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NIST-on-a-Chip: Electromagnetic Field Metrology

Accurate measurement of electrical fields is of urgent importance to both science and industry. Yet at a recent conference, probe manufacturers stated that their devices could measure fields to no better than 10% uncertainty. One reason is the chicken-and-egg dilemma inherent in conventional techniques: To calibrate a probe, you need a known field. But to have a known field, you need a calibrated probe.

NIST researchers are working to eliminate that problem by creating a fundamentally new technology based on atomic-vapor measurements of the sort now used for time, frequency, and length metrology. Directly traceable to the SI, self-calibrating, and capable of absolute measurements on a small spatial scale (i.e., subwavelength) in both the far-field and near-field, the system will have far-reaching applications, including NIST-on-a-Chip transferable E-field standards, new biomedical metrology, new imaging capability, and traceable calibrations above 110 GHz (currently not available).

vapor cell
Credit: Curt Suplee/NIST
A vapor cell in use.

The technique is based on radio frequency (RF) E-field interactions with alkali atoms placed in vapor cells. The atoms are optically excited to Rydberg states. From 200 MHz to over 1 THz, Rydberg atoms have extremely large electric dipole response and can act as a transducer, converting an E‑field to an optical frequency response.

The NIST-on-a-Chip approach utilizes a phenomenon called electromagnetically induced transparency (EIT). The alkali atoms are illuminated by a weak “probe” beam of laser light, and a second, “coupling,” laser is used to excite the atoms to a Rydberg state. In the presence of the coupling laser, the atoms become transparent to the probe laser transmission. When RF radiation is applied, its electrical field causes an atomic transition in the alkali gas and splits the probe beam transmission (the Autler-Townes effect). This splitting of the probe laser spectrum is easily measured and is directly proportional to the applied RF E-field amplitude.

The new approach has several potential benefits over existing E-field measurement techniques. It is directly linked to SI units, and self-calibrating because it is based on atomic resonances. It offers expanded bandwidth compared to current technologies, allowing measurements from 50 MHz to 500 GHz and possibly up to 1 THz. It is independent of current approaches, thus allowing for intercomparisons. And it works at very small spatial resolution (optical fiber and chip-scale) with vastly improved sensitivity and dynamic range over current E-field methods. (<1 mV/m, two orders of magnitude better than current approaches. <10 V/m may be possible.)

When the technique is fully developed, the NIST researchers anticipate that companies and research organizations throughout the world will adopt it as a new international measurement standard, and employ it for the broad range of applications not yet imagined.

NIST-on-a-Chip lab setup
Credit: Curt Suplee/NIST
The lab setup. The horn in the center is the source of electric fields.

The NIST laboratory for this work is fully equipped and operating, collecting data for two different atomic species:  cesium and rubidium. Some recent results:

  1. Researchers have demonstrated the broadband nature of the technique. With one setup, they measured E-field strengths for frequencies ranging from 450 MHz to 208 GHz (including what may be the first quantum-traceable E-field measurements above 110 GHz), and measured field strengths above 1000 V/m, demonstrating the ability to measure very high field levels.
  2. The scientists have performed simultaneous EIT measurements with two different atomic species (Cs and Rb) in the same vapor cell with coincident (overlapping) optical fields exposed to the identical E-field. In effect, this enables two independent measurements of the same E-field strength, providing a relative comparison of the dipole moments of the two atomic species. Dual measurements that help rule out systematic effects and uncertainties.
  3. The researchers have demonstrated that the technique can be used to perform sub-wavelength imaging at microwave frequencies, hence allowing for small spatial resolution measurements and field mapping.

laser beams
Credit: Curt Suplee/NIST
Electromagnetically induced transparency requires two laser beams of different frequencies.

  1. Because this technique requires the two lasers (probe and coupling laser) to overlap inside the vapor cell, all measurements to date have required the vapor cell to be confined to an optical bench. We have recently developed the first fiber-coupled vapor cell that allows the vapor cell to be moved off the optical bench. The figures below show the first prototype fiber-coupled vapor-cell probe. This prototype has been used to: (1) perform near-field imaging, (2) map field on printed-circuit boards, (3) assess current electromagnetic test facilities, and (4) to measure antenna parameters, just a name a few examples.


Created May 18, 2018, Updated November 15, 2019