technology seems to improve year after year, like magic. But behind every incremental improvement and breakthrough revolution is a team of hard-working scientists and engineers.
Professor Ben Mazin of UC Santa Barbara is developing precision optical sensors for telescopes and observatories. In a paper published in Physical Review Letters , he and his team improved the spectral resolution of the superconducting sensor, a major step toward their ultimate goal: analyzing the composition of the exoplanet . .
"We were able to roughly double the spectral resolution of the detector," said first author Nicholas Zobrist, a doctoral student in Mazin's lab.
"This is the largest improvement in energy resolution we have ever seen," Mazin added. "This opens up a whole new path for us to achieve previously unattainable scientific goals."
The Mazin lab uses a sensor called MKID. Most light detectors like the CMOS sensors in cell phone cameras are silicon-based semiconductors. They operate via the photoelectric effect: a photon strikes a sensor, knocking off an electron, which can then be detected as a signal suitable for processing by a microprocessor.
MKID uses superconductors in which current can flow without resistance. In addition to zero resistance, these materials have other useful properties. For example, the semiconductor has a gap energy that needs to be overcome to knock out electrons. The relevant gap energy in superconductors is about 10,000 times higher, so it can detect even weak signals.
What's more, one photon can knock out many electrons from a superconductor, compared to just one electron in a semiconductor. By measuring the number of moving electrons, MKID can actually determine the energy (or wavelength) of the incoming light. "The energy of a photon, or its spectrum, tells us a lot about the physics of the emitted photon," Mazin said.
Leak Energy
Researchers have reached a limit on how sensitive they can make these MKIDs. Upon closer inspection, they discovered that energy was leaking from the superconductor into the sapphire wafer from which the device was made. As a result, the signal appears weaker than it actually is.
In typical electronics , electrical current is carried by moving electrons. But they have a tendency to interact with their surroundings, dispersing and losing energy in what's called drag. In a superconductor, two electrons will pair up, one with spin up and one with spin down, and this Cooper pair, as it's called, is able to move without resistance.
"It's like a couple in a club," Mazin explained. "You have two people paired up, and then they can move together in the crowd without any resistance. However, one person stops and talks to everyone along the way, slowing them down."
In a superconductor, all the electrons are in pairs. "They all danced together and moved around, rarely interacting with other couples as they both stared deeply into each other's eyes.
"The photons hitting the sensor were like someone coming in and spilling a drink on one of the partners," He continued, “This can cause couples to break up, cause one to trip up other couples, and cause turmoil. "This is a cascade of moving electrons measured by MKID.
But sometimes this happens at the edge of the dance floor. The offended party stumbles out of the club without bumping into anyone else. That's fine for the other dancers, but for Not to the scientists. If this happened in MKID, the light signals would appear weaker than they actually are. Mazin, Zobrist and their co-authors found that by placing a layer between the superconducting sensor and the substrate. The thin metal indium greatly reduces the energy leakage of the sensor.Indium basically acts like a fence around the dance floor, keeping the pushed dancers in the room and interacting with the rest of the crowd. They chose indium because it is also a superconductor at the temperatures at which MKID operates, and if adjacent superconductors are thin, they tend to mate with each other. The metal does present challenges for the team, though. Indium is softer than lead, so it has a tendency to clump. That's not great for making the thin, uniform layers researchers need. But their time and effort paid off. The research report states that this technology reduces the wavelength measurement uncertainty from 10% to 5%. For example, using this system, photons with a wavelength of 1,000 nanometers can now be measured to an accuracy of 50 nanometers. "This has real implications for the science we can do," Mazin said, "because we can better resolve the spectra of the objects we are observing." Different phenomena emit photons with specific spectra (or wavelengths) , different molecules absorb photons of different wavelengths. Using this light, scientists can use spectroscopy to determine the composition of objects nearby and throughout the visible universe. Mazin is particularly interested in applying these probes to exoplanet science. Currently, scientists can only perform spectroscopic studies on a small portion of exoplanets. The planet needs to pass between its star and Earth, and it must have a thick atmosphere to allow enough light to pass through it for researchers. Still, the signal-to-noise ratio is low, especially for rocky planets, Mazin said. With a better MKID, scientists could take advantage of light reflected from the planet's surface, rather than light transmitted only through its narrow atmosphere. With the capabilities of the next generation 30-meter telescope, this will soon be possible. The Mazin team is also experimenting with a completely different approach to solving the energy loss problem. While the paper's results are impressive, Mazin said if his team is successful in this new effort, he believes the indium technology may become obsolete. Regardless, scientists are moving quickly toward their goal, he added.
