Professor Ben Mazin discusses superconductors, exoplanets, and dance clubs while explaining sensor technology advances.

The sensor installed in an MKID Exoplanet Camera.
The sensor is installed in an MKID Exoplanet Camera.  Photo Credit: BEN MAZIN

Precision optical sensors for telescopes and observatories are being developed by researchers. The researchers have now improved the spectral resolution of their superconducting sensor, which is a significant step toward their ultimate goal of analyzing the composition of exoplanets.

It may appear that technology advances as if by magic year after year. But behind every incremental advancement and revolutionary breakthrough is a team of scientists and engineers hard at work.

Professor Ben Mazin of UC Santa Barbara is working on precision optical sensors for telescopes and observatories. He and his colleagues improved the spectral resolution of their superconducting sensor in a paper published in Physical Review Letters, a major step toward their ultimate goal of analyzing the composition of exoplanets.

“We were able to roughly double the spectral resolving power of our detectors,” said Mazin Lab doctoral student Nicholas Zobrist.

“This is the largest increase in energy resolution we’ve ever seen,” Mazin added. “It opens up a whole new path to achieving science goals that we couldn’t before.”

The Mazin lab employs a type of sensor known as an MKID. Most light detectors, such as the CMOS sensor in a smartphone camera, are silicon-based semiconductors. These work on the basis of the photoelectric effect: a photon hits the sensor, knocking off an electron, which is then detected as a signal suitable for processing by a microprocessor.

An MKID employs a superconductor, which allows electricity to flow without resistance. These materials have other useful properties aside from zero resistance. For example, semiconductors have a gap energy that must be overcome in order to knock the electron out. Because the related gap energy in a superconductor is approximately 10,000 times lower, it can detect even faint signals.

Furthermore, a single photon can knock many electrons off a superconductor rather than just one in a semiconductor. An MKID can determine the energy (or wavelength) of incoming light by counting the number of mobile electrons. “And the energy of the photon, or its spectrum,” Mazin explained, “tells us a lot about the physics of what emitted that photon.”

Energy leaking

The researchers had reached their limit in terms of how sensitive they could make these MKIDs. They discovered that energy was leaking from the superconductor into the sapphire crystal wafer on which the device is built after much investigation. As a result, the signal appeared to be weaker than it was.

Mobile electrons carry current in typical electronics. These, on the other hand, have a proclivity to interact with their surroundings, scattering and losing energy in what is known as resistance. Two electrons will pair up in a superconductor, one spin up and one spin down, and this Cooper pair, as it is known, will be able to move around without resistance.

“It’s like a club couple,” Mazin explained. “You have two people who pair up and can move through the crowd without encountering any resistance. A single person, on the other hand, stops to talk to everyone along the way, slowing them down.”

All of the electrons in a superconductor are paired up. They’re all dancing together, moving around without much interaction with the other couples because they’re all staring deeply into each other’s eyes.

“A photon hitting the sensor is analogous to someone walking in and spilling a drink on one of the partners,” he went on. “This splits the couple, causing one partner to bump into other couples and cause a commotion.” This is the cascade of mobile electrons measured by the MKID.

However, this can happen at the edge of the dancefloor. The offended party stumbles out of the club without collapsing. Excellent for the remaining dancers, but not for the scientists. If this occurs in the MKID, the light signal will appear weaker than it was.

Keeping them at bay

Mazin, Zobrist, and their colleagues discovered that a thin layer of the metal indium placed between the superconducting sensor and the substrate reduced the amount of energy leaking out of the sensor significantly. The indium functioned essentially as a fence around the dancefloor, keeping the jumbled dancers in the room and interacting with the rest of the audience.

They chose indium because it is also a superconductor at the temperatures where the MKID will operate, and thin superconductors tend to cooperate. The metal did, however, present a challenge to the team. Because indium is softer than lead, it tends to clump. That’s not ideal for creating the thin, uniform layer required by the researchers.

But their hard work paid off. According to the study, the technique reduced wavelength measurement uncertainty from 10% to 5%. Photons with a wavelength of 1,000 nanometers, for example, can now be measured to a precision of 50 nm using this system. “This has significant implications for the science we can do,” Mazin explained, “because we can better resolve the spectra of the objects we’re looking at.”

Different phenomena emit photons with different spectra (or wavelengths), and different molecules absorb photons with various wavelengths. Scientists can use this light to use spectroscopy to determine the composition of objects both nearby and throughout the visible universe.

Mazin is particularly interested in using these detectors to study exoplanets. Spectroscopy can currently only be performed on a small subset of exoplanets. The planet must pass between its star and Earth, and it must have a thick atmosphere to allow researchers to work with enough light. Nonetheless, the signal-to-noise ratio is abysmal, especially for rocky planets, according to Mazin.

With improved MKIDs, scientists can use light reflected off a planet’s surface rather than light transmitted solely through its narrow atmosphere. With the capabilities of the next generation of 30-meter telescopes, this will soon be possible.

The Mazin group is also experimenting with an entirely new approach to the energy-loss problem. Although the results of this paper are impressive, Mazin believes that if his team is successful in this new endeavor, the indium technique may become obsolete. In any case, he added, the scientists are rapidly approaching their objectives.

Source: Materials provided by University of California – Santa Barbara

Reference: DOI: 10.1103/PhysRevLett.129.017701

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