is the science that deals with the paths of light. In a more modern sense, optics has to deal with the interactions between light and matter, because the paths are strongly dependent on this interaction. For a long time it was thought that interactions between light and matter were linear, that is, the amount of some interaction is directly proportional to the amount of light involved. If one were to observe otherwise, some nonlinear effect would be taking place. Nonlinear optics is a relatively new field. While the first nonlinear effects were seen in the 1960's with the invention of the laser, the effects themselves had been postulated decades beforehand, even in the 1930's. Only since the invention of the laser has it been possible to attain the high intensities required to experiment in this field. Nonlinear optics is now a well-developed science that is used for many technical applications."

and, near the end,

"The mass itself becomes the electrons in the atoms of the matter we put the light through, and these exhibit a restoring force and damping force when displaced."

may be replaced by

"The mass itself is replaced by the electrons in the matter, through which the light passes, these generating restoring and damping forces when displaced."

I think you'll find my versions are also shorter.

Dry.

I'll look into it, Dry.

Thanks,

Euan