A plasmonic lens with metallic chirped circular nanoslits corrugated on Au film supported on quartz substrate for the purpose of superfocusing was put forth and fabricated by means of focused ion beam direct milling technique. Topography of the lens was imaged using an atomic force microscope. After that a near-field scanning optical microscope was employed for optical characterization of focusing performance of the lens. Our experimental results verify the focusing performance and further demonstrate that they are in agreement with the theoretical calculation results. Focusing performance is significantly improved in comparison to that of the non-chirped lens. The lenses are possible to be used for the applications of bioimaging, detection, and inspection in submicron scale resolution.
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With rapid development of plasmonics, many researchers focus on nanofocusing for molecular detection , imaging , and waveguide propagation  etc. Various types of surface plasmon polaritons (SPPs)-based plasmonic lenses for superfocusing were reported theoretically [4–12]. In this paper, we experimentally put forth a plasmonic structure constructed with chirped circular nanoslits which were corrugated through an Au thin film supported on quartz substrate, as shown in Fig. 1(a) . The chirped circular nanoslits here means that slit widths and periods of the rings are changed as described in Table 1 . Theoretically, focusing spot size at site of full width and half maximum (FWHM) of ~λ/3 at near-field and long depth of focus can be realized for the lens with the corrugated structures. Resolution of the chirped structure is higher than that of the constant grating-like structures [11,12]. The focusing region is formed by interference between surface plasmon wave and diffractive wavelets generated through the slits. Each slit here acts as a small lasing cavity with Fabry-Pérot resonance which contributes on the final focusing. In physical mechanism, the transmission intensity at the focal point is synthesized by iteration of each zone focusing and interference each other, and can be expressed as
It is the interference of all the SPP’s from the multiple rings that produces the intensity enhancement. Focusing performance of the proposed plasmonic lens was demonstrated in detail on the basis of our experimental study presented below.
2. Experimental setup
In order to further demonstrate and verify the focusing performance of the plasmonic lens with chirped circular slits experimentally, optical characterization by means of a near-field scanning optical microscope ‖NSOM| was carried out recently in our laboratory for a newly designed plasmonic lens with total 8 chirped circular slits which were etched through an Au film supported with glass. Width of the outmost circular slit (8th) is 95 nm. The wave field at the focal point was mapped using the NSOM. For the NSOM measurement, near-field intensity distributions at different horizontal planes vertical to the optical axis have been obtained and compared with simulation results calculated using the finite difference time domain (FDTD) algorithm . Our NSOM probing results were found to be in agreement with the theoretical calculation results.
Configuration of the plasmonic lens is an asymmetric structure in which a thin film of Au is sandwiched between air and glass, as shown in Fig. 1(a). Information regarding design of the plasmonic lens was described in detail in Ref. 8. The Au thin film of 200 nm in thickness was coated on quart substrate using e-beam evaporation technique. The lens was fabricated using focused ion beam (FEI Quanta 200 3D dual beam system) direct milling technique, as shown in Fig. 1(b) . Geometrical characterization was performed using an atomic force microscope (Nanoscope 2000 from DI company). Figure 1(c) shows topography of the FIB fabricated plasmonic lens. The optical measurement was performed with a near-field optical microscope (MultiView 2000TS from Nanonics Inc. in Israel) where a tapered single mode fiber probe, with an aperture diameter of 100 nm, was used working in collection mode. The fiber tip was raster scanned at a discrete constant height of 500 nm, 1.0 μm, 1.5 μm, 2.0 μm, 2.5 μm, 3.0 μm, 3.2 μm, 3.5 μm, 3.7 μm, 4 μm, 4.5 μm, and 5 μm, respectively, above the sample surface,and allowing us to map the optical intensity distribution over a grid of 256×256 points spanning an area of 20×20 μm2. Working wavelength of the light source is 532 nm (Nd: YAG laser with power of 20 mW). Additionally, a typical lock-in amplifier and optical chopper were utilized to maximize the signal-to-noise ratio. Figure 1(d) shows the measured three-dimensional (3D) electric field intensity distribution of the lens at propagation distance of 2.5 μm.
3. Experimental results and discussion
Figure 2 shows the NSOM mapping images at different propagation distance ranging from near field of 0.5 μm to far field of 5 μm in free space. It can be seen that the incident light focusing at 0.5 μm, 1.5 μm, and 2.5 μm, and dispersed at 5 μm. The long focal depth is confirmed from the NSOM images. Figure 2(d) presents the near-field intensity distribution at a plane as far as 5 μm away from the interface. At such a distance, the detected peak intensity significantly reduced to less than 9% of the intensity at the slit surface. The intensity distribution at Fig. 3(d) further shows a Gaussian shaped cross section but with a much larger FWHM (~1.6 μm) and a much lower peak intensity compared with that shown in Fig. 2(b) and (c). The image in Fig. 2(a) shows an apparent polarization effect along linear polarization direction indicated by the arrow. For linearly polarized light, what we probed electric field intensity is the in-plane component which interferes constructively at the focus, and the Ez component vanishes due to destructive interference .
To further compare the measured results with the theoretically calculated results, we plotted E-field intensity profiles at x-axis together with that of the numerical calculated, as shown in Fig. 3 (a)-(d) and Fig. 4 . In our 3D FDTD calculations, simulation time and mesh size are 150 fs, and Δx = Δy = Δz = 5 nm, respectively. The optical field is p-polarized monochromatic wave with the wavelength of 532 nm in the air. At this wavelength, the Au layer has the refractive index of 0.452+i2.451 . It can be seen that variation tendency of the E-field intensity of the measured is in agreement with the theoretically calculated results. As can be seen from Fig. 3, the measured FWHM of the central lobe is slightly larger than the calculated value for cases (a)-(c), and the difference is large for the case (d) at near-field distance as long as 5 μm. It can be attributed to the background noise signal which is stronger gradually with increasing the probing distance from 0.5 μm to 5 μm. It directly leads to the base intensity increases and causes degradation of the signal-to-noise ratio of the detected optical signal. Thus the difference of FWHM between the measured and calculated data is larger as the probing distance is getting far away from the exit plane. This point is further depicted in Fig. 4 in which the measured intensity vs. propagation distance z is in good agreement to that of the calculated, but FWHM vs. z shows a nearly constant difference due to influence of the background noise which can be regarded as a systematic error in the NSOM system. In addition, the nonlinear variation of the measured data for z>3.5 μm may attribute to increasing of the energy loss of the devergence beam while the probe scans with a limited aperture of 100 nm. The smallest FWHM is 260 nm and 220 nm for the measured and calculated values, respectively at z=0.5 μm. However, even with the presence of the imperfection, the focusing effect is apparent. The measured FWHM of our chirped plasmonic lens is ~310 nm at z=1.35 μm. In contrast, we find reported in Ref.16 a value of FWHM=1.24 μm for the equivalent non-chirped structures. It can be seen that the focusing quality is significantly improved by the chirped plasmonic lens.
Figure 5 is a re-plotted 3D image of the NSOM measured intensity profiles along x-axis probed at the different propagation distance z ranging from 5 nm to 5 μm. It intuitively shows the intensity distribution along propagation distance. It can be seen that the peak intensity is significantly enhanced from 0.01 μm to 1 μm, and then degraded gradually in near-field region because of SPP-enhanced wave propagation on Au surface vanished in free space when z >1 μm. Only the interference-formed beam focusing region exists in near-field region. It is also in agreement with our calculated results.
In summary, a plasmonic lens with chirped circular slits for superfocusing was experimentally verified and confirmed by aid of the techniques of FIB nanofabrication, AFM probing, and NSOM characterization. In comparison to the theoretical calculated results, our experimental results demonstrate that the measured results are in agreement with the calculated results.
The work was supported by the National Natural Science Foundation of China (No. 60877021).
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