Radio waves used to super-cool tiny structures

Posted by Rick C. Hodgin

Ann Arbor (MI) - Researchers working at the National Institute of Standards and Technology (NIST) have discovered a way to use radio frequencies to cool very small objects.  Similar previous techniques have involved laser light.  The use of RF will help researchers push tiny machines toward ever smaller levels.

Relatively speaking these tiny objects are huge compared to the cooling techniques being used.  Previous experiments have cooled groups of atoms and even the smallest of nano-scale devices, using laser light.  Temperatures close to absolute zero were attained on those small structures in this way.  But what researchers are now finding is that by using RF waves instead of laser light, much larger tiny objects can be cooled in the same way.  And some of these are on the order of quadrillions of atoms.

 

 

 

In the example illustrated above, a type of very small silicon cantilever (like a diving board) was created and attached to a "fixed deck."  The board measured only 200 x 14 x 1,500 micrometers.  When affixed on one end it naturally vibrated with a resonance frequency of 7 KHz.  The researchers were able to direct an RF signal at the board, subsequently dampening its atomic motion.  This cooled the device down from room temperature to -228C (-379F), just 46C above absolute zero.  Similar laser techniques have been used to cool much smaller objects to a point much closer to absolute zero.  However, scientists researching the RF technique are looking to use higher and different frequencies.  These may prove useful in eventually pushing the temperature boundaries down to less than 0.2 Kelvin, just barely above absolute zero.  And, it will be able to do so on much larger tiny structures.

RF wave cooling physically differs from laser light cooling in that laser bombards the surface with a blanketed array of photons.  Each photon carries with it an extremely small amount of force.  The cumulative effect of the constant bombardment over time acts uniformly upon the object causing its motion to be altered.  It's much like a bunch of photonic ping-pong balls acting on the atomic bowling balls.  If enough ping-pong balls hit it, it slows.  RF waves act more efficiently, like a swing out of phase with its arc.  If the pushing is setup just right, the swing can go really high with little input effort.  If it's setup just right the other way, then it can bring the swing to a complete stop with relatively little effort as well.  And that's just what the RF solution does.  When the correct frequencies and amplitude are applied, it brings the atomic motion almost to a stop.

Scientists hope to use this research to provide cooling or dampening techniques to quell the massively chaotic world of extremely small devices.  That silicon diving board example is just a very small piece of silicon.  And yet it was vibrating naturally at 7,000 cycles per second at such a small size.  As we push nano-scale science ever further, the tiny constructed devices will all have similar problems.  Materials must be specially chosen to work within these micro-environment they're designed for.  With this kind of technology being applied, it may be possible to create all kinds of nano structures using a wider range of materials than would be otherwise possible in the absence of such a field.  And the ability to overcome limiting factors in practical use may be had.