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We report aluminum based structures for manipulation of surface plasmon polariton (SPP) propagation at short wavelength range. Our simulation shows that aluminum is a good metal to excite and propagate SPPs with blue light and that the SPP wavelength can be reduced from about 465 nm to about 265 nm by monitoring the thickness of a coated Si3N4 layer above the aluminum film. It is also shown that the damping becomes more significant with the increase of the thickness of the Si3N4 layer. We also experimentally demonstrated the SPP wavelength tuning effect for 20nm Si3N4 layer covered Al, which can be explained by the variation of effective permittivity. The proposed Metal-Insulator-Air (MIA) structures with SPP wavelength tuning ability have potential applications in 2D optics.
© 2015 Optical Society of America
1. Introduction
Confining light into a deep subwavelength scale has made surface plasmon polaritons (SPPs) attractive to the fields of modern optics, particularly in metamaterial studies [1–5 ], super-resolution imaging [6–8 ], optical trapping [9, 10 ], and transformation plasmonics [11, 12 ]. The propagation of SPPs can be controlled by monitoring polarization of the incident light [13–15 ]. As quasi particles, SPPs also play an important role in the area of sub-wavelength optics and 2D optics [16, 17 ]. However, the widely used noble metals, e.g., gold and silver, generally do not perform well in the short visible wavelength as compared to longer visible and infrared wavelength. This is mainly attributed to the interband transitions in the visible spectrum, which gives higher damping constant.
For this reason, aluminium is often proposed as an alternative material for plasmonics due to its lower damping constant in the UV and visible range, although with the disadvantage of oxidation [18]. The capability of aluminium for plasmonic applications has been theoretically studied [19, 20 ]. Experimentally, plasmon resonance in various aluminium nanostructures has been investigated [21, 22 ], and the feasibility of plasmonic guiding in Al/SiO2 material system had been discussed [23]. However, there have been a few experimental studies about in-plane SPP properties on aluminium. In this paper, we report SPP propagation in the Al/air and Al/Si3N4/air structures for short visible wavelength. The Si3N4 layer in the Al/Si3N4/air structures is found capable of modifing SPP wavelength and loss in addition to preventin native oxidation of aluminum.
2. Structure design and principles
In the near field optics, the propagation of surface plasmon polaritons (SPPs) is characterized by surface plasmon wavelength (λSPP). For an incident light of wavelength λ0 , the dispersion relation of SPPs along the metal-dielectric interface is given by [24]:
where ε m’ and ε d are the real permittivity of the metal and dielectric, respectively. In the case where the metal is highly plasmonic, |ε m’| >> εd, Eq. (1) can be simplified aswhich suggests that changing dielectric permittivity can readily tune the SPP wavelength.However, the dielectric permittivity as perceived by the propagating SPP may not be the same as the bulk permittivity of the dielectric, particularly when the thickness of the dielectric film is smaller than the SPP penetration depth in the dielectric (t < δ d). This is expected as the SPP electric field overlaps with both dielectric and air, which gives an averaged permittivity according to its overlap integral. The SPP penetration depth in the metal (δ m) and dielectric (δ d) are expressed as [24]
which, in highly plasmonic metal case (such as Al), can be further simplified into the followingwhere k 0 is the wave vector of the incident light in the vacuum.It is clear that the SPP wavelength can be accurately predicted by Eq. (1) when the thickness of the dielectric film is thicker than the SPP penetration depth (t > δd). However, when the dielectric thickness is smaller than the SPP penetration depth (t < δd), the SPP wavelength would be longer than that calculated from Eq. (1). The loss and damping constant will also change as the film thickness is changed. As the leakage increases from the insulator-air interface, the direct measurement becomes possible. In our work, Silicon Nitride (Si3N4) was chosen for the dielectric layer. The SPP penetration depth in Si3N4 is deduced as δd ~90 nm for the incidenct wavelength of λ0 = 473 nm. Figure 1(a) shows the schematic of the Al/Si3N4/air structures with 80 nm thick aluminium and varied thickness of the Si3N4 layer.
Fig. 1 (a) Schematic diagram of an aluminium based MIA structure. (b) Simulated Ez of the MIA structure consisting of 80nm Aluminium film and Air at z-y plane. (c) Simulated Ez of the MIA structure consisting of 80nm Aluminium film, 40nm Si3N4 film and Air at z-y plane. (d) Ez distributions in x-y plane at Si3N4/Air interface of the MIA structures with different thicknesses of Si3N4 layer above the 80nm Al film. The dashed straight lines are used to indicate the wavefronts of SPPs of the MIA structures.
Figures 1(b) and 1(c) show the simulated electric field distributions in z-y plane of the Al-Air structure (80 nm Al) and a typical MIA structure (80 nm Al and 40 nm Si3N4), respectively. The SPPs are excited by linearly polarized light of 473 nm at normal incidence through a slot opening which is about 100 nm wide. The input of the incident electric field is 1 Vm−1, and the permittivities are −30.498 + 7.727i for Al, 4.17 for Si3N4 and 1 for air at the incident wavelength of λ0 = 473 nm [25, 26 ]. As shown from the figures, the SPPs excited by the incident light from the air slit propagate along the y and –y directions symmetrically and the field intensity decay along the propagation directions. It is also seen that for the structure with the 40 nm Si3N4 layer, the SPP field distribution and wavelength are different from those of the structure without the Si3N4 layer. To study the effects of the dielectric layer above the aluminium film, the E z field distributions of the Al/Si3N4/air structures with same Al thickness (80 nm) but varied thickness of the Si3N4 layers from 20 nm to 60 nm are calculated and plotted in Fig. 1(d), where the vertical dash lines are used to indicate the wave fronts of these SPPs. It is clearly seen that the SPP wavelength shrinks with the increase of Si3N4 thickness, which is accompanied by the increasing propagation loss.
For more details of SPP propagation in MIA structure, Fig. 2(a) shows the damping constant of E z fields corresponding to 4 different Si3N4 thicknesses. As apparent from Fig. 2(a), for all the MIA structures studied, the decay becomes progressively faster as the Si3N4 thickness is increased. This observation is consistent with Eq. (4) which describes the relation between permittivity and the SPP penetration depth. Assuming that the Si3N4 is well below its skin depth (t < 90 nm) where the effective permittivity depends on the film thickness, one can see that increasing Si3N4 thickness gives higher effective Si3N4 permittivity, which according to Eq. (4) leads to the increase (decrease) of δm (δd). This in turn gives increased overlap between the field and metal, translating to higher propagation loss. On the other hand, decreasing Si3N4 thickness translates into increasing overlap between the field and the dielectric, resulting in lower propagation loss. To have quantitative description for the decay, the SPP propagation can be expressed as [27]:
where αSPP is the electric field decay rate of SPPs propagating in y direction which can be derived from the E z field damping and shown in Fig. 2(b).Fig. 2 (a) Damping of electric field Ez at Si3N4/Air interface of the MIA structures consisting of 80nm aluminium film and varied thickness of Si3N4 film. (b) SPP wavelength (circle), effective refraction index neff (triangle) and decay rate (cross) as a function of thickness t of Si3N4 film. Here, neff approximately follows a polynomial relation, neff = 1.01 + 0.0021t + 1.81 × 10−4t2.
The SPP wavelength as a function of Si3N4 thickness is presented in Fig. 2(b), where the wavelength has shrunk from 430 nm to 265 nm as the Si3N4 thickness is increased from 20 nm to 60 nm. This also corresponds to the increase of the effective index of the dielectric materials which is consistent to Eq. (2), as shown in Fig. 2(b).
3. Experimental verification
For experimental verification, the thickness of Si3N4 film cannot be too large as it will weaken the near-field signals on the top Si3N4 surface, making the NSOM characterization difficult. For the above reason, we fabricated two structures, one of which has only 80 nm Al and the other has 80nm Al/ 20nm Si3N4 on quartz substrate, as illustrated in Figs. 3(a) and 3(b) . The top views of the two structures with fabricated gratings are shown in Figs. 3(c) and 3(d).
Fig. 3 Schematic of (a) 80-nm thick aluminium film on quartz substrate (Sample 1), and (b) 20-nm thick Si3N4/80-nm thick Al on quartz substrate (Sample 2). The SEM images of these samples integrated with grating coupler: (c) Sample 1, and (d) Sample 2.
The Al sputtering was carried out with 30W DC power and 6.4 sccm Argon gas, while the Si3N4 sputtering was carried out with 80W RF power and 5.5 sccm Argon gas. There was no break of vacuum between the deposition of the two layers. The thicknesses of both layers were verified by a surface profiler. The gratings were then fabricated by focused ion beam (Carl Zeiss AURIGA crossbeam FIB-SEM workstation) using 30 keV at 120pA beam current. The slit width of the gratings is 100 nm, while the periodicities are 470 nm (sample 1) and 400 nm (sample 2). The coupling efficiency varies with the number of grating slit and periodicity. We made 5 grating slits by considering the laser spot size. The periodicity for sample 2 is mismatched with the simulated SPP wavelength. Such mismatch does not significantly affect our analysis as it has no effect on SPP wavelength although the field intensity is not as strong as in the matched case.
The electric field distributions were measured using a near-field scanning optical microscope (NSOM). The linearly polarized 473nm laser was normally incident from the bottom of the quartz substrate. We used a photomultiplier tube (PMT) for signal detection and amplification. The NSOM probe tip (aluminium-coated) is less than 100 nm in diameter and it is positioned about 10 nm above the sample surface, based on shear-force feedback mechanism.
The SPP near field patterns are shown in Fig. 4 , which results from the interference between the in-plane SPP fields and the transmitted out-of-plane incident beam [14, 28 ]. The SPP wavelengths can be deduced as the distance between the fringes, λspp = d/(q - 1), where d is the distance between q fringes. The SPP wavelengths in Figs. 4(c) and 4(d) are about 465 ± 5 nm for the sample with 80nm Aluminium only and about 430 ± 5 nm for the sample with 80nm Aluminium film and 20nm Si3N4 coating. The FEM simulation and NSOM results are also presented in Figs. 4(a) and 4(b), where the fringing can be seen for each condition, showing excellent agreement between the measured values and simulation. Finally, we compare our experimental findings with our simulations in Fig. 2(b), showing excellent agreement.
Fig. 4 Ez distributions in x-y plane of (a) sample 1 and (b) sample 2 for y polarized incidence. NSOM images of (c) sample 1 and (d) sample 2. The dashed arrows in (a) and (b) denote the direction of linear polarization.
4. Conclusion
We proposed aluminium based MIA structures and studied SPP propagation characteristics theoretically and experimentally. Such structures can be used to excite and propagate SPPs of wavelengths from 465 nm to about 265 nm by varying the thickness of a coated Si3N4 layer up to 60 nm, which provides an alternative way to manipulate in-plane SPPs. In addition, with a coated thin Si3N4 layer, the oxidation of Al can be prevented to a certain extent. We believe that the presented aluminium based MIA structures have potential applications in nanophotonics, such as super-resolution focusing and 2D optics.
Acknowledgments
This work is supported by the Ministry of Education (RG86/13), A*Star (1220703063), the Economic Development Board (NRF2013SAS-SRP001-019), Singapore and Asian Office of Aerospace Research and Development (FA2386-14-1-0013). The authors thank X. Shen, B. Cao and C. Zou from SPMS, Nanyang Technological University for their assistance in FIB fabrication.
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