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Mastering Light
thyristor writes "'Researchers at MIT document the ultimate control over light: a way to shift the frequency of light beams to any desired colour, with near 100 per cent efficiency. This technology could revolutionise a range of fields, from turning heat into light, or even into prized terahertz rays - which hold great promise for medical imaging. It could also make it possible to focus a wide range of frequencies into a narrow band, make devices such as light bulbs and solar cells more efficient, and help to keep optical telecommunications networks moving.' These are probably the most exciting results in photonics in the last decade."
Joannopolous was also involved in the development of the "perfect" dielectric mirror, which was mentioned here before.
The approach is destructive of the crystal used for filtering the light, although they hope to be able to use sound waves in the future. Due to the distorion of the crystal lattice structure required, even sound waves may wind up breaking the crystal (remember the old memorex commercials with the singer breaking a crystal wine glass). The approach is very interesting, but there still are some serious design issues that they need to address, otherwise, it will be tough to deploy this for applications such as optical repeaters or switches.
Not much more information than in the article, but here's the abstract. This is pretty similar to Bragg scattering, which is a well known effect that uses sound waves to upshift the frequency of light. Current Bragg cells are very inefficient and are limited to small shifts in frequency. A high efficiency Bragg cell capable of shifting frequency by a large amount would be extremely interesting.
From Physical Review Letters.
Color of shock waves in photonic crystals
Evan J. Reed, Marin Soljacic, and John D. Joannopoulos
Unexpected and stunning new physical phenomena result when light interacts with a shock wave or shock-like dielectric modulation propagating through a photonic crystal. These new phenomena include the capture of light at the shock wave front and re-emission at a tunable pulse rate and carrier frequency across the bandgap, and bandwidth narrowing as opposed to the ubiquitous bandwidth broadening. To our knowledge, these effects do not occur in any other physical system and are all realizable under experimentally accessible conditions. Furthermore, their generality make them amenable to observation in a variety of time-dependent photonic crystal systems, which has significant technological implications.
Physics, Cosmology and
Uh, what exactly is a 'heat wave'?
Heat comes in two flavors - radiated light waves and random molecular motion. The second kind is irrelevant to this discussion. As far as the first kind goes, you can't magically make that radiated light have more energy by converting it up to a higher frequency.
The laws of conversation of energy and thermodynamics would like to have a little word with you out back...
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The efficiency of interest in these types of processes is not the total energy efficiency. For example, if I lose heat because I have to stabalize the temperature of the crystal, I am not worried about that. What is of ultimate interest is the optical conversion effiency - the power in at wavelength one, versus the power out at desired wavelength two.
Optical conversion efficiency is what is important, for example, in wavelength conversion for data transmission. You don't want to lose signal power.
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It's not violating the second law of thermodynamics because to do this sort of upshift requires a stress to be applied to a crystal, thus inputting energy into the system. It's just that this energy is converted into a higher frequency light ray.
Right now you can buy AOTF cristals. It is a bit similar, but works as a filter (Acousto Optic Tunable Filter). What it does is bend off one specific wavelength of light based on which ultrasound you beam through it. By sandwiching a AOTF crystal between a piezzo and an absorber, you get a filter which you can control with a waveform generator. Brimrose will sell you a spectrometer that can scan 16,000 wavelengths per second for a ridiculously cheap 100,000$. Downturn is it throws all other wavelengths out meaning you still need a 35 Watt halogen lightsource to measure anything. If you could "recuperate" or shift the other wavelengths then you could use LED's as a light source and have a completely solid-state spectrometer with > 30000 H MTBF. You would use less power, produce less heat, make it smaller, send it to Mars,....
10 ?"Hello World" life was simple then
I'm not an expert on light frequencies, but those are photography filters, and when you look through the IR filter for example, you see everything in dark red, except that the surfaces which reflect IR are brighter. The eye cannot see pure IR, but it can perceive the near-infrared. Looks cool in any case. Same for the UVX filter, except you see more colors, ie flowers almost glow in contrast with the rest of the scenery.
This certainly sounds like an excellent advance in the field.I have been aware of interesting work with shock waves in other materials, for example, to create hydrogen metal, but it wouldn't surprise me if these claimed results were valid.
There are a couple of problems with the article and its claims, however:
I hope for the best, but remain sceptical; let's hope these new shockwave effects become easier to generate and exploit!
I also commented this story here, but I also previously posted another column on this subject. Please read it if you're interested by the photonic revolution.
The energy for conversion comes from the shock wave, the light is merely bouncing between reflective surfaces as it does in a laser. In a laser, usually all the lasing light is in a single frequency. A laser normally works by using a weak mirror, and the color is whatever is inside the laser (some laser mirrors simply use a hole for the beam, which is an interesting way of having a "weak" mirror). This device instead uses a color-sensitive mirror to let the light out when it reaches the correct color.
I do have respect for the design and engineering of an experiment which will involve bullets as a mechanism. Sometimes brute force is the simplest way to test something, such as when the question is "Is it unbreakable?" versus "How strong is it?".
IAAP (I am a Physicist) and the effect is pretty simple. I think anyone should be able to understand it if it is explained properly.
"Doppler Shift" is a phenomena you are already familiar with. Consider a car honking its horn as he drives by at freeway speeds. As he approaches, the sound is heard at a higher frequency. As he passes by, the frequency shifts, and as he is leaving, the frequency is lower than normal.
Light is like sound in that it is a wave and has a frequency. Let's examine light from high to low frequencies. X-Rays are light at extremely high frequencies. Ultra-Violet light is just above the visible light range. Then we get into the rainbow - blue, then green, then red. Next is infra-red light -- light just below red in frequency. Travelling farther down, we start to reach the radio band. Below that, the frequencies are so low that it no longer is light anymore, but more like a slowly shifting magnetic or electric field.
The Doppler effect works for light as well. The problem is you or the object emanating the light has to be travelling near light speeds to see any noticeable effect. We call this "redshift" in astronomy, because stars seem to be travelling away from us, and so the light emanating from them is lower in frequency (more red). Certainly, attaining near-light-speeds is dangerous and difficult. We're not talking "bullet" fast, we are talking "cosmic ray" fast.
However, there is an oh-so-tiny Doppler shift when *any* motion is involved with light. When your friend walks towards you, the light bouncing off of him is slightly more blue. When he walks away, it is slightly more red. Good luck actually detecting this, however.
Photonic crystals have the strange property of behaving like a piece of glass at one moment, and a mirror the next, depending on how much pressure is applied where.
So, using a proper push on the crystal, it is possible to set up a travelling hall of mirrors. The light appears to be slightly shifted due to the Doppler effect to the mirror, so when it is reflected, the light is shifted, by an oh-so-tiny amount. Multiply that shift by a kazillion reflections, which is quite possible if you make the hall of mirrors very tiny (think atomic scale), and you can control light to almost any frequency, high or low, depending on how you set up the mirrors.
So, the net effect is light goes in at one frequency, and comes out the other end at another, without expending hardly any energy to get it done.
The engineering challenge is configuring the crystal so that it can withstand the forces that need to be applied, and applying the forces in a controllable way. Right now they are doing tests with bullets and crystals, because they only need to record data for the instant that the shock waves are travelling through the crystal, and they don't mind using a cheap, destructive method. In the future, they will probably use sound waves to control the crystal. But how they configure this is left to the imagination.
The applications are numerous, and some of them are listed in the article. Needless to say, if we want to use light to transmit data, the more control we have over the light, the more effective we can be in transmitting that data. Also, doctors will be happy because we can now easily exploit the Terahertz range for X-ray type applications.
The radical sect of Islam would either see you dead or "reverted" to Islam.
And here it is:
This is certainly an interesting result, but its heavily hyped as well.
First of all, there are many many ways to shift the frequency of light, both up and down in frequency, with both linear and nonlinear means, - from the Raman effect in optical fibers (scattering off vibrations of silica molecules) to Optical Parametric Oscillators (nonlinear wave mixing), supercontinuum generation (using a multitude of nonlinear effects to generate broad bandwidth from a single laser) to simple OEO conversion (detect your light with a photodiode and use it to drive another laser at a different wavelength. Contrary to what this article implies, these effects work at modest power levels in todays optical fibers, and many are highly efficient, and work over extremely broad bandwidths. For example, supercontinuum generation can generate light sources with bandwidth covering the entire visible, UV and IR spectrum in one source! If you want to talk about bulk optic techniques for wavelength conversion, the list is even longer.
Now think a minute about what these guys are proposing. They have to shock the crystal. Initial experiments will destroy the sample. Maybe they can refine the technique down the line to nondestructively shock the sample, maybe they can't. Certainly, infinite bandwidth won't be available, since the amount of wavelength shift will depend on the amount of shock. A single shot technique for wavelength shifting, while interesting, isn't all that useful practically.
Second, they are using a shock, so conversion of CW light is out of the question, only pulses can be converted here, or you risk a time dependent wavelength shift, as your shock dies out.
Finally, claims of a completely new physical effect seem somewhat overblown. It is an interesting idea, but Doppler shifting off acoustic shocks, and photonic crystals are well known. Marrying the two together and finding a stable regime of operation is novel, but not quite the same as discovering a new physical princple like relativity or quantum mechanics, for example.
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The unaltered 'band-gap' crystal structure traps some quantum energy states and lets others pass through. When a shockwave meets the crystal, the traversing wave is momentarily 'held-up' if the shockwave is travelling in the opposite direction or 'hastened' if the shockwave is travelling in the same direction, thereby compressing or stretching the frequency. Since 'band-gap' crystals apply to all waves and not just photonic, this same method can be applied to sound and heat waves as well. Check this excellent Wired article for more. This whole field is really elegant.
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No it can't. The scattering effects due to the Earth's atmosphere completely diminish any light that would have reached the moon from such a weak source.
Sorry, the title of my comment was a bit harsh...
The article seems to imply that this effect will initially be temporary ("Initially they will generate shock waves by shooting bullets at photonic crystals. This would destroy the crystal, but not before the light has had time to shift.") The article implies that in the future the technique should probably be able to produce a continuous beam ("Eventually, sound waves should do the job just as well.")
converters IR->visible are widely known: night vision goggles.
converters UV->visible do also exist and are commercially available, they are not as common because they do not have so many applications (one of them is to detect corona discharge in high voltage applications, power lines). They use a stack of a photocathode (UV light->electrons), Micro Channel Plates (amplification) and a Phosphor Screen (electrons->visible light).
Actually, past a frequency threshold, there is no "color". Instruments that enable us to view past this threshold have to be programmed to display abritrarily selected colors for the various infrared frequencies. One example is Kodak's EIR slide film (Ektachrome InfraRed). On the package it says "false color infrared slide film".
Actually, this isn't true. IR Filters FILTER Infrared light, preventing it from impacting the underlying CCD. All CCD and CMOS cameras have them. When removed, you can pick up an enormous amount of noise, including clicks from your remote control, which easily overflow the hit count on the chip and spam the resulting photo.
Infrared-pass filters transmit IR light and absorb all visible light. Silicon CCDs (all commercial digital cameras use silicon) have a peak response in the near IR, so you can get a very bright image through the IR filter. The problem is that color digital CCDs have color filters on them which block IR light.
The radiation selectivity property was discovered by observing the phenomenon of Cherenkov radiation inside the photonic crystal.
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For further more detailed technical information, a PDF of the paper is here [http://physics.ucsd.edu/~drs/publications/2003/l
Photonic crystals fall under a broader family of materials called "metamaterials".
Future research note: Software-programmable metamaterials will create wonderfully exotic applications.
Cheers
Andrew
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Woah there nellie. Your talking about shifting electromagnetic radiation up to energies where its wavelength is of the order of the Planck length (10^-35 meters, where the EM and gravitational fields might be unified). Puhleez. No way with this technology which is basically made of stuff whose characteristic distances if of order 10^-10 m. Don't get me wrong this stuff is really nifty but this is just way overselling it.
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The energy indeed comes from the shock wave. When light is shifted from a lower frequency to a higher frequency, a photon with the lower frequency f1 and an acoustic phonon with the frequency f2 are annihiliated and a new photon with the high frequency f1 + f2 is created. Energy is conserved. This is called stimulated Brillouin scattering. I suspect that the mechanism suggested in the article is a multi-phonon process because usually acoustic waves do not have such high optical frequencies. So multiple phonons are annihiliated in multiple bounces to generate a significant frequency shift.