This paper presents a plasmonic bull’s eye consisting of a micron-sized hole and a concentric nano-antenna metallic ring surrounded by periodic circular grooves on a thin gold film. The unique metallic nano-ring imbedded in the supra-wavelength-sized hole acts as an amplifying and filtering component to simultaneously provide a significantly lower spectral noise and a higher power transmission at the resonance wavelength, in comparison to prior sub-wavelength bull’s eyes. Systematic numerical analyses based on finite-difference time-domain method were carried out to find the impacts of the structural parameters. Experimentally we integrated three proposed plasmonic structure on a cleaved facet of an optical fiber that can act as a spatially and spectrally multiplexed photon sorter. Transmission characteristics of the proposed devices were characterized in terms of the spectral response and signal to noise ratio. Potential applications of the fiber optic photon sorter were also discussed.
© 2013 Optical Society of America
The surface plasmon polaritons (SPPs) on a hole array in metal film have generated intense research interests due to their unique enhanced optical transmission (EOT) and sub-wavelength scalability in optoelectronics . In recent years, plasmonic structures have rapidly expanded their applications in biosensing , nano-scale networking , nano-optical trapping , nano-imaging, super-lensing , high efficient LEDs, solar cells , and spectral imaging , to name a few. In the context of spectral imaging, the sub-wavelength-sized bull’s eye (sub-λ-BE) consisting of a central nano scale hole flanked by concentric periodic circular grooves on a metal film, has been of particular interests . In prior sub-λ-BEs, the hole diameter was kept smaller than the resonance wavelength and the periodic circular grooves act like an antenna for the incoming light by converting it to SPPs to result in the resonant EOT through the hole. Detailed structural optimization and subsequent optical characteristics of sub-λ-BEs have been carried out in prior reports [8–10]. Despite flexible tunability of EOT resonant wavelength, conventional sub-λ-BEs have an inherent shortcoming of very low power transmission due to their sub-wavelength daperture size [8, 9].This paper presents a new category of bull’s eyes that can provide much higher transmission compare to sub-λ-BEs for practical applications.
In this study we introduce a new category of the plasmonic bull’s eye, which consists of a central hole with a supra-wavelength scale diameter exceeding 1μm with a metallic nano-ring antenna concentrically imbedded there within, along with periodic circular grooves on a gold thin film deposited on the facet of a silica optical fiber. Firstly we numerically proved that the proposed supra-wavelength bull’s eye (supra-λ-BE) structure can provide a larger EOT throughput with a significantly less spectral noise by an optimal balance between the functions of the larger central hole and the nano-antenna ring. We further fabricated a single supra-λ-BE and then three supra-λ-BEs multiplexed on the facet of a hard polymer cladding fiber (HPCF) to investigate their optical characteristics, for the first time.
The proposed structure is schematically shown in Fig. 1(a). The proposed supra-λ-BE has the hole size larger than the resonance wavelength enabling us to render significantly larger throughput. The nano-ring embedded in the hole further enhances the resonant nature suppressing the spectral noise. Structural parameters of the proposed device are shown in Fig. 1(b), where we assumed Air/Gold and Gold/Silica interfaces. The parameters are categorized into those related to the hole (d), nano-ring (c, r), and the grooves (a, p, w, s). Figure 1(c) illustrates a fiberized plasmonic photon sorter comprising of three distinctive supra-λ-BEs spatially multiplexed on the metalized end-facet of the HPCF.
Metallic nano-antennas have shown unique optical properties including the optical excitation of surface plasmon resonance, strong localization of energy at nanometer scale, enhancement of electric field, and resonance wavelength tunability [11,12]. Among the metallic nano-antennas, circular rings have been center of research interests because they exhibited a strong localized surface plasmon resonance which can be spectrally tuned simply by varying the ring thickness and its radius . Nano-antenna rings are known to be polarization-insensitive, which would make a good match with polarization-insensitive structures such as bull’s eyes.
Spatial and spectral multiplexing of supra-λ-BEs on an optical fiber offers several advantages over the traditional glass cover slip substrates or free standing films [14, 15]. In contrast to these bulk-optic counterparts, the proposed fiber optic device can efficiently confine the light without additional focusing optics and directly couple the light to SPPs on the metal/dielectric surfaces . Optical fiber is light and flexible to provide a new avenue of applications in the microscopic environment, where prior bulk optic could not reach physically. Furthermore, the proposed fiber optic platform miniaturizes the plasmonic photon sorter to the micron scale making it perfectly compatible to silicon photonic circuits in the integrated optical links [17, 18].
2. Design and characterizationFig. 1(b) are labeled as L. The refractive indices of silica and air are denoted by nsilica and nair respectively.
In this study we applied two strategies to increase the total EOT throughput at the resonance wavelength, Tc(λr). Firstly, the intrinsic transmission of the hole, TH(λr), was enhanced by both increasing the hole size and placing an optimal metallic nano-antenna ring as shown in Fig. 1(b). Secondly, modulation functions A1(λr) and A2(λr) were maximized by optimizing the groove parameters, L .
2.1 Aperture design
In prior sub-λ-BEs, the hole diameter defines the cut-off wavelength and TH(λr)is maximized only if the hole size, d, remains in the sub-wavelength range near d~λr/2 . In contrast, in this supra-λ-BE we increased the hole size as large as d~2λr. However, there is a complicated trade-off between the plasmonic features (A1 and A2), and the aperture feature (TH). It is well-known that the larger the hole size of a bull’s eye is, the lower the impact of SPPs coupling becomes [8, 9, 20]. As the hole size increases the plasmonic local dipole magnitude emerging on the rim of the hole decreases, to result in a weaker coupling between the directly transmitted light and SPPs . We used a commercial finite difference time domain (FDTD) simulation package (Lumerical Solutions Inc.), to find the impact of the hole size on transmission and the results are summarized in Fig. 2. Here we fixed the groove parameters as p = 500 nm, a = 1600 nm s = 120 nm, w = 250 nm, h = 300 nm, number of grooves = 7, and varied only the hole diameter d = 0.5~2.0 μm. Note that in Fig. 2 we did not include the nano-antenna ring yet. The transmission spectra are normalized to that of a grooveless bull’s eye with d = 1.5 μm.
In the sub-λ-BE regime with d = 0.5 μm (black curve in Fig. 2), the plasmonic factors A1(λr) and A2(λr) strongly contributed to the EOT peak located near λr = 730 nm. Yet by increasing the hole size, TH shows a stronger contribution, with a decreasing contrast of the EOT peak against the overall transmission background (green and red curves in Fig. 2). If the hole size further exceeds 1.5 μm, d>2.5λr, the plasmonic feature is lost as in the top blue curve in Fig. 2.
Figure 3 illustrates the contribution of TH as well as plasmonic parameters to EOT at different hole sizes. Solid curves in Fig. 3 are electric field intensities at 30nm above the bull’s eye, and dashed curves are those of the grooveless bull’s eye representing electric field intensities of a weakly plasmonic apertures (A1 and A2 <<TH). As red dashed curves indicated in Fig. 3(a) and 3(c), TH has less contribution in d = 0.5μm, and more dominant contribution in d = 2.0μm. However, at the hole size of d = 1.5μm there is a trade-off between TH and plasmonic parameters to give rise to an unique electric field intensity that can be further optimized by changing structural parameters.
In order to simultaneously suppress the spectral noise and increase the EOT peak, we proposed a new idea to imbed a nano-antenna ring inside the supra-wavelength-size hole, as shown in the inset of Fig. 4.
In theoretical analyses, we found that the nano-antenna ring can increase the EOT peak about 20%, while reduces the background transmission about 22~52%. See red curve in Fig. 4. The nano-antenna ring plays the key role of amplifying and filtering in the supra-λ-BE.
To better understand the role of the nano-antenna ring in the supra-λ-BE, the intensity profile of light was investigated along the propagation axis, z, at the resonance (Fig. 5(a)) and a non-resonance wavelength (Fig. 5(b)) using FDTD. As shown in Fig. 5(a), in Silica/Gold interface, the electromagnetic wave reemitted by nano-antenna ring at the resonance wavelength of λr = 721nm (panel ii) is in-phase with that of the bull’s eye resulting in a stronger filed enhancement compare to the case without the nano-antenna ring (panel i). The intensity distribution in panel ii indicate that both sides of the nano-antenna ring contribute to the electric field enhancement. Here the field enhancement in Air/Gold interface refers to the solely electromagnetic coupling between the inner and outer walls of the ring .
On the contrary, as in Fig. 5(b), at a non-resonance wavelength λ = 689 nm, the electromagnetic wave reemitted by the nano-antenna ring is not phase-matched with that of the bull’s eye (panel ii') giving significantly less throughput than the case without nano-antenna ring (panel i'). Figure 5(c) depicts more clearly the intensity profiles of the beam rendered by the supra-λ-BE for the cases of i, ii, i', and ii' at the distance of z = 1μm. The comparison between i and ii in Fig. 5(c) confirms that the nano-antenna plays an amplifying role to boost the EOT at the resonance wavelength. Comparison between the intensity profile of i' and ii' in Fig. 5(c) also indicates unique role of spectral filtering of the nano-antenna ring to suppress transmission at non-resonance wavelengths.
Figure 6 demonstrates more clearly the mechanism of destructive and constructive electric field interface of the structures in panel (ii) and (ii'). The shaded area in Fig. 6 illustrates the position of the central hole. As shown in Fig. 6(a) the constructive interference of electric fields in Silica/Gold interface results in a stronger field intensity distribution, the red dashed line, near the rim of the hole, x = 0.75~2μm, compared to that of the structure in panel (i) (blue solid curve). However, in Fig. 6(b) the destructive interference at a non-resonance wavelength results in a weaker electric filed intensity, red dashed curve, on the rim compared to that of the structure in panel (i') (blue solid curve).
Further numerical study on nano-antenna rings showed that the filtering strength of the supra-λ-BE can be maximized by properly adjusting the nano-ring parameters (c:thickness and, r: inner radius). Figure 7 shows how the normalized transmission behaves as we vary these nano-ring parameters. The solid lines are the EOT intensities and the dashed lines are the averaged background spectral noise. Here we chose two periodicity p = 450, 500nm. The most efficient spectral filtering could be obtained when the contrast between EOT and the averaged noise is maximized. As Fig. 7(a) and 7(b) indicate the transmission contrast of the supra-λ-BEs can be further maximized with the conditions 1) c = 130nm, r = 240nm for a supra-λ-BE with p = 500nm, and c = 130nm, r = 270nm for p = 450nm. These results indicated that the ring radius should be increased when the grooves period was decreased in order to get maximum transmission contrast.
2.2 Bull's eye design
Tc(λr) was further maximized by the modulation functions A1(λr) and A2(λr) with optimal lateral parameters (L: a, p, w, s). FDTD simulation results showed that at the resonance wavelength of λr = 620~740 nm the maximum transmission was obtained when p = 420~500 nm, w = 250~300 nm, and s = 120 nm, satisfying the optimal approximations: w/p~0.5, s/w~0.4 as already confirmed in prior sub-λ-BEs . Spacing parameter, a, was known to play a key role in the constructive coupling of SPPs . Figure 8 shows the EOT peak variation as a function of the spacing parameter, a, for two different groove periods, p = 500 nm and 450 nm. Maxima in Fig. 8 indicate that the constructive interference of SPPs with transmitted light can be maximized at an optimal spacing parameter (a = amax), which are summarized in Table 1. The wavelength of SPPs, λSP, traveling in the Gold/Silica interface at resonance is given by a dispersion relation :
Here εg and εs represent the frequency dependent permittivity of the gold film and the silica substrate respectively. As shown in Table1, we could confirm an optimal spacing parameter approximation as in prior sub-λ-Bes :
3. Device fabrication
Figure 9 schematically shows the fabrication process of the single supra-λ-BE on silica optical fiber facet. HPCF was cleaved vertically using an ultrasonic cleaver to prepare an optical quality facet. A 150-nm-thick sacrificial gold layer was deposited on the HPCF facet using a thermal evaporation technique to overcome charging problem in the following focused ion beam (FIB) process. Circular grooves with the depth of 120 nm were milled on the fiber facet using a FIB setup (SII Nanotechnology SMI 3050). The sacrificial layer on the fiber was wet-etched by using Potassium Iodide (KI) solution which does not have any reaction with silica, and then the prepared facet was re-coated by 300-nm-thick gold film. Finally the central supra-λ hole and the nano-antenna ring were inscribed at the center of the grooves using FIB.
4. Experiments and results
Figure 10(a) shows the FIB micrograph of a fabricated device with a single supra-λ-BE on fiber, where the inset diagram magnifies the central hole and nano-antenna ring. In order to measure the transmission characteristics of the proposed device, a white light source was coupled to the HPCF’s input end. The normalized transmission spectrum of the supra-λ-BE is shown in Fig. 10(b) (black solid curve), measured by a UV-visible spectrum analyzer (Maya 2000), along with simulation results (red dotted and blue dashed curves). An EOT peak at 725 nm was observed and correlated to SPP sat the Gold/Silica interface, consistent to Fig. 4 and Table 1. In contrast, SPPs at the Gold/Air interface had a negligible influence on EOT peak power, since the evanescent waves at Gold/Air interface are exited through indirect illumination of the incident light. Additionally, due to asymmetry of ambient dielectrics on two sides of the gold film the resonant frequency of SPP in Air/Gold interface and that of the Silica/Gold doesn’t coincide resulting in low contribution of the Air/Gold interface to EOT . Other broad peak observed in the spectral range λ<725 nm are attributed to higher order SPP modes . The background noise in the transmission observed in this range is known to originate from the defects in the grooves . Using FDTD analyses, the EOT peak was predicted at 721 nm (dashed blue curve in Fig. 10(b)) showing a good agreement with the experimental measurements.
In order to quantitatively investigate the contribution of nano-antenna ring to EOT, we defined ‘transmission contrast’, which is the difference in the dB scale between the EOT peak intensity and that of the background in the spectral domain. In Fig. 10(b), the transmission contrast in the fabricated supra-λ-BE with the nano-antenna ring was measured to be 6.7 dB, which is in a good agreement with 6.1 dB predicted by FDTD simulation. While the transmission contrast in the absence of the ring is predicted as low as 3.7 dB (see red dotted curve).
Having successfully established the fabrication process for the single supra-λ-BE on fiber, we further imbedded three distinctive supra-λ-BEs on the HPCF facet, in order to realize a spatially and spectrally multiplexed plasmonic photon sorter on fiber. Figure 11(a) and 11(c) illustrate FIB micrographs of a fabricated three supra-λ-BEs on the end-facet of a HPCF. Figure 11(b) shows the end facet of the fiber when white light was launched into the fiber . Three supra-λ-BEs were spatially multiplexed in a linear array with the pitch of 25 μm on the facet of optical fiber, without any overlapping grooves as shown in Fig. 11(c). Note that this linear structure is different from prior report based on a free standing foil where the sub-λ-BEs were arranged in a triangle with significantly overlapping the grooves . Detailed structural parameters are summarized in Table 2.
The transmission spectrum of the fabricated photon sorter was measured by coupling a white light source to the HPCF input end and the results are summarized in the black solid line of Fig. 11(d). In experiments, we measured three strong peaks denoted by 1, 2, and 3 as shown in Fig. 11(d) and they are attributed to the EOT resonances of three supra-λ-BEs. The FDTD simulations were carried out for each supra-λ-BE and the results are overlaid in dotted lines in Fig. 11(d). The experimentally measured EOT resonance wavelengths are listed in Table 3, along with corresponding simulation results in the parentheses. The measured spectral positions of these EOT peaks showed a fair agreement with the FDTD simulation results and we could confirm that spatially and spectrally multiplexing of supra-λ-BEs were realized on the fiber facet, for the first time. It is also very noteworthy to find that transmission contrast for each EOT also showed a very good agreement between the measurements and FDTD simulation as in Table 3. The difference between the experiments and the simulation are mainly attributed to inevitable structural imperfections introduced in the fabrication processes. Interestingly in the experiment a higher normalized transmission at EOT peaks was observed (3.4~3.8 in the normalized scale on the right axis of Fig. 11(d)) than the numerical prediction (1.6~2.4 in the normalized scale on the left axis of Fig. 11(d)).
Each EOT showed a Gaussian-like beam with the beam diameter of ~2 μm measured at the axial distance of ~1 μm from the exit surface, which is also consistent to FDTD simulations in Fig. 5(c). The three beams were spatially separable within this range.
The spectral domain (627~734 nm) of the device corresponds to the responsive range of Si photo-detector, and the full width at half maximum beam diameter of ~2 μm is highly comparable to Si photonic waveguide devices . In terms of this spectral domain matching and beam diameter, the proposed device can have a high compatibility to Si photonics and the authors are investigating feasibility of further integration of the device with Si waveguides and photo-detectors for optical interconnection applications.
In summary, we both theoretically and experimentally demonstrated a new potential solution to overcome the fundamental bottleneck of prior sub-wavelength bull’s eye devices: inherently low transmission. To challenge this issue we proposed a unique supra-λ-BE structure, which is composed of the central hole enlarged to ~2 times of wavelength and a concentric nano-antenna ring imbedded there within. Optimizing the structural parameters, we theoretically predicted that the supra-λ-BE can provide significantly increased transmission intensity by more than 20% and enhanced peak-to-background ratio over 6 dB at the SPP resonances. Single supra-λ-BE was firstly inscribed on the HPCF facet and we confirmed experimentally the impacts of proposed structure by observing enhanced transmission and peak-to-background ratio. We then further integrated three distinctive supra-λ-BEs to form a 1 × 3 array on the fiber facet multiplexing plasmonic resonance both spectrally and spatially. This fiber-optic plasmonic photon sorter showed enhanced optical transmission peaks in the spectral range from 627~734 nm with a beam diameter of ~2 μm, which are highly compatible to silicon photonic devices. Further applications in optical interconnection in silicon photonics are being pursued by the authors.
This work was supported in part by the Brain Korea 21Project, in part by the NRF of Korea and a grant funded by the Korea government (MEST) (2011-00181613, 2012M3A7 B4049800), in part by the Seoul R&BD Program (PA110081), in part by the Samsung Electro-Mechanics (2013-8-1221), and in part by the LG Display (2011-8-2160).
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