1. Introduction

Plasmonic devices, which can break the diffraction limit of light, have shown tremendous potential applications in highly integrated photonic circuits [1–3]. In recent years, many ultracompact plasmonic circuit components have been proposed and demonstrated [1–12]. In order to realize these functional plasmonic devices, it is vital that surface plasmon polaritons (SPPs) be first generated by the designed structures because of the mismatching of the momentums between the SPPs and free–space light [5–12]. Among these SPP generators, unidirectional SPP generators, which can increase the generation efficiency and satisfy some special applications, attract great interests [5–12]. The unidirectional SPP generators could be realized by launching SPPs on one direction and extincting SPPs on the opposite direction. For example, based on the Bragg reflections of a periodical array of grooves [5] or ridges [6] placed on one side of a conventional SPP source, SPP can be unidirectionally launched to the other side. By manipulating the constructive or destructive interference of two different SPP sources, such as nanoslits [7,8] or nanocavities [9], unidirectional SPP generations can be also realized. In these structures, the additional periodical structures and the interspaces between different cavities or slits considerably increase the SPP generator sizes. This makes the devices bulky and difficult to downscale. Compact structures, such as the single slit [11,12], were also proposed to achieve unidirectional SPP generation due to the nearly complete destructive interference on one side. However, all of these unidirectional SPP generators above can only extinct the SPPs for a narrow bandwidth because they are based on the Bragg reflections or the nearly complete destructive interferences, which have critical requirements for the incident wavelength. This is not preferred for realizing complex functional plasmonic devices of multi–wavelengths. Moreover, these unidirectional generators with critical requirements for the incident wavelengths need high fabrication accuracy, which implies the increased difficulty of the sample fabrication. Therefore, realizing broadband unidirectional SPP generators with high extinction ratios in compact structures, which can exhibit robust unidirectional–generating properties and downscale the plasmonic devices, is very essential for the plasmonic circuits.

In the letter, we experimentally demonstrate a broadband unidirectional SPP generator with a dielectric–film–coated asymmetric single nanoslit. Due to the tight field confinements by the dielectric film [1–3,13] and the deep groove in the asymmetric slit, the transmittance of SPPs in the groove to the left side become low. Thus, the generation efficiency of the left–going SPPs is significantly suppressed for a broad bandwidth. Unlike the previous works, this extinction of SPPs does not have critical requirements on the incident wavelength. Meanwhile, because of the low transmittance of SPPs in the groove, the generation efficiency of the right–propagating SPPs have a large flat spectra range [14]. Therefore, the unidirectional SPP generation can be realized in the spectrum range of >100 nm around λ = 750 nm in the dielectric–film–coated asymmetric single nanoslit with the lateral dimension of only 865 nm.

2. Experimental results

The asymmetric single nanoslit is composed of a symmetric nanoslit in immediate contact with a nanogroove in an optically thick metal film, as shown in Fig. 1(a) [without the PVA (polyvinyl alcohol) film]. When p–polarized light (magnetic vector parallel to the slit) illuminates the structure from the back side, it can generate SPPs propagating along the bottom of the groove. These SPPs would be reflected back and forth off the two walls in the upper part of the asymmetric slit, acting as a Fabry–Perot (FP) resonator [12]. If the groove depth is small, the SPPs propagating along the bottom of the groove can largely transmit to the left side and propagate on the front metal surface. As we know, the SPP field confinement can become better when the metal surface is coated with a dielectric film [1–3,13]. The tighter filed confinement due to the dielectric film together with an enough deep groove would ensure a low transmittance of SPPs in the groove to the left side. This can considerably suppress the left–propagating SPP generation efficiency for a broad bandwidth, which does not have critical requirements on the incident wavelength. Meanwhile, the right–propagating SPP generation efficiency has a large flat spectra range because of the low transmittance of SPPs in the groove [14], too. Based on the above principle, we proposed to coat a dielectric film on the asymmetric single nanoslit with a deep groove to realize the broadband unidirectional SPP generation, which can exhibit robust unidirectional–generating properties and has compact sizes.

 

Fig. 1 (a) Structure of the asymmetric single nanoslit and the geometrical parameters. (b) SEM image of the experimental structure on the Au film. (c) Detailed image of the central asymmetric single nanoslit (w = 190 nm, d = 140 nm, and L_FP = 865 nm).

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To test experimentally our proposal, an asymmetric single slit with a deep groove was fabricated using a focused ion beam on a 200–nm–thick gold film, which was evaporated on a glass substrate with a 30-nm-thick titanium adhesion layer. First, a 20-μm-long nanoslit was fabricated on the gold film. Then, a shallow nanogroove was fabricated in immediate contact with the lower half of the nanoslit, forming the asymmetric nanoslit. Figure 1(b) shows a scanning electron microscope (SEM) image of the experimental structure. In the middle of the image lies a 20–μm–long nanoslit. The lower half part of the nanoslit is the asymmetric nanoslit, while the upper half symmetric nanoslit without the nanogroove acts as an in–chip reference. Figure 1(c) displays the detail of the asymmetric nanoslit. The measured geometrical parameters of the asymmetric nanoslit are: w = 190 nm, L_FP = 865 nm, and d = 140 nm. Here, the FP resonator length of about L_FP = 865 nm can make the broadband unidirectional SPP generator have the smallest lateral dimension. Two decoupling gratings (period of 540 nm and separation of 30 μm) lying symmetrically on the two sides of the nanoslit are solely designed to scatter SPPs. Therefore, the detected far–field signals could be directly used to measure the relative intensities of the SPPs generated by the single nanoslit on the front metal surface [12]. In the last, the entire structure was spin coated with a 160–nm–thick PVA (refractive index of n_d = 1.5) film, which can both satisfy the single-mode condition (in the air-dielectric-metal waveguide system) and ensure the tight field confinement for the measurement wavelength range. The AFM measurement shows that the PVA film settles into the FP resonator and keeps its thickness nearly unchanged, leaving an indentation on the top surface of the sample.

In the experiment, the whole structure was normally illuminated from the back side using a p–polarized laser beam (Ti:sapphire laser) with a radius of about 100 μm, which can ensure nearly uniform incident intensities over the slit. The etched Au film in the groove and the Ti adhesion layer is about 60 nm+30 nm=90 nm, so it can prevent the direct transmission light and support one SPP mode only, which will not be perturbed by SPPs generated at Ti/Glass interface. The light impinging on the nanoslit can generate SPPs, which propagate along the front metal surface and are then scattered by the decoupling grating. The scattered light were collected by a long working distance objective (Mitutoyo 100 × , NA = 0.8) and then imaged onto a charge coupled device (CCD). The decoupling grating has a separation of 15 μm from the slit, so the direct transmission light from the slit can be prevent from affecting the detected signal from the decoupling grating. In the measurement, we observed that the lower half part of the left decoupling grating is very dark for a large incident wavelength range, which reveals that the SPPs primarily propagate to the right side for a broad bandwidth. Figure 2 shows the distribution of the scattered SPP intensities obtained from CCD for the incident wavelengths of λ=710 nm, λ=740 nm, λ=770 nm, and λ=810 nm. In these figures, the center of the CCD images is the direct transmission light from the slit, and the left and right sides are the scattered light from the two decoupling gratings. It is observed that the signal intensities scattered from the upper part of the two decoupling gratings are nearly the same because the “isolated” nanoslit is a symmetric structure. For the lower part, the signal intensities scattered from the left and right decoupling gratings are quite different. The left decoupling grating is nearly dark and the right decoupling grating is bright for λ=710 nm, λ=740 nm, λ=770 nm, and λ=810 nm. This reveals that the broadband unidirectional SPP generation is successfully realized in the experiment.

 

Fig. 2 Scattered field distributions for different incident wavelengths of (a) λ = 710 nm, (b) λ = 740 nm, (c) λ = 770 nm, and (d) λ = 810 nm.

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In order to quantificationally describe the unidirectional–generation effect, the generation efficiency, η, was measured from the quotient between the light intensities scattered from the lower and the upper parts of each decoupling grating (evaluated by integration over a spatial scale on the grating). Figure 3 shows the measured η_R (right–propagating SPPs) and η_L (left–propagating SPPs) for various incident wavelengths (black solid lines). Note that SPP generation efficiency to the left is nearly zero in the wavelength range from λ=710 nm to λ=810 nm. In this wavelength range, the extinction ratio of the SPPs, [10 × log(η_R/η_L)], to the opposite directions is greater than 11 dB. Moreover, the SPP intensity at the lower part of the right grating increases to about 1.4 times that at the upper part. Therefore, the dielectric–film–coated asymmetric single nanoslit not only shows robust unidirectional–generating properties but also has higher generation efficiencies.

 

Fig. 3 Experimental and simulation results of SPP generation efficiencies of the PVA–film–coated asymmetric single slit.

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3. Simulations and analyses

To check the validity of the experimental results above, we have carried out numerical simulations by the finite element method (FEM) with Comsol Multiphysics. In the simulations, the asymmetric single nanoslit [Fig. 1(a)] is coated with a PVA film with the thickness of t_PVA=160 nm and the refractive index of n_d=1.5. The structure parameters are taken from the experimentally measured results: w=190 nm, L_FP=865 nm, and d=140 nm. The SPP generation efficiency to one side, η, is defined as the quotient between the SPP intensities on the front metal surface with and without the nanogroove. The permittivity of gold as a function of wavelength was taken from the literature [15], and expanded using the method of interpolation. The simulation results are displayed in Fig. 3 (red dashed line). It is observed that they match the experimental results quite well. From Fig. 3, it is also seen that the generation efficiency of the right–propagating SPPs (red circle dashed line in Fig. 3) shows resonant behaviors, which reveals the FP–resonator effect in the asymmetric nanoslit [12]. Meanwhile, it presents a large flat spectra range from 710 nm to 810 nm. For the left–going SPPs, the generation efficiency (red square dashed line in Fig. 3) also shows resonant behaviors, but the generation efficiency becomes very small (η_L<0.1) for short incident wavelengths (λ≤810 nm). Therefore, broadband unidirectional SPP generation (> 100 nm) with high extinction ratios of > 11 dB is realized, as shown in Fig. 3. This bandwidth is about three times that in the case without the PVA film (Δλ~35 nm).

In order to further understand the physical mechanism of the phenomenon above, we also have calculated the SPP generation efficiency for various FP resonator lengths at λ = 750 nm when the asymmetric single slit is coated with or without the 160–nm–thick PVA film. In the simulations, the slit width is w = 190 nm, and the permittivity of the gold is ε_Au = –20.2+1.25i at λ=750 nm [15]. Figure 4(a) shows the dependences of the SPP generation efficiencies on the cavity lengths (L_FP) in the asymmetric single nanoslit without the PVA film. It is observed that the SPP generation efficiencies present oscillations of nearly sine–cosine line shapes because of the high transmittances of SPPs in the FP resonator [12], and the left–going SPP generation efficiency always maintains a large value of > 0.8. Moreover, for extincting the right-propagating SPPs, there are critical requirements for the FP resonator length, corresponding to the incident wavelength. Thus, it can only realize narrowband unidirectional SPP generation [12]. Coating a PVA film on the asymmetric single slit, the field confinement of SPPs becomes tighter [1–3,13], which can considerably reduce the transmittance of SPPs in the FP resonator to the left side. So the left–going SPP generation efficiency is significantly suppressed, as shown by the black solid line in Fig. 4(b). Moreover, the left–going SPP generation efficiency can be further decreased by increasing the groove depth, which has little relation to the FP resonator length, as shown by the black solid line in Fig. 4(c). Therefore, the coated dielectric film and the deep groove can considerably reduce the left–going SPP generation efficiency for a broad bandwidth. For the right–propagating SPPs, due to the high reflectance of SPPs in the FP resonator, the curve valleys become narrow, and the peaks becomes broad and flat, as shown by the red dashed lines in Figs. 4(b) and 4(c). This can result in a large flat spectra range for the right–propagating SPP generation efficiency [14], coinciding with our analysis and experimental results above. From Fig. 4(c), it is also observed that the FP resonator length of about L_FP = 865 nm can realize a broadband unidirectional SPP generator to the left side with the smallest device size. So we chose this FP resonator length in the experiment.

 

Fig. 4 Dependences of the SPP generation efficiencies to the left (η_L, black line) and to the right (η_R, red dash dot line) on the cavity lengths (L_FP) at λ = 750 nm for (a) t_PVA = 0 nm and d = 140 nm; (b) t_PVA = 160 nm and d = 100 nm; (c) t_PVA = 160 nm and d = 140 nm.

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To further clarify the above principle and improve the device performance, we have made additional simulations. From the above analysis, we know that the transmittance of the SPPs in the groove to the left side can be further reduced when the groove gets deeper and the refractive index of the dielectric film becomes higher. This can result in a more broadband unidirectional SPP generation with higher extinction ratios. Simulations show that the left–propagating SPP generation efficiency is greatly suppressed from λ = 680 nm to λ = 1060 nm when the groove depth and the refractive index of the dielectric film become d = 155 nm and n_d = 2.0, while the right–propagating SPP generation efficiency nearly maintains a value of > 1.0. Thus, more broadband (> 370 nm) unidirectional SPP generations with higher extinction ratios of > 13 dB can be achieved. This bandwidth is about ten times that in the case without the dielectric film (Δλ~40 nm).

4. Conclusion

In summary, by using a dielectric–film–coated asymmetric single nanoslit, broadband unidirectional SPP generation was experimentally demonstrated. In this structure, the tight field confinement (owing to the dielectric film) and the deep groove considerably reduce the transmittance of SPPs in the FP resonator to the left side. So the left–propagating SPP generation efficiency is greatly suppressed for a broad bandwidth. For the right–propagating SPPs, their generation efficiency has a large flat spectrum range because of the low transmittance of SPPs, too. Therefore, the unidirectional SPP generation was experimentally realized in the spectrum range of >100 nm around λ = 750 nm with the extinction ratio of > 11 dB for the device lateral dimension of only 865 nm. When the groove in the asymmetric single nanoslit gets deeper and the refractive index of the dielectric film becomes higher, the left–propagating SPP generation efficiency can be further suppressed, and thus more broadband (> 370 nm) unidirectional SPP generation with higher extinction ratios (> 13 dB) can be achieved. This bandwidth is about ten times that in the case without the dielectric film. Launching SPPs with robust unidirectional–generating properties in such compact structures is very important for highly integrating plasmonic circuits.