Imaging Sub-Wavelength Features with Radio Waves
In general when trying to make measurements using an optical system, your greatest precision is going to be half the wavelength of the light you are using. This is definitely true when trying to image or map electric fields that oscillate at radio frequencies, as the dipoles, probes, and reference antennas have to be the proper size to efficiently make measurements. Researchers at NIST and the University of Michigan though have found a way to make these measurements at resolutions one hundred times smaller than the RF wavelength.
Instead of measuring the electric field directly, this new approach measures how the oscillating field affects rubidium atoms. These atoms are contained in a glass cylinder and have two lasers beams aimed at them. One is a red laser that energizes the atoms' electrons to an excited state. The second laser is blue and kicks the electrons to an even high energy, Rydberg state, which is very sensitive to electromagnetic fields. By passing the atoms through the electric field, their ability to absorb the red laser light is altered, and this change can be measured and directly related to the field strength. By tuning the blue laser, it is also possible to measure the strength of specific RF frequencies.
The prototype the researchers tested with achieved resolutions between 50 and 100 micrometers, while the RF fields being mapped had wavelengths between 2863 and 17,605 micrometers. The resolution was limited by the width of the laser beams while the RF fields could actually have wavelengths between 600 and 300,000 micrometers, in theory.