Plasmonic metamaterials

Nanoantennas

Optical nanoantennas efficiently convert confined optical energy into free-space radiation. Although plasmonic nanoantennas typically make use of fields confined in dimensions much smaller than the light wavelength, there exists also the possibility to use guided waves in low-loss waveguides. This way, optical nanoantennas would become completely identical to their RF counterpart. Typically, the optical nanoantennas considered so far generate linearly-polarized fields with the electric field confined along the nanoantenna plane. Within this activity, we have been designing optical nanoantennas fed by silicon waveguides in order to generate and detect complex polarization states, such as circular or even elliptical polarization.

Figure 1

Results

Optical nanoantennas generally radiate a fixed polarization at a given direction, which is determined by their geometrical structure. As so, synthesizing a desired polarization is generally unfeasible unless the nanoantenna geometry is mechanically changed. However, this situation can be changed if we consider the following scenario (Fig. 1): a rectangular-shaped nanoantenna attached to a silicon waveguide emits (or detects) linearly polarized waves (for the normal direction) at ±45º depending on the input waveguide [1]. Therefore, antiparallel feeding the antenna by injecting light at opposite sides of the waveguide (with complex amplitudes α and β) allows that a linear combination of linearly polarized states can be emitted [2]. As a result, this system implements a universal method to achieve the synthesis of any arbitrary polarization state of radiated light. The reciprocal situation is also very useful [1]: light impinging in the nanoantenna from the normal direction excites its two output waveguides, with each linear polarization component (at +45° and -45°) of the illumination being sorted into either waveguide. This mechanism allows the analysis of the polarization of the incident light just by looking at the amplitude and phase of the output waveguide excitations. This approach, which inspired by the previous demonstration of directional plasmon guiding on metal surfaces using circularly polarized light [3], opens the door towards the full manipulation of polarization in an integrated platform.

In [1], [2], we have demonstrated a new method to generate arbitrary polarization states with a single nanoantenna coupled to a waveguide. In reception, the nanoantenna can properly sort orthogonally polarized incoming photons. Although the method is universal and can be extended to any wavelength and technology, we perform our demonstrations in a silicon photonics platform, potentially allowing the fast electrical modulation of the amplitude and phase of the feeding signals to achieve an on-chip tuneable polarization synthesizer/analyzer. Figure 2 schematizes our experimental results showing both the radiation of arbitrary polarization states as well as the sorting of linearly polarized photons.

Silicon nanoantenna

Figure 1: Polarization synthesis from a single silicon nanoantenna coupled to a waveguide (picture taken from Ref [2]). A dual-input nanoantenna radiating two orthogonal polarizations (a) and (b) when fed from each of its two inputs. The simultaneous feeding of both inputs results in a superposition of both cases (c).

Experimental results

Figure 2: Experimental results taken from Refs. [1], [2]. (a) Schematic (not to scale) of the polarization synthesis experiment. (b) Experimental measurement results. (c) Schematic of the polarization sorting experiment and experimental results (inset). (d) Scanning electron microscope (SEM) photograph of the fabricated nanoantennas.

Notice that Ref. [2] has been chosen as the Front cover for the May 2014 issue of the journal Laser and Photonics Reviews.

Front cover

References

[1] "Sorting linearly polarized photons with a single scatterer", F. J. Rodríguez-Fortuño, D. Puerto, A. Griol, L. Bellieres, J. Martí, and A. Martínez, OPTICS LETTERS Vol. 39, Issue 6, pp. 1394-1397, March 2014.

[2] "Universal method for the synthesis of arbitrary polarization states radiated by a nanoantennas", F. J. Rodríguez-Fortuño, D. Puerto, A. Griol, L. Bellieres, J. Martí, and A. Martínez, LASER & PHOTONICS REVIEWS, Vol. 8, Issue 3, pp. L27–L31, May 2014.

[3] "Near-Field Interference for the Unidirectional Excitation of Electromagnetic Guided Modes", F. J. Rodríguez-Fortuño, G. Marino, P. Ginzburg, D. O’Connor, A. Martínez, G. A. Wurtz, A. V. Zayats, SCIENCE, Vol. 340, Issue 6130, pp. 328-330, April 2013.