Keep the energy in the room

It may seem like technology is advancing year after year, as if by magic. But behind every incremental improvement and breakthrough revolution is a team of scientists and engineers hard at work.

Professor Ben Mazin of UC Santa Barbara develops precision optical sensors for telescopes and observatories. In a paper published in Physical Assessment Lettershe and his team improved the spectra resolution of their superconducting sensor, an important step in their ultimate goal: to analyze the composition of exoplanets.

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

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

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

An MKID uses a superconductor, in which electricity can 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 to knock out the electron. The related gap energy in a superconductor is about 10,000 times less, so it can detect even weak signals.

What’s more, a single photon can knock off many electrons from a superconductor, as opposed to just one in a semiconductor. By measuring the number of mobile 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 that photon was emitting,” Mazin said.

Leaking energy

The researchers had reached a limit on how sensitive they could make these MKIDs. After much research, they discovered that energy was leaking from the superconductor into the sapphire crystal wafer on which the device is made. As a result, the signal appeared weaker than it actually was.

In typical electronics, current is carried by mobile electrons. But these tend to interact with their environment, disperse and lose energy in what is known as resistance. In a superconductor, two electrons will pair — 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 pairing up, and then they can move through the crowd together without any resistance. While a single person stops along the way to talk to everyone, slowing them down.”

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

“A photon hitting the sensor is like someone coming in and spilling a drink on one of the partners,” he continued. “This breaks the couple apart, causing a partner to bump into other couples and cause a nuisance.” This is the cascade of mobile electrons that the MKID measures.

But sometimes this happens on the edge of the dance floor. The offended party stumbles out of the club without knocking on anyone else. Great for the rest of the dancers, but not for the scientists. If this happens in the MKID, then the light signal will appear weaker than it actually was.

fence them off

Mazin, Zobrist and their co-authors found that a thin layer of the metal indium – placed between the superconducting sensor and the substrate – drastically reduced the energy leaking out of the sensor. The indium essentially acted as a fence around the dance floor, keeping the repressed dancers in the room and communicating with the rest of the crowd.

They chose indium because it is also a superconductor at the temperatures at which the MKID will operate, and adjacent superconductors tend to interact if they are thin. However, the metal presented a challenge to the team. Indium is softer than lead, so it tends to clump. That’s not great for creating the thin, uniform layer the researchers needed.

But their time and effort paid off. The technique reduced 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 to an accuracy of 50 nm. “This has real implications for the science that we can do,” Mazin said, “because we can better resolve the spectra of the objects we’re looking at.”

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

Mazin is particularly interested in applying these detectors to exoplanet science. Currently, scientists can only perform spectroscopy for a small subset of exoplanets. The planet must pass between its star and Earth, and it must have a thick atmosphere to allow enough light to pass through for researchers to work with. Still, the signal-to-noise ratio is terrible, especially for rocky planets, Mazin said.

Better MKIDs allow scientists to use light reflected from a planet’s surface, rather than just being transmitted through its narrow atmosphere. This will soon be possible with the capabilities of the next generation of 30-meter telescopes.

The Mazin group is also experimenting with a very different approach to the energyloss problem. While the results of this paper are impressive, Mazin said he believes the indium technique could be obsolete if his team is successful with this new venture. Either way, he added, the scientists are quickly getting closer to their goals.