Optomechanical Silicon Nitride Beams can be Used as Quantum Thermometers

Integrating the fields of mechanics and optics, physicists have developed microscopic structural beams having a range of influential applications when irradiated by light.

These optomechanical systems have the ability to function in normal ambient conditions, function based on certain unrevealed principles of quantum physics, and can function as intrinsically precise thermometers, or otherwise as a kind of optical shield that redirects heat. The study was conducted by a group of Researchers headed by the Joint Quantum Institute (JQI)—a research partnership between the National Institute of Standards and Technology (NIST) and the University of Maryland.

The functions of the optomechanical system have been reported in two new papers published in the journals Physical Review Letters and Science. The prospective uses of the system include chip-based temperature sensors that can be used in the fields of biology and electronics that eliminate the need for adjusting because they are dependent on fundamental constants of nature; tiny refrigerators with the ability to cool down ultra-modern microscope components for obtaining images of higher quality; and improved “metamaterials” that enable Scientists to control and use sound and light in innovative ways.

The beams are formed of silicon nitride, a material that is broadly used in the photonics and electronics industries. The length of the beams are nearly 20 mm, or 20 millionths of a meter. The beams are transparent, and a sequence of holes is drilled through the beams to improve their mechanical and optical characteristics.

You can send light down this beam because it’s a transparent material. You can also send sound waves down the beam.

Tom Purdy, an NIST Physicist and one of the Authors of both papers

The research team considers that the beams can form the basis of highly accurate thermometers that are omnipresent in many devices at present ‒ for example, cell phones.

“Essentially we’re carrying a bunch of thermometers around with us all the time,” stated JQI Fellow Jake Taylor, Senior Author of the new papers. “Some provide temperature readings, and others let you know if your chip is too hot or your battery is too cold. Thermometers also play a crucial role in transportation systems ‒ airplanes, cars ‒ and tell you if your engine oil is overheating.”

However, the difficulty is that the thermometers are not so precise when produced commercially. They have to be calibrated (i.e. manipulated) to a specific standard. The silicon nitride beam design eliminates this difficulty as it is strongly based on fundamental physics. In order to use the beams as thermometers, Scientists should be in a position to measure even minute vibrations in the beams. The amount of vibration of the beam is directly proportional to the surrounding temperature.

Two types of sources lead to these vibrations. One of the sources of vibration is a normal “thermal” source, for example, gas molecules striking the beam or sound waves that pass through it. The other source of vibration solely belongs to the realm of quantum mechanics ‒ the theory controlling the functioning of matter at the atomic level. The quantum behavior takes place when the Scientists make particles of light (i.e. photons) to go down the beam.

When the mechanical beam is buffeted by light, it reflects the photons and consequently recoils, resulting in the occurrence of small vibrations in the beam. At times, such quantum-based effects are illustrated by means of the Heisenberg uncertainty relationship—the photon bounce assists in acquiring information related to the position of the beam; however, as it transmits vibrations to the beam, it introduces an uncertainty to the velocity of the beam.

“The quantum mechanical fluctuations give us a reference point because essentially, you can’t make the system move less than that,” stated Taylor. The Scientists can compute the temperature by using the values of Planck’s constant and Boltzmann’s constant. Moreover, by using this reference point, if the Scientists measure greater motion in the beam (e.g. from thermal sources), they will be in a position to precisely predict the temperature of the surroundings.

Yet, one drawback is that the quantum vibrations are nearly a million times more faint when compared to thermal vibrations, hence detecting these vibrations is similar to detecting the noise of a dropping pin while bathing.

In their investigations, the research team employed an ultra-modern silicon nitride beam developed by Karen Grutter and Kartik Srinivasan from the NIST’s Center for Nanoscale Science and Technology. When high-quality photons are directed at the beam and photons emitted from the beam are analyzed immediately, “we see a little bit of the quantum vibrational motion picked up in the output of light,” stated Purdy. The measurement technique employed by the research team is adequately sensitive to observe these quantum effects at an ambient temperature for the first time. Their outcomes have been reported in the latest issue of the Science journal.

While the experimental thermometers are just in their proof-of-principle stage, the research team predicts that the thermometers can be specifically valuable in the field of biology as well as in electronic devices, where they are used as on-chip thermometers that do not have to be calibrated at all.

Biological processes, in general, are very sensitive to temperature, as anyone who has a sick child knows. The difference between 37 and 39 degrees Celsius is pretty large.

Jake Taylor, JQI Fellow and Senior Author of the new papers

He predicts futuristic applications of the thermometers in the field of biotechnology if one has to measure changes in temperature of “as small an amount of product as possible,” he stated.

In the case of the second prospective application of the beams, the research team has taken a contrasting method, which is reported in a theoretical paper published in the journal Physical Review Letters.

Rather than allowing heat to strike the beam and enable the beam to function as a thermometer, the research team intends to use the beam to redirect heat away from a delicate part of an electromechanical device.

In the experimental setup intended by the Researchers, they enclose the beam inside a cavity that includes a pair of mirrors that reflect light back and forth. They used light to regulate the vibrations in the beam such that the beam does not reflect back the incoming heat in its regular direction and directs it toward a colder object.

For this usage, Taylor compares the functioning of the beam with that of a tuning fork. If a tuning fork is held and struck, it emits clear sound tones rather allowing the motion to be transformed into heat. The sound tones move down the fork into one’s hand.

“A tuning fork rings for a long time, even in air,” stated Taylor. He further added that the two prongs in the fork vibrate in contrasting directions and makes a way through which energy leaves the bottom of the fork and passes into your hand.

The research team also proposes to use an optically controlled silicon nitride beam in the place of tip of an atomic force microscope (AFM), which investigates forces on surfaces to form atom-level images. An optically controlled AFM tip can remain cool, and consequently perform well. “You’re removing thermal motion, which makes it easier to see signals,” explained Taylor.

This method can also be used to produce superior metamaterials, that is, complex composite objects with the ability to control sound or light in innovative ways and used to produce higher quality lenses better known as “invisibility cloaks” that make light of specific wavelengths to pass around an object rather than being reflected from it.

Metamaterials are our answer to the question, ‘How do we make materials that capture the best properties for light and sound, or for heat and motion? It’s a technique that has been widely used in engineering, but combining the light and sound together remains still a bit open on how far we can go with it, and this provides a new tool for exploring that space.

Jake Taylor, JQI Fellow and Senior Author of the new papers