Keeping the energy in the room

It may seem like technology is magically evolving year after year. But behind every incremental improvement and breakthrough revolution is a team of hard-working scientists and engineers.

Professor Ben Mazin of UC Santa Barbara develops precision optical sensors for telescopes and observatories. In a publication in Physical Verification Lettershe and his team improved the spectral resolution of their superconducting sensor, an important step toward their ultimate goal: analyzing the composition of exoplanets.

“We were able to roughly double the spectral resolution of our detectors,” says lead author Nicholas Zobrist, a PhD student in the Mazin Lab.

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

The Mazin lab works with a type of sensor called MKID. Most light detectors – like the CMOS sensor in a phone camera – are silicon-based semiconductors. These work via the photoelectric effect: a photon hits the sensor and knocks off an electron, which can then be detected as a suitable signal for processing by a microprocessor.

An MKID uses a superconductor that allows current to flow without resistance. In addition to zero resistance, these materials have other useful properties. For example, semiconductors have a gap energy that must be overcome in order to knock out the electron. The associated gap energy in a superconductor is about 10,000 times lower, allowing it to detect even weak signals.

In addition, a single photon can knock many electrons out of a superconductor, as opposed to just one in a semiconductor. By measuring the number of moving electrons, an MKID can actually determine the energy (or wavelength) of the incident light. “And the energy of the photon, or its spectra, tells us a lot about the physics of what emitted that photon,” Mazin said.

Leaking Energy

The researchers had hit a limit on how sensitive they could make these MKIDs. After careful examination, they discovered that energy was leaking from the superconductor into the sapphire crystal wafer on which the device was fabricated. As a result, the signal appeared weaker than it really was.

In typical electronics, current is carried by moving electrons. But these tend to interact with their environment, scattering and losing energy, which is known as resistance. In a superconductor, two electrons pair up — one spin up and one spin down — and this Cooper pair, as it’s called, can move without resistance.

“It’s like a couple in a club,” Mazin explained. “You have two people mating and then they can move through the crowd together without resistance.

In a superconductor, all electrons are paired. “They all dance together, move around without interacting much with other couples because they’re all staring deep into each other’s eyes.

“A photon hitting the sensor is like someone walking in and spilling a drink on one of the partners,” he continued. “This separates the pair, causing one partner to stumble into other pairs and causing interference.” This is the cascade of moving electrons that the MKID is measuring.

But sometimes that happens on the edge of the dance floor. The offended party stumbles out of the club without bumping into anyone. Great for the rest of the dancers, but not for the scientists. When this happens in the MKID, the light signal will appear dimmer than it actually was.

to fence you in

Mazin, Zobrist and their co-authors discovered that a thin layer of the metal indium – placed between the superconducting sensor and the substrate – drastically reduced the energy escaping from the sensor. The indium essentially acted as a fence around the dance floor, keeping the jostled dancers in the space and interacting with the rest of the crowd.

They chose indium because it is also a superconductor at the temperatures at which the MKID operates, and neighboring superconductors tend to work together when thin. However, the metal presented the team with a challenge. Indium is softer than lead and therefore tends to clump. That’s not good for making the thin, even layer the researchers needed.

But their time and effort paid off. The technique reduced the wavelength measurement uncertainty from 10% to 5%, the study reports. With this system, for example, photons with a wavelength of 1,000 nanometers can now be measured with an accuracy of 50 nm. “This has real implications for the science we can do,” Mazin said, “because we can better resolve the spectra of the objects we’re looking at.”

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

Mazin is particularly interested in using these detectors in exoplanet research. Currently, scientists can only spectroscopically a tiny subset of exoplanets. The planet must pass between its star and Earth, and it must have a dense atmosphere so that enough light can pass through it for researchers to work with. Still, the signal-to-noise ratio is abysmal, especially for rocky planets, Mazin said.

With better MKIDs, scientists can use light reflected off a planet’s surface instead of just being transmitted 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 a completely different approach to the energy loss problem. While the results of this release are impressive, Mazin believes that if his team is successful in this new endeavor, indium technology may be obsolete. However, he added, the scientists were fast approaching their goals.

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