Combining cutting-edge nanotechnology with holograms results in a novel way to change the intensity, phase and polarization of light rays. Researchers at Harvard’s School of Engineering and Applied Sciences (SEAS) used the holograms to create an unusual state of light called a radially polarized beam. These beams, which span the visible and near-IR spectra, are important for applications such as high-resolution lithography as well as trapping and manipulating tiny particles such as viruses. This is the first time a single simple device has been designed to control the intensity, phase and polarization of light at once. “Our lab works on using nanotechnology to play with light,” said SEAS research associate Patrice Genevet. “In this research, we’ve used holography in a novel way, incorporating cutting-edge nanotechnology in the form of subwavelength structures at a scale of just tens of nanometers.” (Photo on left) A holographic component was fabricated by ion milling with a focused ion beam a 150-nm-thick gold film deposited on a glass substrate. A laser beam is partially transformed into a radially polarized beam as it traverses the device. The wide grooves create the donut-shaped intensity profile, known as a vortex, while the subwavelength nanometer grooves in the inset determine locally the radial polarization, which is perpendicular to the grooves. (Photo in the middle) The computed characteristic beam cross section: The blue arrows indicate the radial polarization. (Photo on right) Federico Capasso of Harvard University. Photo courtesy of Federico Capasso. The new device resembles a normal hologram grating with an additional nanostructured pattern carved into it. Visible light interacts differently with apertures textured on the nanoscale than with those on the scale of microns or larger. By exploiting these behaviors, the modular interface can bend incoming light to adjust its intensity, phase and polarization. The polarization effect that the new interface has on light could formerly only be achieved by a cascade of several different optical elements. “Now, you can control everything you need with just a single interface,” Genevet said. “We’re gaining a big advantage in terms of saving space.” In recent years, Genevet and researchers in the SEAS laboratory of Federico Capasso have focused on nanophotonics with the goal of creating new light beams and special effects that arise from the interaction of light with nanostructured materials. “When light is radially polarized, its electromagnetic vibrations oscillate inward and outward from the center of the beam like the spokes of a wheel,” Capasso said. “This unusual beam manifests itself as a very intense ring of light with a dark spot in the center. “It is noteworthy that the same nanostructured holographic plate can be used to create radially polarized light at so many different wavelengths. Radially polarized light can be focused much more tightly than conventionally polarized light, thus enabling many potential applications in microscopy and nanoparticle manipulation.” The research was published in Nano Letters (doi: 10.1021/nl402039y). Patrice Genevet participated in a Photonics Media webinar, “Developments in Optics and Optical Components,” on the topic of photonic metasurfaces, in May. Visit photonics.com to watch on demand.
