Quantum-Cascade Polychromatic Lasers 158
eznihm writes: "This article describes a new laser, developed by Lucent and others, that emits a band of light and operates at room temperature. "The result: a beam of high intensity at every wavelength from 6 to 8 micrometers, in the so-called midinfrared range.""
Uh oh, someone find Gordon (Score:2, Funny)
Egads, someone find Gordon, and find him quick! I don't want to be a slave to some bigheaded multidimensonal being! (Bill Gates is bad enough)
Re:Uh oh, someone find Gordon (Score:1)
Isn't this a contradiction? (Score:2, Interesting)
Can someone who know more tell me what's going on here?
Re:Isn't this a contradiction? (Score:2, Informative)
The truth is, lasers (even the standard HeNe) don't have to emit a strait beam or a single wavelength of light to be lasers.
Re:Isn't this a contradiction? (Score:3, Informative)
Of course, these are just properties, and might not actually be a working definition of a laser - maybe if you satisfy the acronym, you're a LASER (light amplification by stimulated emission of radiation, for those that don't know).
I could reach behind me to look at Svelto's "Laser Physics" book, but I'm too lazy.
Re:Isn't this a contradiction? (Score:1)
Even if the term LASER refers to the process of generation of the laser, i think there is one common property of devices that we like to call a "laser":
A laser is capable of emitting a lot of bosons that are in the same state (with some tolerance).
If you get yourself to accept that a single photon can be polychromatic, you will also be able to imagine coherent polychromatic light. (However I am not sure if it applies to the laser at hand.)
Re:Isn't this a contradiction? (Score:1)
Re:Isn't this a contradiction? (Score:1)
Every real photon is polychromatic since the norm of the free monochromatic photon \psi(x,t) = e^{i(kx-\omega t)} would be divergent. A completely monochromatic photon cannot be localized. On the other hand if you have a localized wave-packet, it is not sharp in the momentum space. Remember Heisenberg: \Delta x * \Delta p >= \hbar/2. And for photons, momentum p is reciproc colour. (p = \frac{\hbar}{\lambda})
Less mathematically speaking: don't imagine a photon having a colour. The colour is decided at the measurement. Imagine a photon with a function of wavelengths, some wavelengths will be measured more probably, some will be measured less probably. A conventional LASER would have one peak at a certain wavelength and a fall of on the sides. But with the LASER at hand it will look differently.
Re:Isn't this a contradiction? (Score:4, Interesting)
Each of the individual emmiting layer is monochromatic and coherent, but the resulting beam isn't, you're right.
What would be interesting is to see a diagram which would give the intensity of the light for each wavelength.
I suspect that it isn't totally flat, but more something like this:
/\/\/\/\
Re:Isn't this a contradiction? (Score:1)
Re:Isn't this a contradiction? (Score:2, Informative)
Essentially, some material (usually a crystal) is excited, which causes it to emit photons. Usually, because of properties unique to the material, it will only emit photons of a certain wavelength, but this is not a requirement. Lucent's LASER is simply built using a material that emits photons of many wavelenths.
Re:Isn't this a contradiction? (Score:2, Informative)
The key part of the laser acronym is "Stimulated". What happens is electrons are temporarily pumped to higher energy levels. What would normally happen is that the electron would spontaneously drop to a lower energy level resulting in an emitted photon (this happens in all kinds of material and light emitting devices). In a laser however, an existing photon passing the atom with the excited electron Stimulates the emission of the photon, and in doing so, the two photons will be in phase. This is how you get coherent light out of a laser. Of course, it takes special selection of materials, controlled electron pumping, and good optics for this process to build up to appreciable levels
Furthermore, this:
"some material (usually a crystal)"
is wrong: While the first lasers were cryptals (ruby), and some still are (Nd:Yag and others), I think you'll find that lasers these days are usually semiconductors (as measured by shear number - think CD/DVD players) or some variant of gas / chemical reaction (as measured by total power - think Chemical Oxygen Iodine Lasers (COIL) or HF/DF lasers)
Re:Isn't this a contradiction? (Score:1)
Re:Isn't this a contradiction? (Score:1)
Re:Isn't this a contradiction? (Score:1)
But in both cases, crystals are the source and the amplification medium.
Feel free to pick up a Solid State Physics book, you will see that semiconductors are crystals.
Re:Isn't this a contradiction? (Score:1)
Chapter 6: Types of Lasers
Section 6.2 Solid-State Lasers
Section 6.3 Gas Lasers
Section 6.4 Liquid Lasers (Dye Lasers)
Section 6.5 Chemical Lasers
Section 6.6 Semiconductor Lasers
Should I go on? Of course, most solid-state physics books, including the one I could pick up here, are useless in the context of Lasers Types.
Re:Isn't this a contradiction? (Score:1)
I never said that solid-state/crystal lasers are exactly the same as semiconductor lasers. I only said that crystal lasers and semiconductor lasers both use crystals.
You should also learn that having books on your shelf doesn't make you knowledgable. You try reading them.
Re:Isn't this a contradiction? (Score:1)
Re:Isn't this a contradiction? (Score:1)
I was never trying to appear smarter than you, but I'm not sure what kind of response you were expecting to your previous post. Well whatever, think what you will.
Re:Isn't this a contradiction? (Score:1)
DEVICE
HOW DEVICE WORKS
I'd say that is a definition, albeit an implied one.
YMMV
Re:Isn't this a contradiction? (Score:1)
Hung over and ashamed, Scotch
Re:Isn't this a contradiction? (Score:1)
Re:Isn't this a contradiction? (Score:1)
Good luck
Re:Isn't this a contradiction? (Score:1)
Re:Isn't this a contradiction? (Score:1)
Oh wait, let me.
The crystal is pumped, with light or electrically, and generates light. The generated light leaves the crystal, hits a mirror, and re-enters the crystal. That is when the stimulated emission begins to occur. This effect is reinforced be multiple reflections by the mirros which is how all the light becomes in phase.
In semiconductors lasers, the same exact thing happens except the crystal fills the entire cavity.
Re:Isn't this a contradiction? (Score:2)
Re:Isn't this a contradiction? (Score:1)
Re:Isn't this a contradiction? (Score:2)
Re:Isn't this a contradiction? (Score:1)
By the way, most lasers ARE crystals. As you know, a semiconductor is a crystal.
Re:Isn't this a contradiction? (Score:5, Informative)
Re:Isn't this a contradiction? (Score:1)
Re:Isn't this a contradiction? (Score:2)
There are lots of layers of slightly different sizes, and the size of the layer largely determines the color (wavelength) of the light emitted. In effect it's like there are lots of lasers on different frequencies located in nearly the same space. Since they are all firing at the same time the net effect is a broad distributed band of coherent light, but you can still make out individual peaks [bell-labs.com].
Laser Physics Digest (Score:2, Informative)
Lasing phenomena occurs in some media if you have so called "inverse population" of metastabile states of molecules. "Metastabile" means that those (excited) states cannot de-excite in to the lower energy level by spontaneous photon emission (e.g. momentum conservation forbids singlet-triplet transitions). However, if such metastabile molecule is hit by the photon with the energy that corresponds to the difference between upper and lower energy level of the molecule, a stimulated emission occurs. Emitted photon has the same wavelength, phase and direction as the incident one. In the conditions of inverse population (lots of metastbiles and sparsely populated lower levels), something similar to chain reaction happens. The initial photon gets multiplied in the geometric progression as it propagates trough the medium. This accounts for "Amplification" in the acronym LASER. In many cases upper and lower energy levels are well defined i.e. discrete, but they can be energy bands or even continuum. In the later case the wavelength of the "triggering" photon can lie in the range of values. This is actually the answer to the parent of this thread.
The lasing medium is usually confined in so called resonant cavity consisting of parallel mirrors. The reason for this is to effectively enhance the length of propagation in one preferential direction. The bunch of photons are bouncing back and forth between the mirrors many times and each time they traverse medium their number is increased. One of the mirrors is somewhat transparent and the portion of the beam exits the cavity.
Re:Hmm, no (Score:1)
The previous poster was right, the photons at the lasing wavlength *do* have the same phase.
Power is proportional to the square of the amplitude, not the energy.
The energy at a single wavelength increases because the photons add in-phase. The photons themselves do not increase in energy. If they did they would be a different wavelength. Energy is conserved because you only get as energy out (in the form of coherent light) as you out in (in the form of electrical/optical pumping). What the laser does is force the nearly all of the generated light to occupy the same state, creating the coherent intense beam of light.
Re:Isn't this a contradiction? (Score:2)
Light waves do not interfere with each other.
It's not like sound where you can do active noise cancelling..light waves 180 degrees out of phase will not cancel each other out.
The single phase output of the laser effect has interesting optical properties.. but I don't believe it actually causes it to be any 'stronger'.
10 lasers all out of phase with each other, all focused into the same beam would not weaken anything.
Don't mix up the optical propetries with the energy released... You can pack the same amount of non-coherent light into a beam and get the same results as far as burning or cutting... it's just that devices using the laser effect are much more efficient, and hence, practical.
Of course, I'm talking out my ass... but that all makes sense to me.
Re:Isn't this a contradiction? (Score:2)
Light waves do not interfere with each other.
It's not like sound where you can do active noise cancelling..light waves 180 degrees out of phase will not cancel each other out.
Erm... yes they very much do interfere.
Interference is a property of any wave, and light does exhibit it. Non-coherent light doesn't disappear because there are many small coherent streams of light energy - there is some cancellation (destructive interference), but there is also some constructive interference, and in the end the whole beam averages out - actually flickering millions of times a second.
This gives rise to the famous Young's Fringes and Newton's Rings. Young's Fringes form when two sources of coherent waves (light, sound, anything) interfere to produce light (loud) and dark (quiet) bands.
Re:Isn't this a contradiction? (Score:1)
That's how diffraction works.
But then, this gets into the realm of quantum physics, where the intuitive is usually wrong
In any case, you can be forgiven the misinformation, since you at least disclaimed it.
Re:Isn't this a contradiction? (Score:1)
Actually, the phase locking of many different modes of a laser (=mode locking) is used to generate extremely powerful (>Terawatt) and short laser pulses. These pulses are a direct product of constructive interference of many different frequencies (inbetween the pulses you have destructive interference). Using mode-locking it is possible to generate from a 10 Watt continous wave laser a train of Terawatt peak power laser pulses (of course, average power is the same, or even lower).
Re:Isn't this a contradiction? (Score:1)
Re:Isn't this a contradiction? (Score:1)
Re:Isn't this a contradiction? (Score:1)
Infinite Wavelength, or a combination of lasers? (Score:1, Insightful)
Also, it seems like it would take a lot more development to extend this wideband laser work for something more consumer-oriented, like room lighting, as the light would need to be relatively omni-directional, which it doesn't seem like lasers are very good at producing now.
Re:Infinite Wavelength, or a combination of lasers (Score:2, Informative)
As for consumer applications, don't hold your breath. Unless these are cheaper to produce than your supermarket price-checker, they'll stay in the realm of science for now. Multi-spectrum lasers are useful simply because they're light that all goes in one direction, which makes them useful for observing molecules.
Re:Infinite Wavelength, or a combination of lasers (Score:1)
Re:Infinite Wavelength, or a combination of lasers (Score:1)
Common, but untrue. (Score:2)
Pop the columnating lens out of a laser pointer to see just how omnidirectional it is (and please don't look right at it). It's just like any other light source in that respect.. the only difference is that the light it produces is of one wavelength, and in-phase. Both reasons why it's not an ideal room light. But direction has nothing to do with it.
Lasers are focused using a columnating lens.. just like any other light.
As the article said... this produces laser lightevenly distributed across the mid-infrared range.
Therefore, the question arises... (Score:2)
(This is a serious question. I'd love to hear the answer from a qualified physics type. I'm even willing to burn a karma point to show this at a 2 rating, so I stand a chance of getting a real answer.)
I'm just guessing, but it's probably because the monochromatic nature of most laser pointers means that ALL the light focuses the same thru the collimating lens. This is of course because different frequencies of light have different refraction angles thru a lens. Therefore, with a broad-spectrum source like a light bulb, you cannot sharply focus the beam, because each frequency focuses at a different place. As I write this, it occurs to me that's why a flashlight uses a parabolic reflector - because reflection is not affected by wavelenght like refraction is. But with a parabolic lens, the fully parallel beam can never be smaller than the diameter of the lens.
Re:Therefore, the question arises... (Score:2)
Re:Therefore, the question arises... (Score:1)
OTOH, if your source is too small or you have small apertures (on the order of the wavelength) your beam divergence will be limited by diffraction.
Another issue with flashlights is that the light that directly goes towards the exit and does not hit the reflector will not be collimated and thus contribute to the divergence. Of course, flashlights are optimized for brightness, so you _have_ to use a reflector in order to gather the light that is emitted in the "wrong" direction. A lens would only collimate light that already goes towards the exit of the lamp.
Re:Infinite Wavelength, or a combination of lasers (Score:1)
The point of this is not for a general consumer device any time soon. This device will be used in spectroscopy where there are very few line source lights that operate in the mid-infrared range.
Signal to Noise (Score:1)
Sounds like the 1 electron processor from Signal to Noise. I wonder if anyone has a simple explanation of how it exactly works? It seems that the electrons build up so much energy which pentrates diffrent strata of semiconductors and that creates the diffrent wavelenghes. Is that anywhere close to right?
Multiplex? (Score:1)
It's not clear if the different wavelengths have to appear together, or can be manipulated individually.
If they can be manipulated individually think of the coolness.
So many questions.
Re:Multiplex? (Score:3, Informative)
The trouble is the very power modulation can cause shifts in frequency (due to ohmic heating), leading to unwanted losses and cross talk.
Better to have an external modulator which can change its loss at various frequencies, and use this as the source.
Re:Multiplex? (Score:1)
applications in optical communications (Score:1)
Why am I smacking my forehead. I should be smacking yours. Basically, now they have a laser that can replace the 10 or so lasers that would normally feed an optical fiber. Now they can make things smaller and fit more stuff in those central offices at the phone company and stuff.
Before I mouth off, I should research the terms I'm looking for. Also I should get some sleep.
Sounds like Zippy (Score:3, Funny)
Quantum-Cascade Polychromatic Lasers!
Quantum-Cascade Polychromatic Lasers!
Quantum-Cascade Polychromatic Lasers!
(I took one look at that and thought Zippy. Oh boy. Excuse me while I fix a peanut butter and olive sandwich...)
other uses (Score:2)
Of course, they don't mention the weapons potential
or laser headlights for *really* bright high beams on the car.
Lasers Make Poor Weapons! (Score:5, Interesting)
Argh. It seems there is one of those unwritten Slashdot rules that dictates any story about lasers must be accompanied by posts about laser weapons... usually in the Death Ray variety. For the most part lasers make poor offensive weapons, at least in the Burning a Hole Through X variety.
(Thank you Alien54 for not suggesting such a thing)
I've posted this before [slashdot.org] but here it is again:
***************
It seems inevitable that whenever there is a story regarding lasers we get to see all sorts of silly posts about blasting people with laser.
Even antimissle lasers have a long way to go. Between power requirements, beam handling, divergence, and atmospheric interference, lasers do not make great destructive weapons.
However, they would be damned good for some nasty tricks like blinding the enemy army (or, unfortunately, civilians).
Take this scenario: a bomber/cargo style aircraft has been outfitted with a large infrared laser (similar things have been done). Fly said aircraft over the people you wish to 'zap'. Release some fireworks or other attention getting devices and when the crowd looks up turn on and start scanning the laser.
Since the laser is infrared nobody would know they are being exposed to blinding levels of light, nor would the blink/aversion reaction take place. By the time you noticed anything the permanent damage has been done. Scary huh?
Another scenario under serious consideration by police (at least here in Canada, I've participated in meetings on the subject) is the use of lasers against commercial aircraft. The idea isn't to shoot down the aircraft, but to scan at temporarily blind the pilot during final night approaches. The effect is like someone flashing a camera flash in your face when your in a dark room.
As the few moments prior to landing are the most critical, distracting and flash blinding the pilot could easily lead to the plane crashing.
Worse, new solid state lasers are available in the 3watt (plenty of power to cause permanent blindness) range and can be powered off a car with an inverter. Simply park at the end of a convenient runway at night, plug 'er in and away you go. Ok, so it's not quite that easy, but the concept is...
Doesn't that all just scare you a bit more than some silly death ray?
Note: after saying all that I want to point out that I do not support the insane regulations placed against the use of lasers in the United States by the CDRH. It's totally ridiculous and overzealous.
Re:Lasers Make Poor Weapons! (Score:2, Funny)
Pardon my ignorance of Canadian culture but why on earth would the police want to do that?
Re:Lasers Make Poor Weapons! (Score:1)
Re:Lasers Make Poor Weapons! (Score:2)
Back when the (iirc) MGM Grand in Las Vegas was new, they had a laser show they ran off their roof.
They shut it down when it scanned the eyes of a pilot during takeoff from McCarron.
Here's a link to google's cached page for it (the original is behind a login dialog):
Laser blinds flight crew. [google.com]
Now, if blinding pilots for 5-10 seconds while they're climbing under full power at 500 feet isn't considered "life-threatening risk" to everyone on board and lots of people in the city below, then I must be some sort of chicken...
--Blair
Re:Lasers Make Poor Weapons! (Score:2)
I sure as hell want a massive, Death Star, lasing-zapper-blaster raygun. (If only to blast the god-damned pigeons that nest in my gutters -- I'm just waiting for the GPS-phone-tracking version so I blast telemarketers).
So, I can guarantee that anytime there is a "laser" post, you'll see some "death ray" posts because most of us want a Death Star floating over the house so we can unleash death and destruction on anything that bothers us.
Finally!! (Score:3, Funny)
And when you weren't burning hair off your body, you could use it to super heat your evil Science Teacher's house so that popcorn exploded out of it everywhere...and you wouldn't even need to hijack the computers on a nearby B-1 bomber!!
Finally, my life can emulate Real Genius!! (thanks Cliff!)
bell lab's laser (Score:3, Informative)
Re:bell lab's = Lucent (Score:1, Informative)
Not quite right... (Score:5, Informative)
Technically speaking this isn't quite true - it depends on what your lasing medium consists of. While each colour line emitted will be monochromatic, a single laser is capable of producing multiple lines.
In the case of a Krypton or Helium Neon ion gas laser you will get a single line out (usually, but not necessarily, red for either of these).
However, if I look at an Argon [ufl.edu] laser with apropriate optics you get primarily Blue and Green (514nm "Green" and 488nm "Blue") lines (with combinations in between). If I put a prism to the output of my little American Laser 60x I can see 7 individual lines - 5 are of such lower power as to be virtually useless, but the primary Green and Blue are strong.
Then if you look at a Copper Vapor [laserfx.com] laser which works by evaporating copper you get two lines: an emerald green and *gold* (this type of laser was made famous during the Pink Floyd Division Bell tour).
Newer solid state are very much single line. If you ever see a very harsh green beam you are probably looking at a Nd:YAG laser. The new solid state stuff is really looking promising... much more reliable with a much longer lifespan. Now, if they could just get the Blue solid states more powerful reliable we would be laughing. A low to mid powered white-light lasershow that could fit in a briefcase! On the down side though, typically much lower power output than their ion cousins (and the YAG green is, in my opinion, really nasty).
Could almost make me miss lugging 909's around... :)
Re:Not quite right... (Score:2)
To get blue frequency tripling is often used, but each nonlinear element reduces the intensity greatly.
Re:Not quite right... (Score:1)
So what you're saying is... (Score:1)
Re:So what you're saying is... (Score:2, Insightful)
How to tune one even! (Score:1)
The beam gets repeated by two parallel mirrors, one is opaque, the other is part-way(leaky), which lets the light out, lasing maerial between.
Replace the opaque mirror with a prism, which spreads the spectrum out, pick a color and place a mirror perpendicular to the beam, and it will reenter the lasing material. The stimulation will reinforce the one color, and the others will degrade(filters would reduce the frequency you want due to imperfections), works with multiple frequncies.
What should be really fun would be a EL-display type laser device, which might be tunable with the voltage across.
(As far as rude colors, ever see a neon *plasma* pink?)
Light Amplification by Stimulated Emission of Radi (Score:3, Informative)
In answer to all those going, "huh? i thought the whole point of lasers was that they're coherent/all the same wavelength"
Any device which excites one or more substances electrons to jump up an energy level and then fall back generating a specific wavelength (per substance) photon. these are usually bounced back and forth in a chamber and released at one end.
This article is about a quantum cascade laser, which is a bit more complicated than my simplified (even for normal lasers) explaination.
The point is, that while coherent lasers are the norm and coherence has many uses, this is still a laser and the technology may have many different uses itself.
clarification (Score:2)
Wow, I thought everyone knew how lasers worked! (Score:4, Informative)
It's a limitation we could not overcome until now.
Lasers are coherent.
Lucent has created a multichromatic coherent laser.
Simply put, multiple quantum wells laze at different frequencies. Stacks of these multiple quantum wells create multiple lasers in one cavity, if I understand it correctly.
Each frequency is indeed coherent. You get multiple frequencies, however, in one resonant cavity. I'm guessing here, but the reason why you don't see each frequency shooting emitting from the cavity at different times is because it's either a continuous laser, or because the energy spread between the different colors is much smaller than the energy of activation to escape the cavity.
In either case, an analogy would be to place multiple crystals stacked together into one laser, and stimulating all of them. If you assume that there are no diffraction problems, and that they all emit at roughly the same period, you have a very crude multi-chromatic laser.
Re:Wow, I thought everyone knew how lasers worked! (Score:1)
The timing of the light being emitted by the material does not matter since the resonance cavity, through multiple reflections, and the materials, through gain stimulated emission, will cause the light of a single wavelength to be in phase.
Re:Wow, I thought everyone knew how lasers worked! (Score:2)
Am I thinking about a non-issue?
Re:Wow, I thought everyone knew how lasers worked! (Score:1)
What i did not get (Score:2, Interesting)
Do they really have a polychromatic laser or is it just a laser that allows the user to choose an arbitrary wavelength in a given range? The article explains how they get spontaneous emission with a wide range of wavelengths, however it does not explain how to build a cavity that keeps a polychromatic wavepacket (wide in the momentum space) in a stationary state.
I had the chance to play with a 200mW Argon laser last year. It can laze at many different wavelengths, but i can set the cavity only to a discrete set of wavelengths. This was crucial for deciding between the 351.1nm and the 351.4nm (iirc) line. *g*
Re:What i did not get (Score:1)
In the case of the Argon laser, you were using the cavity to force the source to emit light at a slightly different wavelength. You wouldn't be able to tune too much since the source has limited bandwidth, unlike the quantum cascade laser which uses multiple sources.
*clapping hand on the forehead* (Score:1)
If L is the length of the cavity and l is a wavelength, then the distance of the next wavelength that has a resonance in the cavity is l^2/L and this is VERY small under usual circumstances. I think i should read something about laser basics.
You are right, thank you!
Re:What i did not get (Score:2)
From the ScienceNews Site it doesn't seem that revolutionary to be honest.
Re:What i did not get (Score:2, Funny)
Re:Wow, I thought everyone knew how lasers worked! (Score:2)
Re:Wow, I thought everyone knew how lasers worked! (Score:1)
High bandwidth lasers (Score:5, Interesting)
I'd publish my course notes on the issue, but I'm not certain my prof would be too happy. I'll check with him when I see him Monday.
Re:High bandwidth lasers (Score:1)
Re:High bandwidth lasers (Score:2)
He may become unhappy if my crappy notes make him look bad
Re:High bandwidth lasers (Score:2)
Re:High bandwidth lasers (Score:3, Insightful)
(got an email response)
Re: Intro to LASERs (Score:2)
First off, I'm not a physicist. It's been years since "Physics for Engineers" and I didn't even do very well in that course... (Though I would fare better now, as my calc skills have improved vastly!)
That having been said... That PDF is quite a document! It seems very comprehensive -- is it something you put together, or was it given out by your professor? (If so... God! Wish I got PDF handouts in my day!) I did note the creator info, but "Ian Clark" doesn't mean anything to me. All that work using LaTeX, too!
Impressive. Thanks for the posting it!
Re: Intro to LASERs (Score:1)
Unadultrated Geek Sex (Score:1)
Works once per receipient. (Score:1, Funny)
[Star Trek phaser sound]
"Wow! that was the fastest transfer I've ever seen!!! Sorry about the big hole in your computer though.."
Military uses (Score:4, Informative)
Comments, anyone?
Room temperature infared beams. Useful, but... (Score:1, Offtopic)
Re:Room temperature infared beams. Useful, but... (Score:1, Funny)
Applications (Score:2, Interesting)
Yawn. (Score:2)
Re:Quantum Cascade technology is so passe (Score:2)
(not got my PhD yet)
Re:Cool! (Score:1)