We experimentally demonstrate plasmonic lenses working in the visible range with well controlled focal lengths using nano-slits in an aluminum film. The fabricated lenses were characterized using confocal scanning optical microscopy. Two lenses with a design focal length 3 µm and 6 µm at 633 nm were investigated in detail. The full-width half-maximum beam width at the focal point was found to be 470 nm and 490 nm, and the extension of the light spot was 1.3 μm and 2.3 μm respectively. Lens performance compared extremely well with the expected behaviour from finite-difference time-domain modeling. The focal length from experiment and simulation agreed to within 3.5%. The lens manufacture was found to be insensitive to deviations from the optimum process parameters indicating that lens components can be reliably designed and produced.
©2010 Optical Society of America
Light transmitted through a dielectric lens shows a strong diffraction effect at the edge. Progress towards the miniaturization of devices for integrated optical systems is therefore impeded by diffraction on the sub-micron scale. Metallic plasmon resonant optical structures have the potential to overcome such limitations. Since Ebbesen et al. discovered the extraordinary transmission of light through a single hole (or slit), surrounded by surface corrugations or a sub-wavelength hole array in a metal film, metallic nanostructures have attracted much interest . Such light transmission phenomena may be explained as an excitation of a surface plasmon (SP) mode at the hole or slit entrance that propagates through the aperture before emitting into radiation modes at the exit. Suppression of angular divergence down to ± 3° for light transmitted through a nanohole (or a nanoslit) surrounded by dimples was demonstrated . A convex-shaped metallic film with uniform nano-slits that resembled a glass lens was proposed for light focusing and collimation . Light focusing can be obtained, according to the equal optical length principle, by modulating the phase delay distribution at the metallic device’s surface. Recently, simulation work has shown that a planar metallic lens consisting of nonuniform slits in a silver film can give light focusing since the propagation constants of SP modes in metallic slits are strongly dependent on the slit widths . Based on a similar mechanism, optical device designs for sub-wavelength imaging, light deflection and angle compensation have been developed [5–8]. In addition, a two-dimensional (2D) array of metallic pillars was theoretically investigated for 3D light focusing .
Despite considerable progress in theory, very little experimental work has, however, been reported. Fan et al. fabricated chirped gratings in an optically thick gold film using focused ion beam and observed a focused light spot . However, the measured focal length differed from the design by nearly a factor of four. Furthermore, the line-width in the focal-plane and the extension along the propagating direction was approximately 1.4 and 10 times the illumination wavelength respectively. More recently, Roberts et al. fabricated a 2D plasmonic lens consisting of cross-shaped aperture array and demonstrated a 3D focusing in near infrared . However, the authors found that technique only worked well for a few specific focal lengths.
In this paper, we demonstrate, by simulation and experiment, plasmonic lenses that use nano-slits in an aluminum film made by electron beam lithography and dry etch. The results show an excellent agreement, to within 3.5%, between simulation and experiment, yielding well controlled focal lengths and subwavelength focusing in the visible range. This new, practical, ability to manipulate a beam of light on the nanoscale, will enable the improvement of a wide range of application including high-throughput nanolithography, high resolution scanning optical microscopy, optical data storage and optical antenna [12–15].
The lens design in this paper is similar to that in . The slit widths and positions were modulated to form a phase profile for a certain focal length according to the equal optical length principle. Figure 1 shows scanning electron microscope (SEM) images of typical plasmonic lens structures that were patterned. The lenses are uniform in the y direction and are symmetrical in x = 0. The nanoslits were 10 µm in length and the lens width, or aperture (D), was 9.85 µm for the lens with f = 3 µm, and 10.84 µm for the lens with f = 6 µm. The narrowest slit was 50 nm and the minimum gap that was used was 100 nm. A 100 nm gap is large enough to decouple the SP modes in neighboring slits. The lens with f = 6 µm as shown in Fig. 1(b) has in total 25 slits. From the central slit to the last one on the right hand side, the positions are 0, 0.194 µm, 0.37 µm, 0.567 µm, 2.679 µm, 2.879 µm, 3.079 µm, 3.937 µm, 4.137 µm, 4.337 µm, 4.971 µm, 5.171 µm and 5.371 µm. The first four slits have widths of 50 nm, 54 nm, 68 nm, 125 nm and all the others are 100 nm. An enlarged image of the region inside the solid black rectangle of Fig. 1(a) is shown in Fig. 1(c). The roughness on the sidewalls of the gratings is predominantly due to the large grain size of the evaporated metal. The error of the slit widths of the fabricated slits is within 10%. Aluminum was used instead of the more conventional choices of gold and silver because of its low cost, good adhesion and CMOS process compatibility. A 200 nm thick aluminum film was evaporated on to a clean glass microscope slide using an electron beam evaporator. 360 nm of ZEP520A electron beam resist was spin-coated on to the sample and exposed using a Vistec VB6 UHR EWF electron beam lithography tool with a dose of D 0 = 550 µC/cm2 at 100 keV. After development in o-xylene for 30 s at 23 °C, the sample was etched using SiCl4 in a Plasmalab System 100 at an etch rate of 25 nm/min.
The far-field focusing pattern produced by the lenses was measured using a WITec alpha300S confocal scanning optical microscope (CSOM). A pure confocal mode was used for the experiments because the probe for near-field scanning optical microscopy may have caused a perturbation of the local fields. Sample illumination was with a collimated laser beam operating at 633 nm. The laser source was polarized in the TM mode with its electric field perpendicular to the slits. The light that was transmitted through the sample was collected using a 100 × , NA = 0.9 objective. A multi-mode fiber with a core diameter of 25 µm was used to couple the transmitted light into a photomultiplier tube that had a sample integration time is 0.5 ms. The core of the fiber acted as the CSOM pinhole. The sample was scanned in the x and y directions using a piezoelectric scan table, and the microscope working distance was scanned to obtain the z-axis data. The step size in any direction was 200 nm.
4. Results and discussion
Figure 2(a) and 2(b) show the focusing light pattern in the xz plane measured by the CSOM for lenses designed to have f = 3 µm and f = 6 µm respectively. The diffracted light distribution clearly shows focusing for both lenses. The constructive interference at the focal point can be clearly seen. The positions of the foci and the side lobes agree extremely well with the simulated electric field intensity distribution shown in Fig. 2(c) and 2(d). The simulation results were obtained using Lumerical FDTD Solutions . A 2D simulation was chosen to be an adequate approximation since the length of the uniform 10 µm slits in the y-direction is much larger than the light wavelength. In the simulation, a uniform cell of Δx = Δz = 1 nm was used in the metal slab and a nonuniform cell was used elsewhere. The simulation domain was bounded by perfectly matched layers. As with the experiments, a TM-polarized plane wave source at 633 nm with normal incidence to the lens surface was used. The simulation results showed that the field intensity at the focus of the lens with f = 6 μm was 1.9 times that of the incident light. The overall transmission through the lens at 633 nm is 27% that is much larger than the slit filling ratio of 10%. The significant enhancement of the transmission is mainly due to the excitation of SPs . The relatively stronger intensity of the side lobes observed in the experiment was caused by the tolerance limits of fabrication that most strongly affected the narrower slits towards the lens centre. It can be seen that the transmitted light from the central slits is weaker. Theoretical work has also shown that multilayer metal/dielectric films may give a much stronger light intensity at the focus, but this has not been investigated in this work .
The normalized light intensity distributions through the centre of the focus spot along the x and z axis, is shown for both lenses in Fig. 3 . The experimental focal lengths for the lenses [Fig. 1(a) and 1(b)] were 3.1 µm and 6.1 µm, which compare favorably with the simulated values of 3 µm and 6 µm, respectively. The deviation from the intended focal length is less than 3.5% for both lenses. The full-width half-maximum (FWHM) of the beam extension in the z-direction for the lens with a focal length of 3.1 µm is 1.3 µm, and the light intensity at the focus centre is approximately seven times that of the nearest side lobe. The extension of the focus of the lens with the focal length of 6.1 µm is larger as shown in Fig. 3(b). In the x-direction, a line plot across the focal plane of each lens shows that the light intensity drops quickly with the distance from the optical axis (at x = 0) as shown in both Fig. 3(c) and 3(d). The FWHM line-width for the f = 3.1 µm lens is 470 nm, and for the f = 6.1 µm lens it is 490 nm. The resolution of the measured data is potentially limited by the CSOM scan step size of 200 nm and the resolving power of the objective, but as can be clearly seen from Fig. 3, the measured profiles compare very well with the simulations. As a further test of lens quality, we have compared the line-width at the focal point with what we would expect for a Gaussian limited beam. The Rayleigh limit for the resolving power of a lens is given by ds = λf/D, where ds is a half of the line width, λ is the wavelength, f is the focal length and D is the lens aperture. For the lens with f = 3.1 µm, ds = 200 nm, whereas for the lens with f = 6.1 µm, ds = 356 nm. The results therefore suggest that the f = 6.1 µm lens works close to its theoretical limit.
To investigate the robustness of the fabrication process, and the impact of process variation on lens performance, plasmonic lens structures with a design f = 6 µm were written using electron beam lithography with doses from (1-25%)D 0 to (1 + 10%)D 0, D 0 = 550 µC/cm2 as before. The effect in the slit dimensions was to modify them by 15%. Figure 4(a) –4(c) show the focusing light pattern in the xz plane obtained by the CSOM for the lenses written with doses of (1-25%)D 0, (1-10%)D 0 and (1 + 10%)D 0. We can see that the focal length is relatively invariant for the dose range employed. However, the light intensity of some side lobes becomes larger than the light intensity at the expected focus of the lenses in Fig. 4(a) and 4(c). For the four lenses with doses of (1-25%)D 0, (1-10%)D 0, D 0 and (1 + 10%)D 0, the lightintensity distributions along the x and z directions through the foci of the four lenses were plotted in Fig. 5 . The corresponding focal length was 6.5 µm, 5.8 µm, 6.1 µm and 6.4 µm, respectively. The error of the focal length increases with the dose shift, but does not exceed 10% even with a dose of (1-25%)D 0, at the limits of outright fabrication failure. This indicates that the focal length is mostly determined by slit position, not width. The focus width for each lens in increasing order of dose was 474 nm, 500 nm, 490 nm and 472 nm. The beam extension for each of these lenses was 2.8 µm, 2.4 µm, 2.3 µm and 2.7 µm, respectively. All four lenses work very close to the theoretical resolution limit, but the beam extension is sensitive to fabrication errors that have an effect on slit width such as exposure dose.
In conclusion, we have experimentally demonstrated plasmonic lenses with a design focal length of 3 μm and 6 μm working in the visible range. The lenses used nano-slits in an aluminum film, made by electron beam lithography and dry etch. The difference between the design and measured focal length was only 3.5%. Resolution close to Rayleigh limit was achieved. Focus widths of 470 nm and 490 nm in the focal plane were measured using 633 nm laser light for lenses with f = 3.1 μm and 6.1 μm, respectively. The lens properties could be improved by further optimizing the fabrication process for patterning thick metal films but we find the present devices are robust against process variation. The experimental demonstration of plasmonic lenses with well controlled beam manipulating function in the visible range on the sub-micron scale shows the potential for light control at or beyond the diffraction limit.
We appreciate the help of Dr Tomas Dieing and Dr Elena Bailo from WITech GmbH for their support with confocal scanning optical microscopy. This project is funded by a grant from the UK EPSRC.
References and links
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