Abstract
The modulation of near-field signals has recently attracted considerable interest because of demands for the development of nano-scale optical devices that are capable of overcoming the diffraction limit of light. In this paper, we propose a new type of tuneable plasmonic lens that permits the foci of surface plasmon polariton (SPP) signals to be continuously steered by adjusting the input polarization state. The proposed structure consists of multi-lined nanoslit arrays, in which each array is tilted at a different angle to provide polarization sensitivity and the nanoslit size is adjusted to balance the relative amplitudes of the excited SPPs from each line. The nanoslits of each line are designed to focus SPPs at different positions; hence, the SPP focal length can be tuned by modifying the incident polarization state. Unlike in previously reported studies, our method enables plasmonic foci to be continuously varied with a smooth change in the incident linear polarization state. The proposed structures provide a novel degree of freedom in the multiplexing of near fields. Such characteristics are expected to enable the realization of active SPP modulation that can be applied in near-field imaging, optical tweezing systems, and integrated nano-devices.
Highlights
Continuously tuneable devices; only a partial range of phases can be modulated, or it is difficult to continuously vary the external signal
To the best of our knowledge, no research has been reported in which the continuous tuning of the foci of plasmonic lenses has been achieved by using omnidirectional linear polarization angles
A near-infrared laser with a wavelength of 980 nm illuminated the back side of the silver film after passing through a quarter-wave plate (QWP), a half-wave plate (HWP), a mirror, and a linear polarizer
Summary
Continuously tuneable devices; only a partial range of phases can be modulated, or it is difficult to continuously vary the external signal. To the best of our knowledge, no research has been reported in which the continuous tuning of the foci of plasmonic lenses has been achieved by using omnidirectional linear polarization angles. We propose tuneable plasmonic lenses that continuously control the foci of SPPs by changing the incident linear polarization state. Lateral shifts of the nanoslits generate the SPP focus and determine the focal length for a specific incident polarization state. By means of these accurate designs, the SPP focus can be continuously steered along an arbitrary trajectory with the continuous rotation of the linearly polarized incident light, as the line that dominantly excites the SPPs changes. We fabricated the proposed structure on a silver film and measured the near-field profile using near-field scanning optical microscopy (NSOM)
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