A hybrid spiral plasmonic lens that consists of alternating spiral slot and spiral triangular sub-aperture array can differentiate circular polarization of different handedness and enable a miniature circular polarization analyzer design with high efficiency. The improved performance compared to pure spiral slot lens comes from the fact that the hybrid lens is capable of focusing both the radial and the azimuthal polarization components of a circular polarization, doubling the coupling efficiency. In this paper, the spin-dependent plasmonic focusing properties of a spatially arranged triangular sub-aperture array and a hybrid spiral plasmonic lens are demonstrated using a collection mode near field scanning optical microscope. The coupling efficiency could be further improved through optimizing the geometry of the hybrid lens.
©2012 Optical Society of America
Plasmonic focusing draws continuous research interests due to the strong field enhancement offered by highly localized plasmonic fields. It has been demonstrated both numerically and experimentally that optimal plasmonic focusing can be achieved through matching radially polarized illumination to metallic structures with rotational symmetry [1–4]. Non-diffracting evanescent Bessel beam and longitudinally polarized optical needle field have been obtained with planar and Bull’s eye plasmonic lenses [1,4]. However, the alignment requirement of the local polarization of the illumination to the plasmonic structures prevents it from applying to parallel imaging. Therefore, circular polarization was proposed as an alternative for surface plasmon excitation and focusing. Circular polarization can be decomposed into the combination of radial polarization and azimuthal polarization components with a vortex phase with topological charge of +/−1 . It has been shown that spiral slot lens can focus circular polarization of different handedness into spatially separated plasmonic field [6–9]. If the geometric phase provided by the spiral lens cancels the vortex phase wavefront of the radial polarization component of the illumination, solid focal spot will be obtained. Otherwise, a doughnut spot with a dark center will be observed. Such a spin-dependent plasmonic focusing phenomenon has been exploited in the design of miniature circular polarization analyzer . Due to the elimination of the alignment requirement, the spiral slot plasmonic lens also enables parallel focusing and sensing through arranging the spiral slot in an array format [8,9]. However, in these designs, only the radially polarized component contributes to plasmonic focusing since the azimuthally polarized component is TE polarized with respect to the slot and does not couple to the surface plasmon excitation, leading to a loss of half of the incoming power. In order to harvest the other half power carried by the azimuthal polarization component, spatially arranged triangular sub-aperture array was proposed to improve the plasmonic focusing efficiency . Owing to a geometric phase effect, an isosceles triangular aperture etched into thin metal film may lead to a constructive or destructive interference of surface plasmons excited at the two equal sides under a linearly polarized illumination, depending on the relative orientation of the triangular aperture with respect to the local linear polarization. Through appropriate spatial arrangement of an array of triangular sub-apertures, highly confined focal spot beyond the diffraction limit can be achieved at the geometric center under azimuthally polarized excitation with field enhancement comparable to Bull’s eye plasmonic lens under radially polarized illumination . Similarly, if the triangular sub-aperture array is arranged along a spiral, it can focus circular polarization of one handedness while defocusing the other handedness . The spatially arranged triangular sub-aperture array is capable of focusing both the azimuthal polarization and the radial polarization components of a circularly polarized illumination, while the azimuthal polarization component has higher coupling efficiency. Through combining a spiral slot lens and a spiral triangular sub-aperture array, both polarization components of the circularly polarized illumination can be strongly coupled into surface plasmon excitation. Numerical simulation shows that a hybrid lens with alternating spiral slot and spiral triangular sub-aperture array can increase the power efficiency by 94.69% compared to pure spiral slot .
In this paper, we explored the experimental confirmation of plasmonic focusing properties of spatially arranged triangular sub-aperture array under azimuthally polarized illumination with a collection mode near field scanning optical microscope (NSOM). The plasmonic lens was also found to be able to produce plasmonic focusing under radial polarization illumination. Through simply rotating the orientation of each triangular sub-aperture by 90°, the plasmonic structure defocuses the same azimuthal polarization illumination due to the destructive interference caused by a geometric π-phase difference between the two sides of the triangular sub-aperture and between the adjacent sub-apertures. Then we experimentally verified that spiral triangular sub-aperture array and hybrid lens can serve as a miniature circular polarization analyzer with high efficiency. A focal plane array detector integrated with metal wire grids and hybrid plasmonic lenses can be used to extract the full Stokes parameters for polarimetric imaging applications .
2. Circularly arranged triangular sub-aperture array
The diagram of the experimental setup is illustrated in Fig. 1 . An azimuthally polarized beam was generated through coupling a charge + 1 vortex beam into a few-mode fiber with high efficiency and stability . The optical excitation wavelength was chosen to be 532 nm (Coherent DPSS Nd:YAG second harmonic laser). A 150 nm gold film was deposited onto a glass substrate by e-beam evaporation. This thickness was chosen to prevent far field direct transmission of the laser through the silver layer. Plasmonic lenses that consist of spatially arranged triangular sub-apertures were fabricated with focused ion beam milling (FIB, FEI dual beam SEM-FIB NOVA 200 Nanolab system). Figure 2 shows the SEM images of a triangular sub-aperture array arranged in antisymmetric and symmetric modes. The plasmonic lens structures have 32 isosceles triangular sub-apertures arranged along a circle with a radius of 3 µm. Each triangular sub-aperture has a base length of 400 nm and a height of 400 nm. For antisymmetric triangular sub-aperture array, the axis of symmetry of each triangular sub-aperture is aligned to the azimuthal direction. Under an azimuthal polarization illumination, at each triangular sub-aperture, the local polarization of the illumination is polarized along the axis of symmetry. Therefore, the surface plasmons excited at the two equal sides of a single triangular sub-aperture are in phase. Due to the axial symmetry of both the metallic structure and optical excitation geometry, surface plasmon waves excited by the triangular sub-apertures at all azimuthal directions constructively interfere with each other at the geometrical center, producing a highly confined spot with stronger field enhancement. Because the plasmonic focal field is mostly polarized vertically to the surface, extra caution needs to be taken in order to interpret the NSOM imaging results. The surface plasmon intensity distribution was directly imaged by a collection mode NSOM (Veeco Aurora 3) using a metal coated fiber probe with a nominal aperture size of 50-80 nm. The fiber probe is mounted on a tuning fork and shear force feedback mechanism is applied to regulate the probe/sample distance. The measured 2D intensity distribution of surface plasmon focusing along the air/gold interface of the antisymmetric triangular sub-aperture array is shown in Fig. 3 . The surface plasmons excited at all azimuthal directions propagate towards the geometric center, creating a strongly enhanced local field due to constructive interference. The focusing effect can be clearly seen as the surface plasmon interference fringes getting stronger when they are closer to the geometrical focus. Due to the symmetry of the fundamental mode in the fiber core of the NSOM probe, the detected signal of the apertured NSOM fiber probe is proportional to ||2 . Consequently, the experimental result shows a dark center as expected. If each triangular sub-aperture is rotated by 90°, the resulted symmetric plasmonic structure will defocus the azimuthally polarized illumination due to the destructiveinterference caused by the geometric π-phase difference between the two sides of a triangular sub-aperture and between the adjacent sub-apertures (Fig. 3(b)).
More interestingly, the same structures can also be applied to focus radially polarized beam. As shown in Fig. 4(a) , a plasmonic focus with the same spot size and a lower field enhancement is obtained for antisymmetric triangular sub-aperture array under radially polarized illumination. The antisymmetric triangular sub-aperture array is capable of focusing both the azimuthal polarization and the radial polarization components of a circularly polarized illumination, while the azimuthal polarization component has stronger coupling. The symmetric triangular sub-aperture array can also focus a radial polarization illumination (Fig. 4(b)). Since the local polarization of the illumination is polarized along the axis of symmetry for each triangular sub-aperture, surface plasmons generated at the two sides and adjacent triangular sub-apertures are in phase and produce constructive focusing.
3. Spiral triangular sub-aperture array and hybrid lens
The triangular sub-aperture array arranged in a circular shape can focus an azimuthally polarized illumination into a highly confined spot. However, center of the illumination must be aligned to the center of the plasmonic lens, necessitating a scanning mechanism for imaging applications. In order to achieve parallel imaging and sensing, the combination of spiral lens and circular polarization was proposed. Similarly, if the triangular sub-aperture array is arranged along a spiral, it is capable of focusing circular polarization of one handedness while defocusing the other handedness, which can serve as a circular polarization analyzer . A strongly confined solid spot will be obtained when the geometric phase produced by the spiral triangular sub-aperture array cancels out the vortex wavefront of the azimuthal polarization component of the circularly polarized illumination.
The focusing property of the spiral triangular sub-aperture array was experimentally verified. As shown in Fig. 5(a) , spiral triangular sub-aperture array was etched into a 150 nm gold film with FIB milling. Circularly polarized light illuminates the plasmonic lenses from the glass substrate side. A quarter-wave plate was used for quick switching of the circular polarization handedness. The spiral array has 32 isosceles triangular sub-apertures arranged as a left-handed Archimedes’ spiral (LHS, handedness defined from the point of view of the light source). Each triangular sub-aperture has a base length of 400 nm, a height of 400 nm, and its axis of symmetry is along the azimuthal direction. In the cylindrical coordinates, the LHS structure can be described as:Eq. (1) were set to be r0 = 2µm and Λ = λspp. The size of the overall plasmonic lens is about 5.5 µm.
With a circularly polarized illumination, surface plasmons are excited at the edges of the triangular sub-apertures. They propagate along the air/gold interface, and the field distribution in the vicinity of the spiral center strongly relies on the handedness of the circular polarization. The geometrical phase from the spiral arrangement of triangular sub-aperture array can either cancel or double the topological charge of the circularly polarized illumination. The spiral triangular sub-aperture array focuses a right-handed circular polarization (RHC) illumination into a solid spot, while defocusing a left-handed circular polarization (LHC) illumination into a doughnut spot with a dark center. Similar to the plasmonic focusing effect with a circularly arranged triangular sub-aperture array, the spiral triangular sub-aperture array can focus both the azimuthal polarization and the radial polarization components of a circularly polarized illumination, and the azimuthal polarization component has stronger coupling.
Figures 6(a) and 6(b) show the 2D NSOM images of the surface plasmon energy density distributions at the surface of a spiral triangular sub-aperture array under RHC and LHC illuminations respectively. As we expected, a doughnut spot with a dark center was observed for both RHC and LHC illuminations due to NSOM detection mechanism. However, the peakNSOM signal in the vicinity of focus with RHC illumination is higher than that of with LHC illumination. During the experiment, the illumination power was kept approximately the same since only the handedness of circular polarization was switched by rotating a quarter-wave plate by 90 degrees. Therefore, even though the shapes of the NSOM images look similar for LHC and RHC illumination, their intensity difference indicates the different focusing behavior for these two polarizations. The transverse profiles of the measured energy density distribution and the numerically computed results with a finite element method model on the air/gold interface are plotted in Fig. 6(c), showing an excellent agreement. The surface plasmon interference fringe period is measured to be 255 nm, in good agreement with theoretical prediction.
The plasmonic focusing effect from the spiral triangular sub-aperture array can be applied to increase the power conversion efficiency of the miniature circular polarization analyzer design. A hybrid plasmonic lens that consists of alternating spiral slot and spiral triangular sub-aperture array allows the strong coupling for both the radial and the azimuthal polarization components of a circularly polarized light, leading to higher efficiency compared to a pure spiral slot lens. As shown in Fig. 5(b), a combination of left-handed spiral slot and spiral triangular sub-aperture array is fabricated into a 150 nm gold film. The slot width of the spiral lens is chosen to be 200 nm. The spiral parameter r0 is chosen to be 2 µm. The slot spiral and triangular sub-aperture array spiral have a radial displacement of 380 nm (0.75λspp) between them. Since the azimuthal and radial polarization components of the circularly polarized illumination have a π/2 phase difference , the spacing between spiral slot and triangular sub-aperture array was chosen to cancel the phase difference to provide constructive inference of surface plasmons excited by the two polarization components .
The near field optical images of a hybrid lens under RHC and LHC illuminating are shown in Fig. 7 . A doughnut spot is obtained for both illumination, and the peak NSOM signal in the vicinity of focus with RHC illumination is higher than that of with LHC illumination. In order to visualize the solid focus center, a worn-out NSOM probe with aperture size of approximately 200 nm was used to improve the coupling efficiency of longitudinal field Ez. Figure 8 shows the measured near field energy density distribution at the air/gold interface for a hybrid lens with the worn-out probe. The image has a lower resolution due to a bigger probe. A solid spot was captured at the focus with RHC illumination, while a doughnut spot with a dark center was observed with LHC illumination, confirming that the plasmonic field of a spiral lens depends on the spin of the incident photon. If a detector with a diameter d is placed in the vicinity of the focus, the detected signal will provide differentiation between RHC and LHC polarizations. Hence, such a structure becomes a circular polarization analyzer. Figure 9 shows the measured extinction ratio as a function of detector diameter by integrating the NSOM signals shown in Fig. 8 within a central circular area with different radius. The extinction ratio is above 16 with detector diameter d of 0.3λ. The lower extinction ratio compared to theoretical prediction may be caused by the lower coupling efficiency of longitudinal field into the fiber probe, and the finite aperture size of the worn-out probe.8 For a NSOM probe with an aperture size of 200 nm, the total probe dimension that interacts with the plasmonic field would be even larger if the 50 nm metal coating surrounding the fiber core is considered. The measured optical images are the convolution of the plasmonic field and probe aperture function. Therefore, even when the probe is exactly located at the focus center, the plasmonic field within the overall probe dimension also contributes to the detected signal, giving an elevated minimum for defocusing case.
The coupling efficiency could be futher improved with the optimization of the hybrid lens. The field enhancement of plasmonic lens made of spatially arranged triangular sub-apertures is given by the constructive interference of surface plasmon waves excited at sub-apertures at all azimuthal directions. It is possible to further increase the focal field strength through optimizing the shape and number of the sub-apertures to achieve a higher plasmonic coupling and focusing. The strong spatial confinement with a higher field enhancement is very attractive for near field optical imaging and sensing in material characterization and biological applications.
4. Discussions and conclusions
In summary, a spatially arranged triangular sub-aperture array enables the coupling of azimuthal polarization into surface plasmon excitation, which can be applied to design a high efficiency miniature circular polarization analyzer. An antisymmetric triangular sub-aperture array can focus both the azimuthal and the radial polarization with a higher coupling efficiency for the azimuthal polarization. The performance of the spiral triangular sub-aperture array and the capability of a hybrid lens to focus circular polarization of different handedness into spatially separated fields have been demonstrated experimentally. An extinction ratio better than 16 can be obtained with a detector diameter of 0.3λ. Unlike the plasmonic focusing with spatially variant polarization, the hybrid lens does not require the alignment of the structure to a singular point of the illumination. An array of such hybrid lens may find applications in polarimetric imaging, parallel near field imaging and sensing.
References and links
1. W. Chen, D. C. Abeysinghe, R. L. Nelson, and Q. Zhan, “Plasmonic lens made of multiple concentric metallic rings under radially polarized illumination,” Nano Lett. 9(12), 4320–4325 (2009). [CrossRef] [PubMed]
8. W. Chen, D. C. Abeysinghe, R. L. Nelson, and Q. Zhan, “Experimental confirmation of miniature spiral plasmonic lens as a circular polarization analyzer,” Nano Lett. 10(6), 2075–2079 (2010). [CrossRef] [PubMed]
9. Z. Wu, W. Chen, D. C. Abeysinghe, R. L. Nelson, and Q. Zhan, “Two-photon fluorescence characterization of spiral plasmonic lenses as circular polarization analyzers,” Opt. Lett. 35(11), 1755–1757 (2010). [CrossRef] [PubMed]
12. Z. Wu, P. E. Powers, A. M. Sarangan, and Q. Zhan, “Optical characterization of wiregrid micropolarizers designed for infrared imaging polarimetry,” Opt. Lett. 33(15), 1653–1655 (2008). [CrossRef] [PubMed]
13. L. Novotny and B. Hecht, Principles of Nano-Optics (Cambridge Univ. Press, 2006).