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We propose a switching method for optical beaming generated from a metal slit surrounded by surface gratings. The principle of the method is based on the interference of diffracted surface plasmon polaritons from the gratings which are controlled by the relative phases of two oblique incident beams that are illuminated on the metal slit. By adjusting the relative position of the interference pattern of the incident beams with respect to the metal slit, beaming from the proposed structure can be switched from the on- to the off-mode by virtue of the change in the symmetry of the generated surface plasmon polaritons. An experimental demonstration of the method is presented in which an electrically controlled interferometric configuration is used.
© 2014 Optical Society of America
1. Introduction
Since the discovery of extraordinary transmission (EOT) phenomena through a subwavelength metal aperture array [1], various studies have been reported on surface plasmon polaritons (SPPs), which are often referred to as quasi-particles that are formed by the coupling of photons with surface conduction electrons at metal-dielectric interfaces [2, 3]. Among these, studies of nanostructures prepared using SPPs have attracted a great deal of interest, since SPPs can overcome the diffraction limit and significantly enhance optical transmission [4, 5]. In particular, the beaming technique using SPPs, having a high transmittance and concentration within a small angular divergence of approximately ± 3°, has progressed to an extensive extent. These beaming techniques have attracted intense attention in the fields of the optical data storage devices, optical interconnectors and optical sensors [4–8]. After the first demonstration of the plasmonic beaming [9], various types of modified structures for improving beaming characteristics have been reported [10–19]. For instance, studies dealing with off-axis directional beaming arising from the asymmetric gratings have appeared [10–12]. Furthermore, tunable directional beaming by changing the permittivity of metal-dielectric composite surface gratings was reported [13–15], and a method for rendering bundle beams by manipulating the constructive or destructive interference of beams diffracted from multiple subwavelength slits was reported [17]. However, the beaming characteristics of these previous methods were determined by their structures. For example, asymmetric surface gratings can be used for generating off-axis plasmonic beaming, but its direction cannot be tuned until the grating properties are changed. Tunable beaming, based on the oblique incidence of a single beam, can only be achieved in practice when the overall sample is rotated. Such approaches are inappropriate to chip scale applications. Nevertheless, less interest has been devoted to the practical possibility of the switching of the plasmonic beaming without changing its structure. To control plasmonic beaming with high on-off ratio, a novel type of scheme is required in order to modulate surface plasmon excitation by the simple interaction of external signals.
In this paper, we present a novel approach to switch the beaming characteristics from a metal nanoslit surrounded by surface gratings. By changing the excitation phase of SPPs from the nanoslit, which can be controlled by the relative position of the interference pattern of incident beams with respect to the slit, on- and off-modes of beam generation can be determined. For numerical analysis, the rigorous coupled-wave analysis (RCWA) method is used to investigate the characteristics of a switchable beam with the design of an appropriate structure [20–23]. In order to fabricate the designed device, the Ag film is deposited on a SiO2 substrate using an E-beam evaporator. The nanoslit and surrounding metal gratings are then fabricated using a focused ion beam (FIB). To generate a switchable beaming from the proposed structure, two oblique incident beams having a wavelength of 532 nm are used to illuminate the sample. The switching characteristics of the beam, while changing the relative phase difference between the two beams by means of a piezo-stage, are measured using a charge-coupled device (CCD).
2. Principle and numerical results of switchable beaming
In this section, we discuss the physical principles of the beam switching method from a narrow slit surrounded by metal gratings. The main concept of the switching mechanism is based on controlling the phases of two oblique incident beams on a metal slit. The relative position of the interference pattern with respect to the metal slit can be adjusted by controlling the phase of the two incident beams that impinge on the metal slit.
As shown in Fig. 1(a), when two incident beams interfere with the same phase below the slit, the antinode of the interference pattern is located at the center of the metal slit. The x-directional electric field below the slit should then have a symmetric distribution, hence only the symmetric metal-insulator-metal (MIM) plasmonic mode is excited inside the slit. Since the proposed structure is devoid of asymmetry along the x-direction, these symmetric phase profiles inside the slit can be maintained until the MIM plasmonic mode is coupled to the SPP mode at the transmitted side, even after the SPP mode is diffracted by the gratings. Therefore, the beams that are radiated to the free space from the gratings on both sides of the slit are constructively interfering along the axis normal to the slit [9], which will be referred to as the on-mode. On the other hand, as shown in Fig. 1(b), when the two incident beams are out of phase, the node of the interference pattern is located at the center of the metal slit. As a result, coupling to the symmetric MIM mode is prohibited but coupling to the anti-symmetric MIM mode occurs [24]. Compared to the on-mode case, diffracted light from the left- and right-side of the gratings are in destructive interference when anti-symmetric profile of an x-directional electric field is maintained, which corresponds to the off-mode of the beaming. These field scattering characteristics are described in Figs. 1(c) and 1(d), which are the x-directional electric field profiles near the slit for the on- and off-mode cases, respectively.
Fig. 1 Schematic of a switchable beaming structure is shown when two oblique incident beams that illuminate the metal slit are (a) in phase and (b) out of phase, respectively. Red-solid and blue-dotted arrows denote the direction of electric fields caused by right- and left-side coupled SPPs. The images represented in (c) and (d) show the Ex field distribution for each case of (a) and (b), respectively.
In numerical simulations, we assume two obliquely incident p-polarized beams which have incident angles of θin and –θin with a free space wavelength of 532 nm. Slit conditions such as metal thickness (t) and slit width (w) are determined as the values for which the amplitudes of excited surface plasmons coupled from the anti-symmetric and symmetric MIM modes in the slit are in a similar scale. They are taken as t = 300 nm and w = 540 nm [25]. The period of gratings surrounding the metal slit is determined by the Bloch phase-matching condition [4], as shown in Eq. (1):
where θdiff is the angle of diffracted light, λ is an incident wavelength, m is an arbitrary integer number, and Λ is the grating period, respectively. λsp is an effective wavelength of SPP mode which can be calculated as . In our case, θdiff is set to zero for on-axis beaming, hence the period of grating Λ is determined to be λsp/m. For the case of an incident wavelength of 532 nm, the grating period is taken as Λ = 500 nm for m = 1. The height of the gratings is taken as h = 100 nm, being close to the optimal point for maximizing on-axis beaming intensity [26]. As shown in Fig. 2, a peak for the intensity does not identically appear for h = 100 nm due to differences in structure and measuring location. Nevertheless, the intensity does not change significantly for a height in excess of 100 nm, also it is not significantly sensitive to the number of grooves when N is larger than six. The refractive index of the substrate and permittivity of the sliver layer are given as n = 1.5 and εm = –10.19 + j0.83, respectively [27].Fig. 2 Calculated intensities for the on-mode case at x = 0 μm and z = 10 μm for N grooves surrounding a central slit according to the height of grating are presented.
The other remaining design parameters which can affect beaming performance are the fill factor and the length of the offset. Here, we define the fill factor as the ratio of the gratings occupied by metal (f = lm/Λ), and the length of the offset (los) is defined as the distance between the edge of the slit and the nearest extrusion of the grating. To find a better condition for switchable beaming, the intensities of the generated beam for the on-mode and off-mode cases as a function of these two parameters are presented in Fig. 3. The findings show that the intensities of the beam generated for the on-mode case have local maxima per each λsp/2 of the offset length. The physical origin of these periodic changes can be explained by the effect of Fabry-Pérot resonance of the generated SPPs within the region between the left- and right-side nearest the extrusion of the gratings [5]. Thereafter, the optimized value is determined by changing the fill factor. As shown in Fig. 3, the maximum value of the intensity for on-mode is achieved for a fill factor of 0.5, which is in general agreement with findings in a previous report regarding the effects of fill factor in plasmonic beaming structure [28]. Since a change in the fill factor does not influence the distance between the left- and right-side nearest extrusion of the gratings, no significant difference is shown for the location of local maxima according to a change in fill factor. On the other hand, the intensities of the off-mode are close to zero and there is almost no change with the length of the offset and the fill factor. Therefore, only the case of f = 0.5 is plotted for the case of the off-mode.
Fig. 3 The intensity values of a generated beam for on-mode (solid line) and off-mode (dotted line) cases at x = 0 μm and z = 10 μm according to the length of offset and the fill factor are presented.
Figures 4(a) and 4(b) show the intensity distribution of optical beam generated by the diffraction of SPPs with the optimized slit and grating parameters for the on- and off-mode, respectively. Note that the z-direction is reversed in the figure. For the on-mode case, the findings clearly show that a beam with a strong radiation is formed along the z-axis as shown in Fig. 4(a). However, in the case of the off-mode, the diffracted field from the gratings is separated into two branches which radiate obliquely from the slit. The sum of the field radiation along the on-axis is noticeably decreased, as illustrated in Fig. 4(b). The polar plot of Fig. 4(c) shows the angular spectra of power flow for both the on- and off-mode. Near the zero degree of diffraction, the power flow calculated at the on-mode state is significantly higher than that for the off-mode. We defined the on/off ratio of switchable beaming as the rate of the transmitted power that passes through the full-width half-maximum (FWHM) of the on-mode beaming divided by that for the off-mode. In the simulation condition shown in Fig. 3, the on/off ratio is determined to be 8.95.
Fig. 4 Diffraction field distribution of (a) on- and (b) off-mode beam generated from the proposed structure, respectively, with w = 540 nm, t = 300 nm, h = 100nm, N = 6, Λ = 500 nm, los = 250nm, f = 0.5 and θin = 31.74°. (c) Angular distribution of transmitted power for on-mode (blue solid line) and off-mode (red dotted line) cases are shown respectively, with the same conditions as (a) and (b).
3. Experimental results for switchable beaming
Figure 5(a) shows the configuration of the experimental setup used for the generation of a switchable beam. A laser beam with a 532 nm wavelength is expanded to form a collimated plane wave. The beam is then divided to two beams using the beam splitter. In order to carefully equalize the power of the two beams, a beam attenuator is used for tuning the power of a separated beam. Each beam passes through different optical paths. The incident light 1 illuminates the sample directly, whereas the incident light 2 passes through a specific mirror (M2), the location of which is controlled by means of a system with piezo stage (Piezosystem Jena, TRITOR38). Both of the beams pass through a half wave retarder to ensure a TM polarization state. These two beams propagate in parallel before reaching the sample.
Fig. 5 (a) Schematic diagram of the optical setup to measure the switchable beaming. M1, M2, M3 and M4 are mirrors. (b) Optical path in the prism. (c) SEM image of sample which is fabricated by FIB.
A prism as shown in Fig. 5(b) is used to change these parallel beams to an obliquely incident configuration before they illuminate the sample. The angle of the incident light within the prism and dielectric substrate of the sample can be determined by the apex angle of the prism using Eq. (2):
where θp is the apex angle of prism and θin is the incident angle of light. Here, we used a prism which has the apex angle of 40.1°, which gives the incident angles of beam 1 and 2 of 31.74° and –31.74°, respectively. Moreover, a carefully designed iris is used in order to precisely merge the two beams at the same point of the sample, which can force the distance between the two incident beams as defined in Eq. (3):In the case of ls = 1 mm, lp = 15.05mm, θp = 40.1°, and θin = ± 31.74°, d is 3.68 mm. The use of an iris can also prevent a multiple reflection of the incident beam inside the prism.For the fabrication of the proposed switchable beaming structure, we use a focused ion beam (FIB) machine (Quanta 200 3D, FEI Corp.) to pattern a narrow metal slit and gratings after evaporating a 400 nm (t + h) Ag layer on the SiO2 substrate. The length of the slit and gratings is 15 μm. Other design parameters are based on simulation results of Fig. 4. A scanning electron microscope (SEM) image of the fabricated structure is shown in Fig. 5(c).
For the measurement, the beam that is radiated from metal slit and gratings to free space is captured by the CCD. When the applied voltage to the piezo stage is switched from 1.6V to –1.6V by applying a square wave signal produced by a function generator, as shown in Fig. 6(a), it represents the condition of a 2π phase shift of incident light 2. In the insets of Fig. 6(a), CCD images are shown for both the cases of the on- and off-mode of switchable beaming. Figure 6(b) shows the measured intensity along the solid white line of the captured image of Fig. 6(a). In the case of on-mode, the maximum value of the intensity appears at x = 0 μm, which is the center of the slit. In comparison with the intensity of the off-mode, the intensity of the on-mode is higher in all areas. The objective lens is located a few centimeters above the sample. Therefore the angle observed by the CCD is less than 1°. As shown in Fig. 4(c), the side lobes for the off-state are emerging strongly from angles of ± 15° or more. For this reason, in Fig. 6(b), the side lobes cannot be shown in the experiments. The on-off ratio is calculated by integrating the field intensity of the on-mode case, and then dividing it by that of the off-mode case. Here, the regions of integration for both cases are the FWHM of the beam generated by the on-mode case, the same as the numerical calculation. The average value for the on-off ratio measured in experiment is 5.86.
Fig. 6 (a) Captured CCD images for the on-mode and off-mode cases as a function of the voltage applied to the piezo stage. (b) Comparison of the intensity of the on-mode (blue solid line) and off-mode (red dotted line)
Further improvements of the proposed structure are possible. We expect that the use of a dielectric grating (for example PMMA on metal) around the nanoslit rather than a metal grating can improve the diffraction uniformity. A dielectric grating produces less light scattering than a metal grating, and can achieve uniform diffraction over the entire gratings [4]. Another possible way to increase the switching efficiency is the use of an input grating similar to the output grating [9]. The coupling efficiency of incident light into the metal nanoslit can be increased when such an input grating is used. As a result, this approach can improve the performance of beaming for the on-mode case. In addition, the use of parallel identical slits would increase the switching efficiency.
4. Conclusion
In conclusion, we propose a method for switchable beaming, achieved by controlling the phase difference between two obliquely incident beams. The key factor for the switching of the generated beam from a nanoslit surrounded by metallic gratings lies in adjusting the phases of the two obliquely incident beams, which can change the relative position of the interference pattern with respect to the metal slit. The geometrical parameters are optimized for efficient beaming and the characteristics of the generated beam are investigated based on the on/off ratio. The on/off ratio of the proposed structure is experimentally determined to be approximately 5.86. Although our switchable beaming scheme uses a mechanical device such as a piezo stage, the actual switching mechanism is based on the slight phase control of the incident light. Hence the scheme has the potential to be achieved without the need for a bulk mechanical device; for example, by using active materials such as quantum dots [29]. Furthermore, it would be expected that the switchable beaming configuration described herein can be widely applied to integrated photonic devices.
Acknowledgment
This work was supported by the National Research Foundation of Korea funded by Korean government (MSIP) through the Creative Research Initiatives Program (Active Plasmonics Application Systems).
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