Atoms are not always small. When excited to a Rydberg state—in which the outermost electron is placed in a diffuse orbital that’s barely bound to the nucleus—an atom can have macroscopic dimensions of a millimeter or more. Their large size and polarizability make Rydberg states an important tool of atomic physics because they allow researchers a way to controllably switch interactions on and off between atoms in neighboring optical traps. (See the article by David Weiss and Mark Saffman, Physics Today, July 2017, page 44.)
But if an atom is to be stably excited into a Rydberg state, it needs to be held at least several millimeters away from any solid surface, including the surface of the lens that’s used to focus the trapping light. For a typical trap size of 1 µm, that’s like confining an object to a spot the size of a golf ball from the opposite end of a football field.
Conventional optics can meet that demanding requirement. But the complex multielement or aspheric lenses that are tailor-made for each new experiment take months to design and manufacture. And because they take up a lot of space, they often can’t fit inside the vacuum chamber with the atoms, so their focal lengths must be even longer.
Now Cindy Regal of JILA in Boulder, Colorado, and her colleagues have shown that there might be another way. They’ve created an array of optical traps for single rubidium atoms by focusing light not with smoothly curved glass lenses but with a metasurface, the orange square in the photo above. As shown in more detail in the figure below, the metasurface is made of a thin layer of fused silica topped with subwavelength-sized pillars of amorphous silicon. When the pillars scatter incident laser light, they create an interference pattern that mimics the focusing effect of a conventional lens.
Metasurface optics have been used for years to focus and manipulate light. (See the article by Kai Wang, Maria Chekhova, and Yuri Kivshar, Physics Today, August 2022, page 38.) But designing them to the specifications of atom-trapping experiments was uncharted territory, largely because atomic physics and metasurface photonics are the domains of separate research communities. Scott Papp, Regal’s collaborator at NIST in Boulder, recognized the needs and capabilities of both fields. So he connected her cold-atom research group with Amit Agrawal, of the photonics and optomechanics group at NIST in Gaithersburg, Maryland, who fabricated the metasurface.
At just 4 mm on a side, the metasurface lens is easily small enough to fit inside a vacuum chamber, and its focal length of 3 mm and focus size of 1.1 µm or less are suitable for atom trapping. Because metasurfaces can be designed and fabricated more quickly and in larger numbers than conventional multielement lenses, they could make it much easier for atomic physicists to adapt their labs to new experiments. (T.-W. Hsu et al., PRX Quantum 3, 030316, 2022.)
