Questions RE; Short Pulse Laser Technology

[thorin]

So I was reading this bit over @ wired:
http://www.wired.com/science/d...2007/06/raydiance_side

They talk about knocking electrons out of their orbits. My question is what happens to the "left overs"? Even if everything is broken apart the constituent pieces still have to be there (somewhere), plus maybe some form of energy?

Do the newly disassociated electrons and the nucleus (protons & neutrons) just fall through subatomic spaces in the material now that they're busted apart and smaller?

 

[Idontcare]

Thorin, the article was written by a rank-amatuer in regards to the attempt to employ science in describing the functions of the device. Do not attempt to rationalize the statements, they are not founded in physics or chemistry.

A photon couples to matter via electromagnetic interactions. Just because they ionize the atoms does not mean they have caused matter to convert to energy here, nor have they magically created sub-sub-atomic particles (which would contradict accepted quantum physics as well as the definition of sub-atomic particles) by use of photons.

Quite simply the device is creating structural defects, moving atoms around, such that the resulting material (changes in density) has a slightly different index of refraction thereby scattering light slightly such that the human eye observes "haze" inside the peice of glass.

This is not rocket science, any more these days at least, and the inventor of the laser (the story being it's cost, not the physics behind it's use) no doubt is horrified by the inaccuracies of the novice author's efforts.

 

[f95toli]

Well, the article is pretty accurate; I can't find any majpr errors.

The whole point of using femtosecond lasers is that they duration of the pulse is so small (femtoseconds, hence the name). This means that you can use pulses in the gigawatt range which can quite litterarly e.g. break chemical bonds by "knocking out" electrons (assuming you can tune the frequency of the laser); but the TOTAL energy is still very small since the duration of the pulse is so short.
This means that you do not really need to dissipate much heat.
Pulsed gas lasers have been used for this type of applications for a very long time and for the same reasons (large energy but short time). However, the pulse duration from a gas laser is pretty long meaning they still create a lot of heat at the target.

 

[f95toli]

I guess I didn't answer the question...

The answer is: It depends.
What usually happens is that you are left with a bunch of ions that, if they can, will simply move around until they find a a place where they can form a new bond. The electrons will just fly off and join the "cloud" of electrons you will find in most materials (with the exception of really good insulators).
However, if the laser penetrated well below the surface and their diffusion length is short (the diffusion length depends on the material and the temperature) the ions might get "stuck" in the wrong place (aDsorbed, as opposed to aBsorbed) which means that you end up with a charged surface or one with a rather weird stochiometry (if ions moving around on a surface get "desperate" enough they might form compounds which are normaly unstable).

Also, if you hit a surface with a powerfull laser you WILL create an "explosion" (the LOCAL temperature might be quite) which means that some of the material will simply be ejected from the surface as a plume WITHOUT changing the stochiometry. If you place a bare substrate in the right position you can use the matrial in this plume to deposit a thin film.
This is reason why laser ablation is so useful for depositing thin films of very complicated materials (e.g. many oxides) where ordinary evaportation (thermal or e-beam) does not work. This can be done with high precision, one atomic layer at a time.

 

[thorin]

So you're saying that aren't actually removing anything from the inside of the object they're just changing the form of the matter that is present?

Is an atom still an atom if it has no electrons?

 

[f95toli]

Yes and no.
The fact that they are able to selectively break chemical bonds means that they can change the structure of the material making it much weaker, so I guess you could say that it "falls apart" (changing some of the constituents into a gas).
Moreover the pulse will, as I pointed out above, also create a small "explosion" on the surface which will eject material. I suspect that the latter is much more important.

An atom which is missing one or more electrons is called an ion, on a "subatomic" level there is little difference between an ion and and atom; what goes on in the "cloud" of electrions has very little effect on the nucleus. Hence, iron is still iron even if you remove an electron.
Also, in metals the outermost electrons of each atom is free to move around in the whole crystal , hence the crystal lattice is actually made up of ions surrounded by an "electron gas". This means that the electrons are very mobile, which is of course why metals are good conductors.

 

[jagec]

Originally posted by: thorin
So you're saying that aren't actually removing anything from the inside of the object they're just changing the form of the matter that is present?

Is an atom still an atom if it has no electrons?

The atoms haven't lost all of their electrons--in fact, I'm guessing that most atoms in the affected area of the glass slide didn't lose any, and were simply blown apart by local thermal gradients. Those that were ionized probably only lost one or two electrons. I can guarantee that not a single particle was lost, just moved.

I don't understand what's supposed to be so revolutionary about this laser, though...I assume it's just a standard CPA femtosecond laser, as found in research labs for years now. Our old Q-switched mode-locked Nd:YAG is a dinosaur now, with an enormous 2 ns pulse width. Blech!

 

[Nathelion]

The new part in the particle seems to be that those lasers are cheaper and smaller.

However, I'm not sure that the article knows what it's talking about. I don't know. But I do know that an attosecond laser was recently built, they might actually be talking about that instead, but just got the terminology mixed up?

 

[Super Nade]

f95toli: "assuming you can tune the frequency of the laser"

Why? It is already broadband. If it has an ultrashort time signature, the fourier transform (to the frequency domain) make it a step function.

 

[wwswimming]

are you interested in short pulse lasers at ultraviolet frequencies ?

these are some of the specs for a medical laser -
"Bausch & Lomb FDA Filing for the Technolas 217 Ophthalmic Laser
Laser Wavelength 193 nm
Laser Pulse Duration 18 nanoseconds
Laser Head Repetition Rate 50 Hz
Effective Corneal Repetition Rate 12.5 Hz"

basically vaporizes protoplasm (the human body) at the rate of
about 225 cubic inches every 30 seconds.

good thing it runs pulsed and not CW !

that's just part of the spec.

 

[f95toli]

Originally posted by: Super Nade
f95toli: "assuming you can tune the frequency of the laser"

Why? It is already broadband. If it has an ultrashort time signature, the fourier transform (to the frequency domain) make it a step function.

I don't think that is correct. Remember that the frequency of a laser is the same thing as the energy of the photons it is emitting (once you have multipled it by h). Hence, in this case I don't think "simple" EM theory is applicable (you probably need to use quantum electrodynamics to get the right answer), the pulse is probably best seen as an ensemble of photons and unless their energy (frequency) matches that of the level spacing in the molecule you are trying to break up; you will simply heat the target.

 

[Super Nade]

Mate, Fourier analysis is the heart of quantum mechanics. There is more to a pulse than just a bunching of photons, yet it can be treated semi-classically. QED/Second quantization is not always needed to explain light matter interaction. A semiclassical description works most of the time (unless you are dealing with very weak light).

The current experiment we are working on involves generation of freq entangled photon pairs by PDC. The pump laser is a femtosecond laser. The second harmonic generation stage clearly shows the dependence of conversion efficiency on bandwidth of the pulsed laser.

 

[f95toli]

I am not an laser expert (I work with single photon generation using circuit-QED, so my photons are in the GHz range) and maybe I am missing somehing, but isn't the bandwidth of the laser just a describtion of the energy distribution of the photons in this case?
Of course a pulse will always have a some spectral width, but from my understanding the pulse shape is just the ENVELOPE of the field (in a semi-classical picture); the photons arriving during the duration of the pulse are still of a certain wavelength (or to be more specific, a distribution of wavelengths centered around the centre frequency of the laser).

I.e. what I have in mind is something similar to manipulating a 2-state system by pulsing it at its Rabi frequency, in many cases the pulses are very short and have very fast rise times, but since all you are doing is turning a "sinusoidal excitation" on and off (i.e using a rectangular envelope) the photons still have a relatively well-definined frequency and that frequency can be significantly lower than the frequencies that belong to the spectrum of the pulse envelope.

My point is that if you want to break chemical bond by exciting an electron you need photons of a certain energy range (that falls within the bandwidth of that transition) meaning a "photon picture" is very useful here. Normal laser ablation generally relies on just heating the target but that is not what they were refering to in the article.

 

[Super Nade]

I understand what you are saying. Perhaps I misunderstood the original context.

BTW, what is circuit QED? Is is some kind of super-high finesse based cavity QED ?

 

[jagec]

Originally posted by: f95toli
I am not an laser expert (I work with single photon generation using circuit-QED, so my photons are in the GHz range) and maybe I am missing somehing, but isn't the bandwidth of the laser just a describtion of the energy distribution of the photons in this case?
Of course a pulse will always have a some spectral width, but from my understanding the pulse shape is just the ENVELOPE of the field (in a semi-classical picture); the photons arriving during the duration of the pulse are still of a certain wavelength (or to be more specific, a distribution of wavelengths centered around the centre frequency of the laser).

I.e. what I have in mind is something similar to manipulating a 2-state system by pulsing it at its Rabi frequency, in many cases the pulses are very short and have very fast rise times, but since all you are doing is turning a "sinusoidal excitation" on and off (i.e using a rectangular envelope) the photons still have a relatively well-definined frequency and that frequency can be significantly lower than the frequencies that belong to the spectrum of the pulse envelope.

My point is that if you want to break chemical bond by exciting an electron you need photons of a certain energy range (that falls within the bandwidth of that transition) meaning a "photon picture" is very useful here. Normal laser ablation generally relies on just heating the target but that is not what they were refering to in the article.

Generally you start with a Q-switched laser that produces short bursts of high-power, broadband light, and then add the mode-locker which breaks each pulse into a pulse train of very fast, narrowband light packets of successively higher frequency. You can then use CPA to basically pump the power of the different frequency components individually without burning out your optics, and then put them back together for a series of very high-energy pulses on an extremely short timebase.

At least, if I understood the head researcher's explanations properly.

 

[Blefuscu]

There is nothing particularly revolutionary about this laser. The only unusual feature is the center wavelength (medical applications tend to use infrared). It is more of an engineering feat because it is half to 1/4 the size of similar systems. They do not give detailed specs on their website:

http://www.raydiance-inc.com/productspec.htm

800fs is not a particularly short pulse. Physicists have been using lasers in the attosecond regime for years now. The pulses are so short that they are only a single optical cycle long. However, this company seems to be more concerned with ablation, so pulse energy is important, too. Commercially available Ti:sapphire regenerative amplifiers for modelocked lasers can give you 150fs pulses at the same pulse energy as the one in the Wired article (i.e. peak power levels about 7.5 times higher). However they are huge (take up a whole optical table) and cost about half a million dollars each for a complete system.

http://www.coherent.com/Lasers...ction=show.page&ID=940

Continuum lasers also makes a version of this.

As far as the actual physics of what happens when these pulses are focused onto matter, it is just a matter of making the light come to a focus inside the glass so that the peak intensity at the focus spot gets higher than the damage threshold of glass (something like 10GW/cm^2). In answer to the OP's question about the "leftovers", nothing really goes anywhere. The structure of the glass just changes. Bonds are broken and new bonds are formed. Glass has many metallic impurities as well as OH- which probably make the process not so simple to analyze, but I bet it has been studied, perhaps by the guys at Livermore where they are trying to do nuclear fusion with lasers. Internal damage is visible. You can buy internal "sculptures" made this way

http://www.bathsheba.com/crystal/

A guy at work has one made from some photos of his kids. (his kids' faces in 3d inside the glass).


As far as this company's final market, it is the medical industry. The idea is to use it for surgery or something. The difference between this and previous medical lasers is that because the pulse length is short, the pulse energy required to reach high intensities is lower and the effective ablation spot size is smaller compared to longer pulse lasers.

 

[thorin]

Wow thanks for the all the replies you guys. Sorry it took me so long to get back to this thread I was out of country for a while.