Optical Control of Light on a Silicon Chip

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Optical Control of Light on a Silicon Chip

Posted by samzenpus on Wednesday October 27, 2004 @09:45PM from the light-turns-it-on dept.

An anonymous reader writes "Researchers at Cornell University have demonstrated a device that allows one low-powered beam of light to switch another on and off, on silicon, a key component for future "photonic" microcircuits in which light replaces electrons for propagating signals. It is highly desirable to use silicon--the dominant material in the microelectronic industry--as the platform for these photonic chips. The approach developed confines the beam to be switched in a circular resonator, greatly reducing the footprint required on the chip and allowing a very small change in refractive index to shift the material from transparent to opaque."

38 of 129 comments (clear)

Shedding some light... by rooijan · 2004-10-27 21:50 · Score: 5, Funny

This certainly sheds some light on the future of technology - hell, you could even say they are going to light the path of progress!

...sorry, couldn't help myself.

--
Daar is nie 'n lepel nie
1. Re:Shedding some light... by Darth23 · 2004-10-28 02:15 · Score: 2, Funny
  
  That wasn't exactly a glowing endorsement.
  
  --
  -------- In Soviet Russia, "Soviet Russia" sigs hate Slashdot.
Optronic gates by lisaparratt · 2004-10-27 21:51 · Score: 5, Interesting

I thought diffraction and interference was to be the answer to switching light. Does anybody know what happened to this technology?
1. Re:Optronic gates by willijar · 2004-10-27 23:59 · Score: 3, Insightful
  
  Diffraction and interferences are linear processes - you need a nonlinear process (such as the change in index used in the devices) to have one signal modify another.
2. Re:Optronic gates by Optics+Geek · 2004-10-28 05:39 · Score: 4, Informative
  
  Interference is still key to this. The nonlinear optic effect here is the refractive index change of the resonator material due to the beam controlling the switch. What's different here is the circular resonator, that basically make the path in the material with the index change extremely long, so a very small index change can induce the necessary phase change for the beam to switch. The resonator sits in one path of a (waveguide) Mach-Zehnder interferometer. When the phase shift induced by the resonator path is 0, you have the "on" state. When the phase shift induced by the resonator path is pi, you have the "off" state.
Can somebody explain ... by YeeHaW_Jelte · 2004-10-27 21:54 · Score: 2, Interesting

What is the exact use for this? Is it's advantage that there's no need to switch back & forth between electric signals & optic signals in e.g. a optical router, or is a computer based on solely optical signals faster than one based on electrical signals?

--

---
"The chances of a demonic possession spreading are remote -- relax."
1. Re:Can somebody explain ... by DigitumDei · 2004-10-27 22:06 · Score: 5, Informative
  
  correct.
  
  from the article itself.
  
  What are the applications of this device?
  
  These structures will find their first application in routing devices for fiber-optic communications. At present, information that travels at the speed of light through optical fibers must be converted at the end into electrical signals that are processed on conventional electronic chips. These electrical signals can in turn be converted back into optical signals for re-transmission, which in the end makes this an extremely slow process. The all-optical switch enables routing signals without the need of conversion to electronics.
  
  --
  East Coast Brewers
2. Re:Can somebody explain ... by frankvl · 2004-10-27 22:30 · Score: 5, Interesting
  
  Light travels about 10x faster than electrons in their optimal medium, so the potential processing speed limit is increased.
  
  Also, light processing does not necessarily generate heat, so there is no cooling needed to preserve the hardware, unlike the electro solution.
3. Re:Can somebody explain ... by Anonymous Coward · 2004-10-27 22:41 · Score: 5, Informative
  
  This isn't for optical network switches, this is for processor cores.
  
  IAAEE, so here goes a simple explanation of why optical is more desirable for a processor.
  
  1: Faster signal propagation. In the GHz region propagation delay can cause major timing headaches in synchronous computers (one reason your system bus is always slower than your CPU: the physical length of the clock lines on the motherboard introduce too much delay to properly synchronize at really high speeds).
  
  2: Higher slew rates. Another limit on clock speed is the rate that the logic gates can change state, which is proportional to the power consumption (it takes more power to change the state of a logic gate more quickly). Theoretically, an optical switch uses the same amount of power regardless of speed because youre switching an optical state rather than energizing (or de-energizing) a circuit.
  
  3: Lower power consumption. Because you aren't using ever-higher currents to force electrical states at higher speeds, your driver circuitry doesn't need to be as robust. This also leads to:
  
  4: Lower cost. Less circuitry to push around large signals means you can save die area on the chip.
4. Re:Can somebody explain ... by Gherald · 2004-10-27 22:55 · Score: 2, Insightful
  
  > Light travels about 10x faster than electrons in their optimal medium, so the potential processing speed limit is increased.
  
  What?? I thought electrons traveled at the speed of light! AFAIK the advantage of optical over electrical is that the paths of photons can cross w/o interfering with each other, thus potentially allowing for smaller processors.
  
  --
  The unofficial /. digest
5. Re:Can somebody explain ... by amorsen · 2004-10-27 23:03 · Score: 4, Informative
  
  I thought electrons traveled at the speed of light!
  Think again. Electrons have rest mass, therefore they do not travel at the speed of light. In fact they travel really slowly in a wire, perhaps a meter per hour on a good day.
  
  --
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6. Re:Can somebody explain ... by MoP030 · 2004-10-27 23:15 · Score: 2, Insightful
  
  Ah, there was a discussion about this in a recent thread... can't remember which though. The electrons cannot travel with the speed of light because they have mass. But they don't need travel much anyway, because the information is transmitted via the electrostatic force which can be explained as the exchange of virtual (light-speed fast) photons. So the first electron in the wire gets pushed a bit and in turn pushes the second electron in the wire and so forth, much like when you push one end of a stick and automagically the other end moves too. In both cases the effect is not light-speedy but speedy nonetheless.
  
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7. Re:Can somebody explain ... by TheRaven64 · 2004-10-27 23:27 · Score: 4, Informative
  
  Light travels about 10x faster than electrons in their optimal medium, so the potential processing speed limit is increased.
  Umm, the speed of electron travel is irrelevant. I assume you've seen a Newton's cradle (a set of 4 or more balls on string arranged in a row. You swing the end one or two and when it hits the stationary ones the corresponding ones at the far end swing). The balls in this are only moving at a few meters per second, while the signal (when the balls collide) is moving at the speed of sound. In a chip, the individual electrons move relatively slowly, but the signal moves at the speed of light.
  The problem with using electrons is that two electrons can collide. This means that your circuit paths can not cross. With something the complexity of an IC, this means that a lot of space is wasted just routing electrical paths around each other. The analogy I was given when I saw something like this demonstrated a few years back was that designing an electronic chip was like trying to lay out the road system in Great Britain without any roads crossing. Photons, on the other hand, can pass right through each other without interfering (quantum mechanics is magic like that). This means that signal distance between any two components on a chip is the same as the straight line distance (on an electronic chip it can be significantly further). This is good news, because we are starting to get close to the light speed limit with current ICs. A 3GHz chip must pass data from one pipeline stage to the next 3,000,000,000 times every second. Light can travel (roughly) 10 cm in this time. Scale this up by a few orders of magnitude and you start to get some real problems with component density.
  
  --
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8. Re:Can somebody explain ... by Antique+Geekmeister · 2004-10-28 00:31 · Score: 5, Interesting
  
  "Think again" is right. The electrons are involved in propagating a wave of electromagnetic energy, in ways that are fun to examine. But what you are describing is the average rate of travel of an electron, much like the average rate of travel of a lake: only a little bit of water goes in and out of it, on the average, so the average speed is very slow.
  
  The *wave* in the lake, however, is much faster, carried by particles that bounce around each other much faster. Typical propagation speeds of electrical signals in network cable is a significant fraction of the speed of light, roughly 75% of the speed of light for 75-Ohm coax cable as one example.
  
  Optical propagation in fiber-optic cable, which is what this new technology will be used for, is also limited to less than the speed of light. There, you get interesting effects because it's being transmitted through glass (or plastic for short cables), but still a significant fraction of the speed of light in vacuum.
9. Re:Can somebody explain ... by Anonymous Coward · 2004-10-28 00:43 · Score: 2, Informative
  
  I like your explanation why information is transported with approximately speed of light in conductors, but the reason why electrons travel much slower is different:
  
  The reason why electrons travel at a finite (rather slow) speed is scattering with the crystal lattice. If you apply a voltage, i.e. create an electric field along a metallic wire you would in principle continously accelerate the electron along the wire to an kinetic energy that corresponds to the applied voltage (e.g. 1eV for 1V of applied voltage and 1eV corresponds to 600000 m/s for an electron!).
  
  However, this acceleration is stopped every few ten femtoseconds, when the electron collides with a nucleus of the crystal lattice. So basically, instead of constant acceleration (like in vacuum) you have a stop and go motion, which results in a net drift velocity on the order of millimeters per second. The collisions with the nuclei are also the reason why conductors heat up if you run a current through them, because part of the kinetic energy of the electron is transferred to the nuclei (remember that for matter, temperature corresponds to vibration of the nuclei around their nominal positions).
  
  Now the difference between different conducting materials is just the average time between two collisions and the density of electrons (how many of them can move), this is what determines the resistivity.
10. Re:Can somebody explain ... by barawn · 2004-10-28 02:12 · Score: 4, Informative
  
  The *wave* in the lake, however, is much faster, carried by particles that bounce around each other much faster.
  
  Actually, the wave in the lake is carried by something akin to phonons (heck, they might be phonons - I hate fluid mech). That is, the wave is "transmitted" by quanta of the intermolecular forces, not by any particles in the medium itself.
  
  Strangely enough... as you suggested, the exact same thing happens in electrical signals, except there, the wave is "transmitted" by the inter-electron forces, which we call "electromagnetic" forces. Quanta of the electromagnetic field are, of course, photons, and the reason that electrical signals travel at 75% the speed of light is because that is the speed of light in that material, roughly.
  
  So, in a very real way, signals on chips have always been carried by photons. It takes power to shove electrons around, though, whereas photons will just propagate. So transmitting a signal purely by photons (rather than by photons through electrons) is lower power.
11. Re:Can somebody explain ... by Plammox · 2004-10-28 02:19 · Score: 2, Insightful
  
  Of course it's for "optical" switching!
  Well, for the optical modules in a switch, anyway.
  
  There's a much more obvious application for this than optical CPUs.
  It's every optical networking component maker's wet dream to be able to modulate light on silicon, as this would bring down costs of optical modules for 10 Gb/s, 2.5Gb/s, etc. In principle, you could live without the expensive optical components (pin-diodes, EAMs) and do it all this on one single piece of silicon.
  
  Now we just need to find a clever way of emitting light on silicon as well as finding a cost effective way of packaging ICs with optical fibres coming out of them ;-).
  
  PS: Didn't Intel demonstrate optical modulation on Silicon already??
12. Re:Can somebody explain ... by cheese_wallet · 2004-10-28 02:29 · Score: 2, Insightful
  
  "What is the exact use for this?"
  
  How many pins are on the latest AMD64? 939? 940? something like that. Optical interconnect could reduce that to single digits.
  
  I'm not sure what loading concerns there are with optics... one problem I run into in my designs is needing to connect to many other devices, and that slows things down.
13. Re:Can somebody explain ... by CRepetski · 2004-10-28 02:43 · Score: 2, Interesting
  
  Additionally, an optical circuit has the advantage that two beams of light can cross each other without interfering harmfully with each other. Obviously you couldn't do this with in an electric circuit. This allows optical circuit designers to make more compact designs, and it's a lot easier to do. With circuits on microchips today being so complicated, you need some pretty hefty programs to actually to the designing. The same optical circuit could be much smaller and eaiser to design.
Great commercial slogan potential! by GozzoMan · 2004-10-27 22:10 · Score: 3, Funny

"FASTER THAN LIGHT COMPUTING!" ... uh, "fast-AS-light" in fact. damn, never mind.
Why silicon? by ChrisMDP · 2004-10-27 22:12 · Score: 2, Interesting

It is highly desirable to use silicon...
What the poster and the article both neglect to mention for us simpler types is why silicon is desirable.
Is it simply because it requires less modification to the production pipeline, or is there another more scientific reason?
Perhaps a scientific slashdotter can enlighten us. Ahem.
1. Re:Why silicon? by amalcon · 2004-10-27 22:21 · Score: 2, Interesting
  
  IANA...well, I am not a person whose speculation on this matter should be taken seriously. Nonetheless, I would wager that the reason for this is: Silicon is very common on Earth. As I recall, it's the most common element which is solid at room temperature. This makes it inexpensive.
  
  --
  -Amalcon
2. Re:Why silicon? by flyingman · 2004-10-27 22:41 · Score: 5, Informative
  
  Because silicon is well established in the semiconductor industry and therefore cheap to obtain easy to process into semiconductor devices.
  
  On the other, almost all optical devices (LEDs, laser diodes) are made from III-V compund semiconductors like Galliumarsenide (GaAs), InAs, AlAs, GaN, GaP and so on. These are not available as large crystalline blocks and thus there are no such things as 300mm wafers. They are usually fabricated by expensive methodes. However, they are the only practical solution because the are so-called direct semiconductors - you just cannot do optics with indirect band-gap semiconductors like silicon.
  
  Now, if you find THE technological trick to do optics with silicon, you benefit from the cheap silicon technology and are ready to build optical computers with cheap fabrication technology. There are some tricks around already like mixing silicon with germanium (SiGe) or putting in nano-crystals so the silicon are catching up in doing optics.
3. Re:Why silicon? by hopey · 2004-10-27 23:25 · Score: 5, Informative
  
  My research area is silicon based optoelectronics and we are trying to fabricate efficient light emitting silicon based components. Basic components are made from MOS-structures with incorporated excess silicon to the silicon dioxide layer. After this the device is annealed at high temperature and the excess silicon forms so called nanocrystals inside the oxide. This allows the direct electron transition like in III-V group semiconductors.
  
  In basic structures the efficiency is however very poor. All kinds of tricks are needed in order to get the efficiency in range of direct bandgap semiconductors. We do not know yet if it is possible :)
  
  One of the reasons to use silicon for IC technology is its very good native oxide. You can produce dielectric with breakdown voltages of 10MV/cm with only annealing in oxygen. Think about it 100 nm of silicon dioxides breakdown voltage is over 100 V!
How good? by F'Nok · 2004-10-27 22:12 · Score: 3, Interesting

These structures will find their first application in routing devices for fiber-optic communications.
That's a fantastic use...

But I'm more interested in optical computing.

In theory extrememly low power chips should be possible, but what is the absorption rate like, especially in terms of heat, and quantity of reused light.

That is ofcourse, assuming that this CAN be used for more sophistication chip design.

Has there been any suggestion of other uses, and if so, what possibilities are there available for such technology?
Re:Light switching CPU mentioned before? by amalcon · 2004-10-27 22:18 · Score: 3, Interesting

(must need a huge heatsink).

Actually, one of the major benefits of optical computing is that you don't need a heatsink at all. This is because the heat put out by a CPU is due to inefficiency (in other words, because they are not room-temperature superconductors). There is little to no inefficiency in modern optical cable, especially compared to copper wiring.

--
-Amalcon
Many Hands... by zenmojodaddy · 2004-10-27 22:20 · Score: 3, Funny

... make light work.
1. Re:Many Hands... by corngrower · 2004-10-28 01:34 · Score: 2, Funny
  
  Quite right, it wasn't really Tom Edison that invented the light bulb, it was his assistant, a native american, Many Hands.
Re:Light switching CPU mentioned before? by Laser+Dan · 2004-10-27 22:29 · Score: 2, Insightful

There is very little loss in the FETs in a CPU either, until you start switching them really fast.

I'm pretty sure there will be switching losses in optical switches as well, especially while they are changing state. Optical CPUs probably won't need a heatsink until they become very advanced and operate way above the speeds achievable now, but its likely they will eventually. After all, the first few computers I had didn't need a heatsink either.

-Daniel
It took a team of 17 people, bravo all... by joelethan · 2004-10-27 22:37 · Score: 4, Funny

Because as we all know:
"Many hands make light work!"
The Cornell Nanophotonics Team
/JE
1. Re:It took a team of 17 people, bravo all... by Enigma_Man · 2004-10-28 02:09 · Score: 2, Informative
  
  You just copied the post above you, you bastard. Why do people keep doing this?
  
  -Jesse
  
  --
  Nothing says "unprofessional job" like wrinkles in your duct tape.
Functional casemods? by Spykk · 2004-10-27 22:52 · Score: 2, Funny

Finally, a use for all those colorful tubes of light.
Re:Light switching CPU mentioned before? by jannic · 2004-10-27 22:53 · Score: 5, Informative

This is not true, at least for this kind of optical switch. In the article, the authors state that it takes 0.15pJ to generate the free carriers. This sets a single switch to 'on', a single time, for about 500ps. If you assume that a switch is turned on, on average, 50% of the time, a single switch would consume 0.15mW. An optical CPU with one million switches would therefore need 150W, at 2 GHz. If you want a faster switch, you must reduce the carrier lifetime. Therefore you need more pump power to keep the switch turned on. So power consumption would increase linearly with clock speed.
And these numbers do not include any other losses, and assume that you can recover all the pump light which is not absorbed in the ring. If you don't recover that pump light, power consumption goes up by a factor of 166. (So you'd need 25kW for the 2GHz CPU with 10^6 switches...)
Christmas lights? by Danathar · 2004-10-27 23:26 · Score: 2, Funny

Ooooo.....This should make my Christmas tree which uses fiber optics MUCH more interesting!
Switching time?? by TooTechy · 2004-10-27 23:33 · Score: 2, Interesting

What's the darn switching time? Can't find it. The really important measurement and I can't find it.

Herriot-Watt were doing this on a physically bigger scale back in the 80s and managed something like a 10ms switch speed.
1. Re:Switching time?? by pkhuong · 2004-10-28 00:03 · Score: 3, Informative
  
  From TFA:
  
  To turn the switch "off," a second beam of light with a wavelength in the same spectral range is sent through the system. This wavelength is absorbed by the silicon through a process known as two-photon absorption creating many free electrons and "holes" (positively charged regions) in the material. This changes the refractive index of the silicon and consequently shifts the resonant frequency of the ring enough that it will no longer resonate with the 1555.5 nanometer signal. The process can theoretically take place in a few tens of picoseconds.
  
  Very interesting stuff... It's kind of like EIT, but much more sensitive.
  
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A Fair comparison by slackerny · 2004-10-28 01:20 · Score: 2, Interesting

I do not see any use for optics in processing even though photons theoritically travel faster than light. (Remember photons also do not travel at 3*10^8 in a waveguide eg silicon: velocity = c/refractive index and refractive index of silicon ~= 3.5)

although this would boost the oppurtunity for optics in processing... I do not believe it would be usefull in high speed processing simply because it would be drain lot of power (wall-plug efficiency is being worked on to improve right now!) but this could change..But one thing that cannot change is that the waveguides and devices (need to be atleast as big as the wavelength) are very big compared to the electronic devices...

here is a fair comparison of wavelengths.
-optical wavelength = 1.1 microns. electronic wavelength
-(electrons can be compared in energy to an x-ray photons and so wavelength of x-ray photon - this concept is used in electron microscopy) this is in nanometers 2 orders smaller.

so the electronic device sizes are 2 orders smaller and so lot more dense.

make love
make: *** No rule to make target `love'. Stop.
Re:My God by TeknoHog · 2004-10-28 01:50 · Score: 2, Funny

"My God, it's full of stars!"
"Nope, that's just a Beowulf cluster of optical Linux boxen. Nothing to worry about."

--
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