1. Introduction

Since the directional beaming effect of a subwavelength metal aperture surrounded with the periodic grooves was firstly observed by H. Z. Lezec et al [1], which is generally interpreted as the interactions of surface plasmon polaritons with metal surface corrugations [2, 3], there has been great interest in manipulating light based on surface plasmon polaritons (SPPs) [4–13]. In recent years, types of slit-grooves structures are demonstrated to generate a subwavelength-sized focusing spot with the focal length of several micrometers in simulations and experiments [14–17], which have great application potential in imaging, optical storage and integrated optical circuits. In this letter, we experimentally demonstrate a focusing plasmonic device with a focal length of several wavelengths, which is designed using the method reported in ref [17]. The plasmonic device consists of the fixed-width and depth grooves surrounding the central slit, which is much easier to fabricate compared with the focusing structures consisting of the stepped grooves [14, 16]or the chirped dielectric surface gratings [15]. The micron-focusing device is fabricated using the focused ion beam (FIB), and the transmitted fields are detected with the scanning near-field optical microscope (SNOM) and compared with the simulation results.

2. Principle and simulation

The geometric configuration of the plasmonic device is shown in Fig. 1 . On the SiO₂ substrate are the Au/Cr dual metal layers which have the same thickness of 100 nm. The central slit has the width of 200 nm; the width and the depth of the grooves are 150 nm and 100 nm respectively. The p-polarized incident lights with a wavelength of 633 nm are illuminated on the slit. As described in ref [17], the SPPs waves are excited at the slit exit and then propagate along Au film surface. The grooves scatter the SPPs waves into radiation waves with the specific phases. To focus the transmitted lights with the focal length of f, the radiation waves from grooves and the slit should interfere constructively at the focusing spot, so the phase from the slit (denoted as ${\phi }_o$) and the phase from the groove with the position of x (denoted as ${\phi }_x$) should satisfy the following equation:

 

Fig. 1 The schematic diagram of the plasmonic device.

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According to the method reported in ref [17], the dependence of the phase difference ${\phi }_x-{\phi }_o$ on the groove position x is explored by investigating the simplified structure which is composed of a pair of grooves symmetrically surrounding the central slit. The phases at the centers of the slit exit and the groove exit are used to represent the phases of radiation lights from the slit and the groove. With the grooves positions varying from 440 nm to 3240 nm with the step of 80 nm, the phases of E_x field (${\phi }_x$ and${\phi }_o$) at the centers of the groove exit and the slit exit are calculated with finite-difference time-domain (FDTD) method. The dependence of the phase difference ${\phi }_x-{\phi }_o$on the groove position x is shown in Fig. 2 . Under linear approximation, the relation is expressed in the following formula:

 

Fig. 2 (a) The dependence of the phase difference on the groove position and (b) the linear fit

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Therefore by solving Eqs. (1) and (2), the grooves positions of the focusing device with a specific focal length of f can be obtained easily. Here a focusing device with a focal length of 1.8 μm as an illustrative device is designed. The transmitted field through the device is simulated with FDTD method and shown in Fig. 3 . The simulation results indicate that the transmitted field is focused efficiently by the designed device with the focal length of 1.65 μm and the focusing spot has the size of 400 nm.

 

Fig. 3 The electric field intensity distribution of the transmitted lights through the plasmonic device.

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3. Experiment

In this section, we experimentally investigate a micron-focusing plasmonic device which is designed based on above analysis and simulations.

Two metal layers of 100-nm-thick chromium (Cr) and 100-nm-thick gold (Au) are deposited successively by the electron beam evaporation on the SiO₂ substrate. The plasmonic device with a focal length of 1.8 μm corresponded to the incident wavelength of 633 nm is fabricated using the FIB. The structure parameters of the slit and the grooves are adopted the same as those in the simulations. The SEM image of the focusing plasmonic device is shown in Fig. 4 .

 

Fig. 4 The SEM image of a micron-focusing plasmonic device with a focal length of 1.8 μm.

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The He-Ne laser beam ($\lambda =633nm$) is incident normally on the plasmonic device with the polarization vertical to the long edges of the slit and the grooves. The SNOM (NTEGRA, NT-MDT) is employed to detect the transmitted field at the horizontal planes with different heights from the device surface. Figure 5 show the field distributions at the planes with the heights of z=0.5 μm, 1.6 μm (corresponding to the simulation result of the focal length), 2.5μm and 3.5 μm. It can be seen that with increasing the distance from the plasmonic device, the transmitted lights focus and then diverge. A line-shaped apparent focus spot with a stronger intensity is formed at the plane z=1.6 μm. At the plane z=2.5 μm the focus spot has a decreased intensity and a low contrast; at the plane z=3.5 μm the optical field diverges and no focus spot is formed. Figure 5(e) demonstrates the field profile along the solid line in Fig. 5 (b) and the corresponding simulation results. The experimental results show a basic agreement with the calculated values. The full width at half maximum of the focus spot is 550 nm, a little larger than the calculated results.

 

Fig. 5 The intensity distribution of the transmitted lights at the horizontal planes with the distances of (a) z=0.5 μm, (b) z=1.6 μm, (c) z=2.5 μm, and (d) z=3.5 μm away from the device surface. (e) the intensity distribution along the solid line in Fig. 5 (b).

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To verify the focal length of the plasmonic device, we use the SNOM to measure the field intensity at a center point of the slit at the different heights from the device surface (along the optical axis of the device). The dependence of the field intensity on the distance from the device surface is displayed in Fig. 6 . For the distance z<0.5 μm, the field intensity decreases with the distance increasing which is due to the attenuation of the evanescent waves. For the distance z> 0.5 μm, the strongest intensity is observed at the point with the height of z=1.5 μm, which corresponds to the focus spot and has an accordance with simulation results.

 

Fig. 6 Intensity distribution along the optical axis of the plasmonic device.

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4. Conclusions

In this work, a focusing plasmonic device with a focal length of 1.8 μm is designed and fabricated. The transmitted fields through the plasmonic device are measured by the SNOM, and the measured results show agreement with analysis and simulation results.