Using light waves instead of electric current to transfer data, photonic microcircuits for light – advanced basic research in many areas from timekeeping to telecommunications. But for many applications, the narrow beams of light that intersect these circuits must be significantly extended in order to connect to larger systems outside the crystal. Wider light rays can increase the speed and sensitivity of medical images and diagnostic procedures, security systems that detect trace amounts of toxic or volatile chemicals and devices that depend on the analysis of large groups of atoms.
Scientists at the National Institute of Standards and Technology (NIST) have developed a highly efficient converter that increases the diameter of the light beam 400 times. NIST physicist Vladimir Aksyuk and his colleagues, including researchers from the University of Maryland NanoCenter at College Park, Maryland and Texas Tech University in Lubbock, spoke about their work in the journal Light: science and applications,
The transducer expands the cross-section or area of the beam in two successive stages. Initially, light passes through an optical waveguide, a thin transparent channel whose optical properties limit the beam diameter to several hundred nanometers, less than one thousandth of the average length of a human hair. Because the waveguide channel is so narrow, a portion of the moving light extends outward beyond the edges of the waveguide. Using this broadening, the team placed a rectangular plate consisting of the same material as the waveguide, at a thin, accurately measured distance from the waveguide. The light can jump over the tiny gap between the two components and gradually seep into the slab.
The slab maintains a narrow width of light in a vertical (top to bottom) size, but does not provide such limitations for side or side measurement. As the gap between the waveguide and the plate gradually changes, the light in the plate forms a precisely directed beam 400 times wider than a beam diameter of about 300 nm.
In the second expansion stage, which increases the vertical size of the light, the beam passing through the slab collides with the diffraction grating. This optical device has periodic solutions or lines, each of which scatters light. The team developed the depth and distance between the rows in order to vary, so that the light waves combined to form a single wide beam, directed almost at a right angle to the chip surface.
It is important to note that the light remains collimated or exactly parallel in the whole two-stage expansion process, so that it remains on the target and does not spread. The area of the collimated beam is now large enough to cover the large distance required to study the optical properties of large diffuse groups of atoms.
Working with a team led by John Kitching from NIST in Boulder, Colorado, researchers have already used a two-stage converter to successfully analyze the properties of about 100 million gaseous rubidium atoms when they jumped from one energy level to another. This is an important proof of the concept, since devices based on the interaction between light and atomic gases can measure quantities such as time, length, and magnetic fields, as well as applications in navigation, communications, and medicine.
“Atoms move very fast, and if the beam controlling them is too small, they move so fast and come out of the beam that they are difficult to measure,” said Kitching. “With large laser beams, the atoms stay in the beam longer and allow more accurate measurement of the atomic properties,” he added. Such measurements can lead to improved wavelength and time standards.
Trap atoms, not spaceships, with tractor rays
Sangsik Kim et al. The photon waveguide in the converter of the extremal regime of a Gaussian beam in free space, Light: Science and Applications (2018). DOI: 10.1038 / s41377-018-0073-2