Physicists Build Diode for Electromagnetic Waves
A device that allows polarized microwaves to travel in one direction but not the other has exciting potential.
The electronic diode is a device that allows current to travel in one direction but not the other. That makes them handy things to have around. So handy, in fact, that you’d be hard pressed to find an electronic device that doesn’t contain one. It’s no exaggeration to say they have become one of the fundamental building blocks of the modern world.
Physicists have known for some time that it is possible to make diode-like devices for electromagnetic waves. The mathematics of electromagnetic wave propagation suggests that certain types of material should allow polarized waves to pass in one direction but not the other when bathed in a magnetic field. Engineers can readily build such a device but its effect is what physicists call linear, meaning that the amount of light you get out is proportional to the mount you put in.
That’s not really how an electronic diode acts. Its behaviour is nonlinear meaning that a small change in the input can have a mssive change in the output. For example, a small change in the electronic current can make the output current fall to zero. This highly nonlinear behaviour is what makes electronic diodes so useful.
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So it’s no wonder that physicists have searched for ways to do something similar with electromagnetic waves. They know for example, that lithium iodate crystals behave like this, except that the effect is tiny. Before the invention of the laser, physicists thought the light intensity necessary to see the effect could only exist inside stars.
Today, Ilya Shadrivov at the Australian National University in Canberra and buddies say it is possible to do much better than this thanks to the nonlinear magic of matematerials, stuff that has been engineered to manipulate the behaviour of light travelling through it.
Metamaterials are built using repeating arrays of electronic components like resistors and capacitors of various shapes that together interact with electromagnetic waves. These components are like molecules, or metamolecules–they are the stuff from which metamaterials are made.
Shadrivov and co say it is possible to create diode-like behaviour using metamolecule made of two wires separated by a dielectric sheet and rotated relative to each other.
A microwave passing through wires generates currents in each that tend to interact. At certain frequencies these currents re-reinforce or cancel out. Adding a nonlinear diode to one of the wires makes the effect of the metamolecule nonlinear too.
The result is a device that acts as a diode for right-handed polarized light of a specific frequency, but is entirely transparent to left-handed polarized light. The direction of permissible transmission depends on the frequency of the microwaves. And the chirality of the device can be reversed by changing the relative angle of the wires.
Plot this behavior and you can immediately see the complexity of the response curve. The transmission curves say Shadrivov and co have “a remarkable similarity with the I-V response of an electronic diode.”
This is a strong effect, unlike that seen in lithium iodate. The transmission in one direction can differ from that in the other by 18dB, that’s a factor of 65. And in total, the new effect exceeds that found in lithium iodate by 12 orders of magnitude, say Shadrivov and pals. Not bad!
But this device may be more significant still. The transmission curve shows a hysteresis effect. That means the actual intensity of the transmitted light takes different values for the same input, depending on the history of the device.
If that sounds familiar it’s because a very similar type of behaviour occurs in memristors.
Memristors are one of the fundamental building blocks of electronic circuits, along with resistors capacitors and inductors. Their existence was thought to be entirely theoretical until just a few years ago when engineers at HP Labs in Palo Alto stumbled across them.
Their great promise is that they can be used to process and store information since they can have more than one output given a certain input.
One hope is that memristors will make electronic logic circuits simpler and therefore cheaper. A more ambitious line of thought is that memristors will enable entirely new types of information processing circuitry that more closely mimics the way the brain works. We’re still waiting to see. But there’s no shortage of people trying.
The idea that similar type of device could operate on electromagnetic waves, if that turns out to be the case, could trigger a similar kind of gold rush. Watch this space!
Ref: arxiv.org/abs/1010.5830 : Electromagnetic Wave Analogue Of Electronic Diode
