‘Quantum optical antennas’ offer more powerful measurements at the atomic level

Similar to the way a radio antenna picks up a broadcast from the air and concentrates the energy into a song, individual atoms can collect and focus light energy into a strong, localized signal that researchers can use to study the basic building blocks of matter.

The stronger the gain, the better the antenna. But researchers have never been able to use the potentially large intensity enhancements of some “atomic antennas” in solid materials simply because they were solid.

“Most of the time when you have atoms in solids, they’re interacting with the environment. There’s a lot of disorder, they get shaken by phonons and face other disruptions that reduce the coherence of the signal,” said UChicago Pritzker School of Molecular Engineering Asst. . Prof. Alex High.

In a new paper published in Photonics of nature, a multi-institutional team led by the High Laboratory has tackled this problem. They have used hollow centers of germanium in diamonds to create an optical power gain of six orders of magnitude, a challenging regime to achieve with conventional antenna structures.

This million-fold improvement in power creates what the paper calls an “exemplary” optical antenna and provides a new tool that opens up entirely new areas of research.

“It’s not just a breakthrough in technology. It’s also a breakthrough in fundamental physics,” said PME Ph.D. candidate Zixi Li, co-first author on the paper. “While it is well known that an excited atomic dipole can generate a large intensity, no one has demonstrated this in an experiment before.”

From theory to practice

The main characteristic of an optical antenna is that it creates an oscillating electronic dipole when excited at resonance.

“Optical antennas are essentially structures that interact with electromagnetic fields and absorb or emit light at certain resonances, like electrons moving between energy levels in these color centers,” High said.

The electron oscillates when it goes between an excited state and a ground state and concentrates a relatively large amount of energy, making an atomic optical dipole in a solid an excellent antenna—in theory.

What held that theoretical ability was the fact that atoms were in solids, subject to all the shocks, electronic interference, and general noise that comes from being part of a tightly packed structure. Color centers—tiny defects in diamonds and other materials with interesting quantum properties—provided the team with a solution.

“Something that has been observed over the last seven or eight years is that certain types of color centers can be immune to these environmental effects,” High said.

This opens up intriguing research possibilities, said co-author Darrick Chang of the Institute of Photonic Sciences in Barcelona, ​​Spain.

“To me, the most interesting aspect of a color center is not only the field enhancement, but also the fact that the emitted light is essentially quantum mechanical,” he said. “This makes it intriguing to consider whether a ‘quantum optical antenna’ might have a different set of functionality and working mechanisms compared to a classical optical antenna.”

But turning this theory into a practical antenna took years, collaboration with researchers around the globe, and theoretical guidance from UChicago’s Galli Group.

“The collaboration between theory, computation and experiments initiated by Alex High not only contributed to the understanding and interpretation of fundamental science, but also opened up new lines of research on the computational side,” said PME Liew Family Prof. Guilia Galli, a co-author. on paper. “The cooperation has been extremely fruitful.”

“The Magic of a Color Center”

Imaging at the atomic level is a combination of amplification and bandwidth—the strength of the signal and the amount of signal you can study. Because of this, co-first author Xinghan Guo sees the new technique as complementary to, rather than replacing, existing techniques.

“We offer much higher amplification, but our bandwidth is narrower,” said Guo, who recently completed his Ph.D. in PME and is now a postdoctoral researcher at Yale. “If you have a very selective signal that has a narrow bandwidth but requires a lot of amplification, you can come to us.”

The new technique offers other benefits than just a stronger signal. While existing techniques such as single-molecule Raman and FRET spectroscopy induce the signal by bursting it with light, this technique requires only nanowatts of energy to activate. This means a strong signal without the bleaching, warming and background fluorescence that excess light creates.

Germanium hollow cores also do not dissipate energy while in use, unlike conventional plasmonic antennas.

“The magic of a color center is that it is both point-like and avoids the losses of a plasmonic material, allowing it to retain its extreme field broadening,” Chang said.

For High, the exciting part isn’t the new shape of the antenna, but the potential breakthroughs it will make.

“What’s exciting is that this is a general feature,” High said. “We can integrate these color centers into a huge range of systems, and then we can use them as local antennas to grow new processes that build new devices and help us understand how the universe works.”