Slashdot Mirror
Optical Control of Light on a Silicon Chip
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."
from the article itself.
East Coast Brewers
One: Its faster than a normal circuits.
Two: It consume less power.
Timang tinggi tinggi
parang sudah asah
alang alang mandi
biar sampai basah
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.
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.
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...)
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.
Finally! A year of moderation! Ready for 2019?
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!
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.
I am TheRaven on Soylent News
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.
Try Corewar @ www.koth.org - rec.games.corewar
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.
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.
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.
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.