NIST miniaturizes laser cooling

Technology News | January 25, 2021

By Rich Pell

IoT Wireless Communications Optoelectronics Analog Sensing / Conditioning Materials & processes Test and Measurement

Cooling atoms is equivalent to slowing them down, which enables researchers to measure the particles’ energy transitions and other quantum properties accurately enough to use as reference standards in a myriad of navigation and other devices. Scientists have traditionally cooled atoms by bombarding them with properly prepared laser light, which typically requires an optical assembly as big as a dining-room table.

As a result, say the researchers, it limits the use of these ultra-cold atoms outside the laboratory, where they could become a key element of highly accurate navigation sensors, magnetometers. and quantum simulations. The new NIST compact optical platform, on the other hand, is only about 15 centimeters (5.9 inches) long and cools and traps gaseous atoms in a 1-centimeter-wide region.

Although other miniature cooling systems have been built, say the researchers, this is the first one that relies solely on flat, or planar, optics, which are easy to mass produce.

“This is important as it demonstrates a pathway for making real devices and not just small versions of laboratory experiments,” says NIST researcher William McGehee.

While the new optical system is still about 10 times too big to fit on a microchip, say the researchers, it is a key step toward employing ultra-cold atoms in a host of compact, chip-based navigation and quantum devices outside a laboratory setting. The apparatus consists of three optical elements.

First, light is launched from an optical integrated circuit using a device called an extreme mode converter, which enlarges the narrow laser beam, initially about 500 nanometers (nm) in diameter, to 280 times that width. The enlarged beam then strikes a carefully engineered, ultrathin film “metasurface” that’s studded with tiny pillars, about 600 nm in length and 100 nm wide.

The nanopillars act to further widen the laser beam by another factor of 100. The dramatic widening is necessary for the beam to efficiently interact with and cool a large collection of atoms. Moreover, by accomplishing that feat within a small region of space, the metasurface miniaturizes the cooling process.

The metasurface, say the researchers, reshapes the light in two other important ways: by simultaneously altering the intensity and polarization (direction of vibration) of the light waves. Ordinarily, the intensity follows a bell-shaped curve, in which the light is brightest at the center of the beam, with a gradual falloff on either side.

The researchers designed the nanopillars so that the tiny structures modify the intensity, creating a beam that has a uniform brightness across its entire width. The uniform brightness allows more efficient use of the available light. Polarization of the light is also critical for laser cooling.

The expanding, reshaped beam then strikes a diffraction grating that splits the single beam into three pairs of equal and oppositely directed beams. Combined with an applied magnetic field, the four beams, pushing on the atoms in opposing directions, serve to trap the cooled atoms.

Each component of the optical system – the converter, the metasurface and the grating – was developed at NIST but was in operation at separate laboratories on the two NIST campuses, in Gaithersburg, Maryland and Boulder, Colorado. McGehee and his team brought the disparate components together to build the new system.

“That’s the fun part of this story,” he says. “I knew all the NIST scientists who had independently worked on these different components, and I realized the elements could be put together to create a miniaturized laser cooling system.”

Although the optical system will have to be at least 10 times smaller to laser-cool atoms on a chip, say the researchers, the experiment is proof of principle that it can be done. For more, see “Magneto-optical trapping using planar optics.”

NIST