Photonic Thermometry

June 16, 2014

In most cases, a degree in temperature doesn't matter that much in our daily lives. As a practical matter, baking bread at 351°F isn't that different from baking at 350°F. Most cooking ovens will only control temperature within five or ten degrees of the setpoint, and not all portions of the oven interior will be at the same temperature. A warm summer's day at 81°F doesn't feel any different than one at 80°F.

Normal human body temperature is taken to be 98.6°F (37.0°C), but small differences from this number aren't significant. The body temperature of a healthy adult will vary almost a degree Fahrenheit (~0.5°C) throughout the day. Your temperature is usually lower when you arise in the morning, and higher in late afternoon after you've been active.

In some physical processes, a very small temperature change is critical. Not surprisingly, these are characterized by a "critical temperature," generally symbolized as TC. These temperatures mark the boundary between different phases of a material, one example being the superconducting critical temperature, the temperature below which a material is superconducting.

As science fiction author, Arthur C. Clarke's third law states, "Any sufficiently advanced technology is indistinguishable from magic."

This is a lighted candle being levitated on a magnet above a YBCO superconductor cooled to liquid nitrogen temperature.

The levitation is caused by the Meissner effect.

(Photograph by Julien Bobroff and Frederic Bouquet, LPS (Orsay, France), via Wikimedia Commons.)

Superconductors are one instance for which a small fraction of a degree in temperature makes a big difference. In 1911, when Heike Kammerlingh Onnes discovered superconductivity while making resistivity measurements on mercury, he found that the resistivity transitioned from normal to zero when cooling from 4.2 K to 4.19 K, a temperature change of just 0.01 K. Subsequent measurements with more accurate thermometry showed that the transition width is far smaller than this.

When you're interested in measuring temperature to high precision, what thermometers will suit your needs? There are quite a few types of thermometers used in the laboratory, including the traditional mercury thermometer, and more advanced electrical devices, such as thermopiles, thermocouples and RTDs.

Every thermometer is based on some temperature-dependent material property. For thermocouples, it's the Seebeck effect; and for RTDs, such as platinum resistance thermometers, it's the variation in electrical conductivity. Electrical engineers have struggled with the temperature-dependence of quartz crystal oscillators for more than a century, but that same effect is desirable in the quartz thermometer, which has a 0.0001°C resolution.[1]

One type of thermometer, popular because it can be built into any silicon integrated circuit, is the silicon bandgap temperature sensor. The forward voltage of a silicon diode is temperature-dependent, and a combination of two such diodes with appropriate circuitry will give a linear voltage change with temperature. Such a thermometer is useful over the operating range of silicon devices, from -50°C to 125°C.

It's also possible to measure temperature using light. The decay time of an excited phosphor has a temperature dependence, so phosphors can be incorporated into optical fibers to form temperature sensors using phosphor thermometry. Other optical properties, such as birefringence, have a temperature dependence.[2]

The refractive index is another temperature-dependent optical property, primarily because the thermal expansivity of materials modifies their atomic spacing. Scientists from the University of Western Australia (Western Australia) , the University of Adelaide (South Australia), the University of Queensland (Brisbane, Australia) and the Australian National University (Australian Capital Territory, Australia) have used the refractive index to make a thermometer with the highest claimed precision of 30 billionths of a degree.[3-4] This is three times more precise than the previous best.[4]

Their thermometer also requires dispersion, another optical effect, to function. Dispersion is the material property that light of different colors travels at different speeds; that is, the refractive index, which is defined as the ratio of the speed of light in vacuum to the speed of light in the material, changes as a function of wavelength. An example of this can be seen in the figure, below.

The wavelength dependence of the ordinary index of refraction for the rutile phase of TiO2.

(Highly modified Wikimedia Commons image.)

This thermometer operates by launching rays of two light colors into a highly polished crystalline disk that acts as a whispering-gallery. I wrote about optical whispering-galleries in a previous article (Resonant Light Conversion, December 14, 2011). The two colors of light are the 1064 nm light from a Nd:YAG laser and its frequency-doubled counterpart at about 532 nm.

Illustration of the sensing element of the Australian optical thermometer. The light beams are infrared and green.

(University of Adelaide image by Dr James Anstie.)

The frequency difference between the resonant modes of these two colors is an ultrasensitive indicator of the resonator temperature.[3] This two color approach also solves some problems, since it automatically suppresses the affects of thermal expansion and vibration of the resonator.[3] Says project leader, Andre Luiten of the University of Adelaide School of Chemistry and Physics, "We believe this is the best measurement ever made of temperature - at room temperature." More sensitive temperature measurement can be done near absolute zero.[4]

As the disk resonator heats, the infrared beam slows with respect to the green beam. Since these light beams circulate thousands of times around the edge of the whispering-gallery resonator disk, there's a large phase difference between the beams when they exit. This allows the high precision demonstrated.[4]

This optical technology can be used to measure things besides temperature, such as pressure, humidity, force, and the presence of a chemical compound.[4] This research was supported by the Australian Research Council and the South Australian Government's Premier's Science and Research Fund.[4]

References:

1. This is an example of the "platformate effect." When chemists were devising means of catalytically modifying the organic constituents of oil into better gasoline, they hit upon a nice catalyst for the job. The problem was that it slightly colored the traditionally water-white motor fuel. The business solution to the problem was, "Sell the color!"
2. Devlin M. Gualtieri, Janpu Hou, William R. Rapoport and Herman van de Vaart, "Birefringent-Biased Sensor Having Temperature Compensation," U.S. Pat. No. 5,694,205, December 2, 1997.
3. Wenle Weng, James D. Anstie, Thomas M. Stace, Geoff Campbell, Fred N. Baynes and Andre N. Luiten, "Nano-Kelvin Thermometry and Temperature Control: Beyond the Thermal Noise Limit," Phys. Rev. Lett., vol. 112 (April 21 2014), Document No. 160801, DOI:10.1103/PhysRevLett.112.160801.
4. World's best thermometer made from light, University of Adelaide Press Release, June 2, 2014.

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