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New device expands light beams 400 times, expanding scientific and technological possibilities

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By using light waves instead of electrical current to transmit data, photonic chips, circuits for light, have advanced basic research in many fields from time measurement to telecommunications. But for many applications, the narrow beams of light that pass through these circuits need to be significantly expanded to connect to larger, chipless systems. Larger beams of light could increase the speed and sensitivity of medical imaging and diagnostic procedures, security systems that detect traces of toxic or volatile chemicals, and devices that rely on large-group analysis. d & # 39; atoms.

Scientists from the National Institute of Standards and Technology (NIST) have come up with a highly efficient converter that can expand 400 times the diameter of a light beam. NIST physicist Vladimir Aksyuk and his colleagues, including researchers from the University of Maryland's NanoCenter University in College Park, Maryland, and Texas Tech University in Lubbock, described their work in the journal Light: science and applications.

The converter widens the beam section or surface in two consecutive steps. Initially, the light travels through an optical waveguide, a thin transparent channel whose optical properties limit the beam's diameter to a few hundred nanometers, less than one-thousandth of the average diameter of a hair. human. As the channel of the waveguide is so narrow, some of the light that travels extends outward beyond the edges of the waveguide. Taking advantage of this expansion, the team placed a rectangular slab made of the same material as the waveguide, at a tiny and accurately measured distance from the waveguide. The light can pass through the small space between the two components and infiltrate the slab.

The slab maintains the narrow width of the light in the vertical dimension (from top to bottom), but it does not provide such stresses for the lateral or lateral dimension. As the gap between the waveguide and the slab changes progressively, the light in the slab forms a precisely directed beam 400 times wider than the diameter of about 300 nm. of the original beam.

In the second stage of expansion, which enlarges the vertical dimension of light, the beam passing through the slab meets a diffraction grating. This optical device has rules or periodic lines, each diffusing light. The team designed the depth and spacing of the rulers to ensure that the light waves combine, forming a single, wide beam directed almost at right angles to the surface of the chip.

It is important to note that the light remains collimated, or precisely parallel, throughout the two-stage extension process, so that it stays on the target and does not spread out. The area of ​​the collimated beam is now large enough to travel the long distance required to probe the optical properties of large diffuse groups of atoms.

Working with a team led by John Kitching of NIST in Boulder, Colorado, researchers have already used the two-step converter to successfully analyze the properties of some 100 million gaseous rubidium atoms while they went from one level of energy to the other. This is an important proof of concept, since devices based on the interactions between light gases and atomic gases can measure quantities such as time, length and magnetic fields and have applications in navigation. communications and medicine.

"The atoms move very fast and if the beam that monitors them is too small, they come in and out of the beam so quickly that it becomes difficult to measure them," Kitching said. "With large laser beams, the atoms stay longer in the beam and allow a more accurate measurement of atomic properties," he added. Such measures could improve wavelength and time standards.

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More information:
Sangsik Kim et al. Photonic waveguide to Gaussian extreme beam mode converter in free space, Light: Science & Applications (2018). DOI: 10.1038 / s41377-018-0073-2

Journal reference:
Light: Science & Applications

Provided by:
National Institute of Standards and Technology

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