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NIST-on-a-Chip: Atomic Vapor - Temperature

doppler broadening

As a beam of light passes through an atomic vapor, the atoms will absorb a certain range of the beam’s frequencies. The range depends on the velocity distribution – that is, on the temperature. Atoms at lower temperatures (blue curve) absorb fewer frequencies than that higher temperatures (red curve). This phenomenon, called Doppler broadening, can be measured spectroscopically and thus can serve as a thermometer.

Atomic vapor-cell technology can also be adapted to create a chip-scale, self-calibrating, quantum-based thermometer that works by measuring “Doppler broadening” of spectral absorption lines as an indicator of temperature.

The Doppler shift is the familiar phenomenon in which waves of sound or light produced by a source moving toward an observer have a higher frequency/shorter wavelength, and those coming from a receding source have lower frequency/longer wavelength. Those effects can be directly measured in a population of atoms in a vapor cell by shining laser light through the vapor and detecting the photons that pass through without being absorbed while varying the laser frequency.

Because of the Doppler effect, an atom moving away from the beam will absorb photons of a lower frequency, and an atom approaching the laser light will absorb those of a higher frequency. The range of the atoms’ velocities is a function of temperature.

Thermodynamic temperature is a measure of the average total energy in the motion of a collection of atoms or molecules as they travel, vibrate, and/or rotate. The higher the energy content, the greater the range of atomic velocities and the higher the temperature. A greater range of velocities in a collection of atoms means that photons traveling through the vapor will be absorbed in a wider range of frequencies. That is, the absorption spectrum is broadened as temperature increases. The degree of broadening is thus a sensitive measure of temperature.

This phenomenon is based on well-known relationships between physical invariants including the Boltzmann constant, upon which temperature will be defined in the impending redefinition of the SI base units.

In practice, the vapor-cell unit would be placed against an object whose temperature is to be measured, with a fiber-optic connection leading back to a separate package containing the laser and measurement optics. NIST scientists believe that eventually the vapor-cell thermometer could measure temperatures with an accuracy as low as 100 mK.

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Created July 7, 2017, Updated November 15, 2019