Controlling Fluorescence with Nanofibers

Fluorescence near an optical nanofiber depends on the shape of light used to excite the atoms. (Source: E. Edwards, JQI)

Scientists can play tricks with an atom’s sur­roundings to tweak the relaxation time for high-flying electrons, which then dictates the rate of fluores­cence. Now, researchers at the Joint Quantum Institute observed that a tiny thread of glass, an optical nanofiber, had a signi­ficant impact on how fast a rubidium atom releases light. The research showed that the fluores­cence depended on the shape of light used to excite the atoms when they were near the nanofiber.

“Atoms are kind of like antennas, absorbing light and emitting it back out into space, and anything sitting nearby can poten­tially affect this radiative process,” says Pablo Solano, a University of Maryland graduate student at the time this research was performed. To probe how the environ­ment affects these atomic antennas, Solano and his colla­borators surround a nanofiber with a cloud of rubidium atoms.

Nanofibers are custom-made conduits that allow much of the light to travel on the outside of the fiber, enhancing its inter­actions with atoms. The atoms closest to the nanofiber within 200 nanometers felt its presence the most. Some of the fluores­cence from atoms in this region hit the fiber and bounced back to the atoms in an exchange that ulti­mately modified how long a rubidium atom’s electron stayed excited. The researchers found that the electron lifetime and sub­sequent atomic emissions depended on the wave charac­teristics of the light. Light waves oscillate as they travel, sometimes slithering like a side­winder snake and other times corkscrewing like a strand of DNA. The researchers saw that for certain light shapes the electron lingered in the excited state, and for others, it made a more abrupt exit.

“We were able to use the oscil­lation properties of light as a kind of knob to control how atomic fluores­cence near the nanofiber turned on,” Solano says. The team originally set out to measure the effects the nanofiber had on atoms, and compare the results to theo­retical predic­tions for this system. They found disagree­ments between their measure­ments and existing models that incor­porate many of the complex details of rubidium’s internal structure. This new research paints a simpler picture of the atom-fiber inter­actions, and the team says more research is needed to understand the discre­pancies.

“We believe this work is an important step in the on-going quest for a better under­standing of the inter­action between light and atoms near a nanoscale light-guiding structure, such as the optical nanofiber we used here,” says JQI Fellow and NIST scientist William Phillips, who is also one of the lead inves­tigators on the study. (Source: JQI)

Reference: P. Solano et al.: Alignment-dependent decay rate of an atomic dipole near an optical nanofiber, Phys. Rev. A 99, 013822 (2019); DOI: 10.1103/PhysRevA.99.013822

Link: Joint Quantum Institute, University of Maryland, College Park, USA

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