Nanoscale optical antennas are nothing new. Their behavior when driven near and far from resonance has been well-understood for over a decade. But there remained a key physical trait of these resonators that had been left relatively unexplored up until now: their phase-response.
“It’s difficult to measure phase [of an electromagnetic wave] directly,” said Mikhail Kats, a graduate student in the Capasso Group and a researcher on the project. “Amplitude you can measure directly, you just put a detector in front of it. It’s quite a bit harder to experimentally play with phase.”
But researchers in the group discovered there was incredible power and precise control afforded if they could do exactly that. Light which scatters off each optical resonator acquires a phase depending on the orientation and geometry of the antenna. Find a set of resonators able to provide the full range of possible phase shifts and you can build up any arbitrary wave-front pixel-by-pixel.
This was somewhat tricky as conventional nanoscale antennas have only limited phase response. The group got around this fact by building custom V-shaped resonators with varying interior angles. “If you look at the phase response of a linear antenna, it only goes from 0 to pi,” Kats said. “By exploiting the fact that there are two resonances and two orthogonal polarizations [in our custom antennas] we were able to design a structure that could control the phase of light from 0 to 2pi.”
The rest is just tiling. Set down a linear phase gradient, and one could send normally incident light reflecting and refracting into arbitrary, anomalous angles. Choose an azimuthally varying phase, and one could generate a vortex beam. “We can design light in an almost unnatural way. At each pixel you can control the amplitude, phase and polarization of light,” Kats said. Arrays of these structures are capable of reproducing almost any optical component out there for almost any wavelength light.
But unlike conventional optics, which rely on the principles of refraction or diffraction, and in turn imply a certain minimum operational thickness on the order of several wavelengths, these plasmonic devices are effectively infinitely thin. The beam is already fully formed less than a wavelength above the array.
This work, beginning about two years ago in a collaboration between post-docs Nanfang Yu and Patrice Genevet, graduate students Mikhail Kats and Francesco Aieta and visiting scholar Zeno Gaburro, currently seeks the creation of a variety of ultra-thin optical components — for focusing, polarizing, redirecting light — to augment or even eventually replace their optical counterparts. The work was the cover of the October 21st issue of Science Magazine. The full list of relevant publications is below.
- Light propagation with phase discontinuities: Generalized laws of reflection and refraction
Nanfang Yu, Patrice Genevet, Mikhail A. Kats, Francesco Aieta, Jean-Philippe Tetienne, Federico Capasso, Zeno Gaburro
Science 334, 333 (2011)
- Effect of radiation damping on the spectral response of plasmonic components
Mikhail A. Kats, Nanfang Yu, Patrice Genevet, Zeno Gaburro, Federico Capasso
Optics Express 19, 21749 (2011)
- Ultra-thin plasmonic optical vortex plate based on phase discontinuities
Patrice Genevet, Nanfang Yu, Francesco Aieta, Jiao Lin, Mikhail A. Kats, Romain Blanchard, Marlan O. Scully, Zeno Gaburro, Federico Capasso
Applied Physics Letters 100, 13101 (2012)
- Out-of-plane reflection and refraction of light by anisotropic optical antenna metasurfaces with phase discontinuities
Francesco Aieta, Patrice Genevet, Nanfang Yu, Mikhail A. Kats, Zeno Gaburro, Federico Capasso
Nano Letters 12, 1702 (2012)
- Modeling nanoscale V-shaped antennas for the design of optical phased arrays
Romain Blanchard, Guillaume Aoust, Patrice Genevet, Nanfang Yu, Mikhail A. Kats, Zeno Gaburro, Federico Capasso
Physical Review B 85, 155457 (2012)
- Giant birefringence in optical antenna arrays with widely tailorable optical anisotropy
Mikhail. A. Kats, Patrice Genevet, Guillaume Aoust, Nanfang Yu, Romain Blanchard, Francesco Aieta, Zeno Gaburro, Federico Capasso
PNAS, in press.
- Tiny Antennas Could Have Big Impact – Technology Review