Quantum Physics

Using spatially structured polarized light to create new structured spin states for information storage.

Electric and magnetic fields that oscillate perpendicular to one another make up light. Polarized light is produced when these oscillations are constrained, for example, along a plane. In optical communications, polarized light plays a significant role and has the potential to similarly revolutionize information storage.

Information is stored in current electronic devices using electronic charge. However, spin, an electron-specific quantum property, provides an alternative. Polarized light can be used to control the spin and store information. Spin-polarized electrons, or spins aligned in a particular direction, are produced when a polarized light beam interacts with electron spins in a semiconductor. Up to this point, just consistently energized light, i.e., light with a spatially uniform polarization, has been taken advantage of to control electron turns. However, electron spins that are spatially structured can be produced by the polarization if it has an additional spatial structure (variation), opening up new storage methods.

To this end, a gathering of scientists, led by junior academic partner Jun Ishihara and including graduate understudy Takachika Mori, graduate understudy Takuya Suzuki (at the hour of the exploration), and Teacher Kensuke Miyajima from Tokyo College of Science (TUS), Japan, has now devised a strategy for creating such spatially organized electron turns utilizing an organized light with a spatially fluctuating polarization profile. Researchers from Chiba University, Tohoku University, and Tsukuba University in Japan collaborated on the study, which was published in the journal Physical Review Letters.

“We used vortex half-wave plate and quarter-wave plate devices to create a doughnut-shaped structured light—a vector optical vortex beam with orbital angular momentum (OAM)—from a basic Gaussian beam. The electron spins trapped within a gallium arsenide/aluminum gallium arsenide semiconductor quantum well were then excited using this beam. In turn, these spins produced a helical spatial structure in the form of a circle.”

Dr. Ishihara.

“Using vortex half-wave plate and quarter-wave plate devices, we created a structured light in the shape of a doughnut—a vector optical vortex beam with an orbital angular momentum (OAM)—from a basic Gaussian beam in this work. The electron spins contained within a gallium arsenide/aluminum gallium arsenide semiconductor quantum well were then excited by this beam. “In turn, these spins created a helical spatial structure in the shape of a circle,” Dr. Ishihara explains.

Interestingly, the beam with an OAM number of two produced a helix with four such alterations, whereas the beam with an OAM number of one produced a helix with two spin periods—spin up and spin down—around the circle. These findings suggested that the semiconductor’s electron spins absorbed the optical vortex’s spatial polarization structure, as determined by the OAM. A higher spin repetition rate around the circle was also suggested as a way to increase information storage capacity by increasing the OAM number.

In addition, the researchers used a single beam to simultaneously generate two spin waves with opposing phases in the vertical direction by utilizing the effective magnetic field of the spin-orbit interaction acting on electron spins in the quantum well. This suggested that structured light beams and effective magnetic fields, which are found in solids, could be used to create a variety of spin states with spatial structures.

The researchers discuss their work’s potential for the future in light of these exciting findings. “Elemental technologies for higher-order quantum media conversion and information capacity enhancement using spin textures are expected to result from the conversion of the spatial polarization structure of light into a spatial structure of spin, as well as the generation of new spin spatial structures in combination with effective magnetic fields in solids,” says Dr. Ishihara.

More information: Jun Ishihara et al, Imprinting Spatial Helicity Structure of Vector Vortex Beam on Spin Texture in Semiconductors, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.130.126701

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