A four-fold increase in the scattering of microwave light has been observed from a cylindrical rod structured on the subwavelength scale. The “superscattering” structure could offer exciting new opportunities for applications ranging from highly-efficient antennas to devices that harvest energy from infrared radiation.
Light scattering is a well known optical phenomenon that happens when light interacts with structures smaller than its wavelength. The incident light perturbs electrons in the structure to form oscillating dipole moments that re-emit the light in many different directions. The intensity and direction of this scattered light depend both on the incident wavelength and specific properties of the structure, including its size, shape, and refractive index. Ultimately, these values are quantified by a parameter known as the scattering cross-section – which, until recently, appeared to have a fundamental upper limit.
In a 2010 study, however, physicists Zhichao Ruan and Shanhui Fan at Stanford University in the US proposed that the supposed limit in scattering cross-section could be overcome by inducing oscillations of higher-order electric moments in the structures, including quadrupoles. If these additional modes have the same resonant frequency as the dipole oscillations, Ruan and Fan said that they could work together to increase the overall scattering cross-section.
To induce these oscillations, the duo proposed a theoretical design for a metamaterial rod containing multiple nanolayers. However, their structure was too complex to build experimentally, and would have led to a loss of scattering power due to finite metal conductivity at optical wavelengths. Now, an international team led by Hongsheng Chen at Zhejiang University in China, and Baile Zhang at Nanyang Technological University in Singapore, has used new calculations to further investigate the ideas raised in Ruan and Fan’s research.
In their study, the team fabricated a rod made from three concentric cylindrical metasurfaces, separated by dielectric materials. These metasurfaces consisted of copper strips with far smaller widths and periodicities than microwaves, allowing surface waves with microwave frequencies to form when illuminated with microwave light. At a result, light scattered by the structure should be polarized along the length of the rod, which minimizes power loss. By fine-tuning the periodicity and width of the strips, the team ensured that the metasurfaces could support dipolar and quadrupolar modes resonating at the same frequency.
Chen and colleagues tested their rod using microwaves of two different frequencies, and clearly demonstrated superscattering in both the near- and far-field regions. Compared with a continuous copper rod of the same size and shape, their induced scattering cross-sections were four times greater than those achievable with dipole moment oscillations alone.
The result shows a substantial improvement in light–matter interactions on subwavelength scales. Chen’s team proposes that the high scattering efficiency and directionality produced by their rod could allow it to be used as a highly-efficient antenna, for applications including wireless communication, data transmission, and remote sensing. In the future, the researchers hope to expand the rod’s superscattering abilities to infrared wavelengths, which could lead to extensive opportunities in areas such as imaging, sensing, optical displays, and energy harvesting.
- Full details are reported in Physical Review Letters
