Researchers at the National Institute of Standards and Technology (NIST) have increased the sensitivity of their nuclear radio a hundredfold by closing the small glass cylinder with cesium atoms in what looks like personalized copper “headphones.”
The structure – a square upper circuit connecting two square panels – increases the incoming radio signal or electric field applied to the gaseous atoms in the flask (known as the steam cell) between the panels. This improvement allows the radio to detect much weaker signals than before. The demonstration is described in new paper.
The structure of the headphones is technically a split ring resonator that acts as a metamaterial – a material designed with new structures to produce unusual properties. “We can call it a metamaterial-inspired structure,” said NIST project manager Chris Holloway.
NIST researchers previously demonstrated the atom-based radio. The atomic sensor has the potential to be physically smaller and work better in noisy environments than conventional radios, among other possible advantages.
The steam cell is about 14 millimeters (mm) long and 10 millimeters in diameter, about the size of a fingernail or computer chip, but thicker. The upper contour of the resonator is about 16 mm on the side and the ear caps are about 12 mm on the side.
The NIST radio relies on the special state of atoms. Researchers are using two different color lasers to prepare the atoms contained in the steam cell in high-energy (“Rydberg”) states, which have new properties such as extreme sensitivity to electromagnetic fields. The frequency and strength of the applied electric field affect the colors of the light absorbed by the atoms, and this results in the conversion of the signal strength into an optical frequency that can be measured accurately.
A radio signal applied to the new resonator creates currents in the upper loop that produce magnetic flux or voltage. The dimensions of the copper structure are smaller than the wavelength of the radio signal. As a result, this small physical gap between the metal plates has the effect of storing energy around the atoms and amplifying the radio signal. This increases the efficiency of productivity or sensitivity.
“The loop captures the incoming magnetic field, creating tension through the gaps,” Holloway said. “Because the gap separation is small, a large electromagnetic field develops through the gap.”
The dimensions of the contour and the gap determine the natural or resonant frequency of the copper structure. In NIST experiments, the difference is just over 10 mm, limited by the outer diameter of the available steam cell. Researchers used a commercial mathematical simulator to determine the size of the circuit needed to create a resonant frequency close to 1,312 GHz, where Rydberg’s atoms switch between energy levels.
Several outside collaborators helped model the design of the resonator. The modeling suggests that the signal can be made 130 times stronger, while the measured result is approximately one hundred times, probably due to energy losses and structural imperfections. A smaller gap would lead to more gain. Researchers plan to study other resonator designs, smaller steam cells and different frequencies.
With further development, atom-based receivers can offer many advantages over conventional radio technologies. For example, atoms act as an antenna and there is no need for traditional electronics that convert signals to different delivery frequencies, as atoms do the work automatically. Atomic receivers can be physically smaller, measuring micrometers. In addition, atom-based systems may be less susceptible to certain types of interference and noise.
The study was funded in part by the Agency for Advanced Defense Research Projects and the NIST on a Chip program. Modeling assistance was provided by collaborators from the University of Texas, Austin; City University of New York, New York; and Sydney University of Technology, Australia.
Papers: CL Holloway, N. Prajapati, AB Artusio-Glimpse, S. Berweger, MT Simons, Y. Kasahara, A. Alu and RW Ziolkowski. Improving the Rydberg-based field with a split ring resonator. Letters in Applied Physics. Published online May 20, 2022. DOI: 10.1063 / 5.0088532