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John O | July 2017

Versatile optomechnical beams can act as thermometer or as heat shield

collaboration between the joint quantum institute (jqi) at the university of maryland (umd) and the national institute of standards and technology (nist) in gaithersburg, maryland has led to the development of microscopic structural beams that work at room temperature as inherently accurate thermometers or as an optical shield to divert heat.


artist's rendition of a quantum thermometer, a micron-scale mechanical device that can
provide highly accurate temperature. (emily edwards/joint quantum institute)


according to a report on the nist website, “the potential applications include chip-based temperature sensors for electronics and biology that would never need to be adjusted since they rely on fundamental constants of nature; tiny refrigerators that can cool state-of-the-art microscope components for higher-quality images; and improved ‘metamaterials’ that could allow researchers to manipulate light and sound in new ways.”


the transparent beams are made of silicon nitride, which the article calls “widely used” in electronics and photonics, and are 20 microns in length. holes are drilled through the beams to enhance their properties.


“to use the beam as a thermometer, researchers must be able to measure the tiniest possible vibrations in the beam,” the article continued. “the amount that the beam vibrates is proportional to the temperature of its surroundings.”


these vibrations can be caused by ordinary thermal sources such as gas molecules or sound waves, but can also be caused by passing photons (particles of light) down the beam.


“struck by light, the mechanical beam reflects the photons, and recoils in the process, creating small vibrations in the beam,” the article said. “sometimes these quantum-based effects are described using the heisenberg uncertainty relationship: the photon bounce leads to information about the beam’s position, but because it imparts vibrations to the beam, it adds uncertainty to the beam’s velocity.”


using boltzmann’s constant and planck’s constant allows the researchers to determine the temperature. these vibrations caused by the photons are a million times smaller than the thermal vibrations, forcing researchers to use a novel measurement system that was sensitive enough to record the quantum effects at room temperature for the first time.


the research on the beams being used as thermometers was published in science. the abstract stated:


“quantum back action—the “reaction” of a quantum mechanical object to being measured—is normally observed at cryogenic temperatures, where it is easier to distinguish from thermal motion. purdy et al. managed to tease out the effects of quantum back action at room temperature by using a mechanical oscillator and probing it with light (see the perspective by harris).


“the fluctuations of the force produced by the light probe caused correlated changes to the motion of the oscillator and the properties of the transmitted light. these correlations revealed the effects of the back action, which allows the system to be used as a quantum thermometer.”


the second option for the beam is to divert heat away from an electromechanical device. to test this potential application, researchers enclosed the beam in cavity with mirrors bouncing light back and forth. researchers used the light to control vibrations in the beam and keep it from radiating heat towards the colder object.


the researchers compared this to a tuning fork. the article explained, “when you hold a tuning fork and strike it, it radiates pure sound tones instead of allowing that motion to turn into heat, which travels down the fork and into your hand.”


it was suggested that this could lead to the beam being used as the tip in atomic force microscopes or in composite metamaterials with fine-tuned properties.


this research was published in physical review letters. the abstract from this report stated:


“optomechanical systems show tremendous promise for the high-sensitivity sensing of forces and modification of mechanical properties via light. for example, similar to neutral atoms and trapped ions, laser cooling of mechanical motion by radiation pressure can take single mechanical modes to their ground state.


“conventional optomechanical cooling is able to introduce an additional damping channel to mechanical motion while keeping its thermal noise at the same level, and, as a consequence, the effective temperature of the mechanical mode is lowered.


“however, the ratio of the temperature to the quality factor remains roughly constant, preventing dramatic advances in quantum sensing using this approach. here we propose an approach for simultaneously reducing the thermal load on a mechanical resonator while improving its quality factor.


“in essence, we use the optical interaction to dynamically modify the dominant damping mechanism, providing an optomechanically induced effect analogous to a phononic band gap. the mechanical mode of interest is assumed to be weakly coupled to its heat bath but strongly coupled to a second mechanical mode, which is cooled by radiation pressure coupling to a red-detuned cavity field.


“we also identify a realistic optomechanical design that has the potential to realize this novel cooling scheme.”

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