Light is ubiquitous and vital, but also incredibly strange - and it's possible we'll never exhaust the opportunities to learn more about it. Case in point: researchers at Harvard have developed a material that can generate and maintain completely new and more complex states of light. The tool uses polarization to generate structures such as swirling vortices, spirals, and corkscrews that not only help explore light's properties, but also have potential practical applications, such as high-powered imaging. Discoveries about light are still being made. It was only in 2015 that scientists took the first-ever photograph of light behaving as both a particle and a wave.
And it hasn't even been that long - just 1992, 25 years ago - since light was discovered to have orbital angular momentum. This is angular momentum based on the shape of its wavefront, rather than its orientation. The new tool - a type of metasurface - uses this along with second type of angular momentum called spin angular momentum (also known as circular polarization).
Harvard's Leah Burrows writes in a statement, "Think about orbital angular momentum and circular polarization like the motion of a planet. Circular polarization is the direction in which a planet rotates on its axis while orbital momentum describes how the planet orbits the sun."
It's previously been established that a single beam of light can exhibit both types of angular momentum, and that connecting them and using polarization to control the OAM can result in beams with new and complex shapes, such as the aforementioned corkscrew. According to the researchers, until now there was a significant limit on this. Only certain polarizations could connect to certain OAMs. This is where the new tool comes in - it allows any polarization to be converted to any OAM, which means it can create spirals and corkscrews and vortices of any size.
Co-first researcher Antonio Ambrosio, Principal Scientist at Harvard Center for Nanoscale Systems said, "This is a completely new optical component. Some meta-surfaces are iterations or more efficient, more compact versions of existing optical devices but, this arbitrary spin-to-orbital conversion cannot be done with any other optical device. There is nothing in nature as well that can do this and produce these states of light."
(Capasso Lab/Harvard SEAS)
Orbital angular momentum already has several proposed uses, such as high-speed data transfer, and encoded communications. Researchers have even figured out how to transmit the OAM of individual photons using entanglement. Other previously proposed applications include the manipulation of microscopic objects, and imaging systems. This is where Harvard's device could prove practical. The meta-surface could be used to shape optical tweezers to manipulate objects as small as atoms and molecules. Changing the polarization could change the direction of the applied force. It could also be used for high-powered imaging, because the black hole down the center of the vortex can be used to take images of features smaller than the diffraction limit, the researchers said.
Co-first researcher Noah Rubin explained, "There is interest in these beams in quantum optics and quantum information. On the more applied side, these beams could find application in free-space optical communication especially in scattering environments where this is usually difficult. Moreover, it has been recently shown that similar elements can be incorporated into lasers, directly producing these novel states of light. This may lead to unforeseen applications."