Single Asymmetric Plasmonic Antenna as a Directional Coupler to a

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Single plasmonic V-antenna as a directional coupler to a dielectric waveguide Dries Vercruysse, Pieter Neutens, Liesbet Lagae, Niels Verellen, and Pol Van Dorpe ACS Photonics, Just Accepted Manuscript • Publication Date (Web): 25 Apr 2017 Downloaded from http://pubs.acs.org on April 27, 2017

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Single asymmetric plasmonic antenna as a directional coupler to a dielectric waveguide Dries Vercruysse,1, 2 Pieter Neutens,1 Liesbet Lagae,1, 2 Niels Verellen,2, 1 and Pol Van Dorpe1, 2

1

2

imec, Kapeldreef 75, B-3001 Leuven, Belgium

KU Leuven, Dept. of Physics and Astronomy, Celestijnenlaan 200 D, B-3001 Leuven, Belgium

ABSTRACT: Directional in-coupling of light into a single mode SixNy waveguide is demonstrated using a single sub-wavelength asymmetrically shaped metallic nanoantenna. In simulation as well as experiments, we show that at its quadrupolar resonance the V-shaped antenna can couple near-infrared light in the waveguide with a directivity reaching 25 dB. As an ultra-compact directional coupler in the near-infrared and visible spectral range, the investigated system constitutes an important building block for integrated nanophotonic circuits and sensors.

KEYWORDS: plasmonics, nanoantenna, directional, waveguide, in-coupler, nanophotonics

The integration of plasmonics in dielectric photonic integrated circuits brings many new opportunities for compact optical on-chip devices. Plasmonics provides a means to scale down integrated photonic circuits by enabling sub-wavelength signal processing [1,2], while alternatively, integrated photonic circuits can serve as a compact optical interface to plasmonic

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sensors [3,4]. Coupling between plasmonic and dielectric photonics is therefore a growing field of research where different metal-dielectric configurations are being explored [5-7]. Metal nanoantennas of various complexity have been fabricated on top of, or embedded in, waveguides, producing novel devices for sensing, telecommunication and non-linear optics [4,812].

Plasmonic antennas with highly directional lateral radiation patterns are interesting candidates for integration. Strong lateral scattering (in contrast to forward or backward scattering) can increase waveguide in-coupling efficiency, while directionality can be used to color-route light or to create plasmonic sensors [13]. On a high-index waveguide, a disk antenna with a slit has been shown to demultiplex telecom wavelengths by coupling different wavelengths in opposing directions [14,15]. At optical frequencies, antenna designs such as V- or U- antenna geometries have shown similar free space directional radiation patterns [16,17]. In free space, for example, bimetallic dimer nanoantennas have been used to detect small refractive index changes by simply gauging lateral directionality, rather than analyzing a spectrum. Directional plasmonic antennas are therefore promising candidates for compact integrated plasmonic bio-sensors. [18].

In this work, we study the directional coupling of a plane wave light source to a single mode silicon nitride, SixNy, waveguide mediated by a single V-shaped plasmonic antenna. The directional behavior of such a nanoantenna is the result of the interference of two plasmonic modes, as illustrated in Figure 1a [16]. Radiation of the dipolar and quadrupolar modes interferes constructively in the direction of the antenna’s tip, but destructively in the direction of the antenna’s opening. The resulting combined radiation pattern near the quadrupole resonance is therefor strongly directional [17,19]. First, we examine the antenna's in-coupling cross section


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and directionality in simulation, and take a closer look at the influence of the Si-carrier substrate on the in-coupling spectra. We then experimentally verify the simulated directivity and study the spectral tunability of the directivity spectra by experimentally varying the V-antenna's angle and length.

Figure 1: (a) Radiation pattern of the dipole and quadrupole modes at the quadrupole resonance wavelength of a V-antenna. The phase of the radiated electric field is indicated by the color map and arrows [18]. (b) FDTD simulation setup: the light coupled in by the V-antenna in the center of the waveguide is evaluated by the monitors, Ml and Mr. (c-e) Simulation results for 3 ACS Paragon Plus Environment

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an antenna with a 120° angle and 220nm leg length. (c) Light transmitted through Ml (red) and Mr (blue), total in-coupled light intensity (green) and directivity (black). (d) Antenna optical cross sections: extinction (black), scattering (red), absorption (blue), and in-coupling (green). (e) Electrical field intensity in the xz-plane through the waveguide, the bottom plot shows the same data as the top in a logarithm scale. The orange lines outline the waveguide. The black rectangle covers the high field in the TFSF region.

The simulation of the investigated antenna-to-waveguide coupler is schematically depicted in Figure 1b. The V-antenna is positioned directly on top of the waveguide with the tip of the Vshape pointing along the waveguide. The large dashed box in Figure 1b indicates the simulation space of the FDTD software, Lumerical [20]. A total-field-scattered-field (TFSF) source is used to simulate the incident plane wave on the antenna and is indicated by the small dashed box encompassing both the antenna and the waveguide. The mesh size is 2nm around the antenna, 10nm in the waveguide and 25nm outside the waveguide. In this initial simulation geometry, the Si-carrier wafer is not taken into account; the substrate is thus assumed completely SiO2. For the refractive index of the SiO2 and the SixNy we take 1.46 and 1.89, respectively, while for the dielectric properties of gold the data of Johnson and Christy is used [21]. The waveguide is 500nm wide and 180nm thick. The simulated V-antenna has a thickness of 50nm, has two legs with a length of 220nm and a width of 60nm and exhibits a 120° angle between the legs. Panel c in Figure 1 shows the power that the antenna couples into the different directions of the waveguide. The red curve indicates the power through monitor Ml depicted in panel b, i.e. the power in the direction of the antenna’s tip. The blue curve shows the power in the opposite direction through monitor Mr. The green curve is the sum of the power through both monitors, and thus represents the total in-coupled power. It reaches its maximum value around 780nm,


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where the antenna has its quadrupole resonance peak (see panel d). On the longer wavelength side of this resonance, the directivity, i.e. the ratio of the power transmitted through Ml and Mr expressed in dB, 10log10(Ml/Mr), reaches a maximum. The electric field in the xz-cross-section through the waveguide at this wavelength can be seen in Figure 1e. The top image clearly shows how the scattered light of the antenna is coupled into the left side of the waveguide. A considerable amount of the scattered light nevertheless is also radiated to the far field. In order to better visualize this, the same image is shown below in a logarithmic scale. We can clearly see additional light radiating from the antenna, as well as through leaky waveguide modes further along the waveguide. The latter is particularly visible in the positive x-direction. To better quantify how light scattered by the V-antenna is coupled to the waveguide, we look at the antenna's different optical cross-sections in Figure 1d. At the directionality peak indicated by the dashed line, the scattering cross-section is about 0.021µm2, a little over half of the antenna’s extinction cross-section. The light coupled-in by the waveguide mode is about 5.1% of the total scattered light, which results in an in-coupling cross-section σwg,of 0.0011µm2.


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Figure 2: (a) Dielectric material stack: 180nm SixNy, 2µm SiO2 on a Si-substrate. The red cross indicates the position of the antenna. (b) Electric field intensity at the antenna’s position. (c) FDTD simulation results for an antenna with a 120° angle and 220nm leg length taking the Si-substrate into account: transmitted light power through monitors Ml (red) and Mr (blue) on the left axis, and in-coupling directivity on the right axis (black).

The carrier substrate, which was thus far assumed to be SiO2, also has a strong effect on the incoupling spectra. Experimentally, the waveguides are fabricated on a Si-wafer with a 2µm SiO2 cladding layer between the SixNy waveguides and the carrier Si-substrate. Reflections of the incident light on the Si-substrate will result in Fabry–Pérot standing wave resonances in the cladding layer, causing wavelength dependent variations of the field intensity at the antenna’s position. A simple 1D simulation of the SixNy/SiO2/Si stack, depicted in Figure 2a, shows that the field intensity at the antenna’s position is highly wavelength dependent, as shown in panel b. Within the 300nm spectral window, the field reaches four maxima due to constructive interference and three minima due to destructive interference. These field variations at the antenna’s position are apparent in the antenna’s in-coupling efficiency. The in-coupling spectra of the FDTD simulation shown in Figure 1, with the addition of the Si-substrate, are shown in panel c of Figure 2. The stack’s standing wave response dominates the in-coupling transmission spectra through the Ml (red) and Mr (blue) monitors. The same resonances as seen in panel b are clearly present. These resonances hide the broader quadrupolar resonance of the antenna, which is expected around 780nm based on Figure 1c. Nevertheless, when comparing the power transmitted through the Ml and Mr monitors, the directivity peak is still clearly visible around 790nm (Figure 2c black curve, right axis).


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Figure 3: (a) SEM image of the golden V-antenna on the SixNy waveguide. (b) Schematic overview of the experimental waveguide layout, the green line indicates the dicing line. (c) Schematic overview of the experimental setup. (d) Image of the sample taken with microscope 1. (e) Image taken with microscope 2 showing light being emitted from the two ends of one waveguide. The dotted line indicates the sample surface. (f) Spectrally resolved intensity 7 ACS Paragon Plus Environment

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measured for the left (red) and right (blue) light spot in panel e. The ratio of the spots intensities, i.e. directivity, in dB is shown on the right axis (black). The four black arrows indicate the spectral positions of the intensity dips according to simulations.

An SEM image of a V-shaped nanoantenna fabricated on a SixNy waveguide is shown in Figure 3a. The waveguides on which the antennas are positioned are made in large loops spanning 5mm in length (panel b). Several of these loops are patterned one in another, as shown in panel b, in order to have as many loops and antennas on one sample. Once the full fabrication process was finished the samples were diced in the middle of the loops (green line in panel b). At the open ends of the waveguide, the light emitted in both directions of the waveguide can now be measured. By using these terminated waveguides rather than out-coupling gratings, a much larger spectral window can be efficiently inspected.

The experimental set-up used to evaluate the antenna’s in-coupling spectra is depicted in Figure 3c. It consists of two microscopes: one facing the top side of the sample (Microscope 1), and one facing the diced side of the sample (Microscope 2). The first microscope is used to focus light on the antenna. A bright field image taken with this microscope is shown in Figure 3 d. Five waveguide loops can be seen, with a golden triangle that indicates the middle of the waveguide where the golden antenna is located. Through this microscope, the spectrally filtered light of a supercontinuum laser with a polarization perpendicular to the waveguide is focused to a 4-5µm spot that is positioned on the antenna. An acousto-optical tunable filter (AOTF) is used to filter out a 5nm wide spectral band that can be swept between 600 and 900nm. The second microscope, which images the diced side of the sample, allows us to evaluate the light coupled in the two different directions of the waveguide. An image taken with this microscope is shown in


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panel e. Two spots that correspond to the two ends of one waveguide are observed. The intensity of these two spots over the 600 to 900nm spectral window is plotted on the left axis in panel f. The intensity in the direction of the antenna’s tip is shown in red, the intensity in the opposite direction is shown in blue. Similar to the simulations in Figure 2, four peaks are seen in the intensity spectrum, which correspond to the material stack’s resonances. The quadrupole resonance of the V-antenna with a 190nm leg length is expected to be located around 750nm, in between low in-coupling dips caused by the stack’s destructive interference at 710nm and 800nm (indicated by vertical black arrows). At this wavelength the power in the direction of the Vantenna’s opening reaches a minimum, while the power in the tip’s direction reaches a maximum. The right axis of Figure 3f shows the directivity based on these measurements. Where the in-coupling spectrum reaches its maximum, the directivity peaks at 23dB. Small variations in the weak in-coupling signals detected at the stack’s resonance nodes get amplified in the calculated directivity and consequently result in artifacts in the directivity spectrum. These artifacts can be observed in Fig 3f to coincide clearly with the simulated positions of these nodes (indicated by the black arrows).


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Figure 4: Experimental directivity spectra of the antenna-to-waveguide in-coupling for: (a) antennas with a leg length of L = 190nm and varying angle α, (b) antennas with an angle of α = 120° and varying leg length. In all graphs, dashed lines indicate the spectral windows in which


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the material stack induces a destructive interference.

The antenna’s geometry, defined by the angle and leg length, strongly influences the plasmonic resonances of the antenna, and as such also its directivity. In order to experimentally evaluate the influence of these geometric parameters, a series of V-antennas with varying angles α and leg lengths L were fabricated on the waveguides. The top graph in Figure 4a shows the experimental directivity spectra of an antenna with a leg length of 190nm and an angle of 180°, i.e. a nanobar of 380nm. Since this represents a symmetric antenna geometry, the light couples equally in both directions of the waveguide. The directivity is therefore zero. Some small variation can nevertheless be seen in the regions where the material stack results in a destructive interference pattern at the antenna’s position. When the antenna is bend, a clear peak appears in the directivity spectra of the antenna around 700nm. For α = 150° already a maximum of 13dB is reached. As the angle decreases further, the directivity increases as well, reaching a maximum of 25dB for a α = 120°. For antennas with an even smaller angle the peak starts to decrease again. This optimal angle, 120°, was kept constant for antennas with varying leg length. The directivity for these antennas can be seen in panel b. Since the quadrupole resonances lie at the origin of the directional scattering behavior and since this resonance red-shifts as the length of the antenna is increased, the directivity peak red-shifts likewise. As the leg length increases from 145nm to 265nm, the directivity peak shifts from λ = 652nm to λ = 830nm. Within this interval, the directivity peak coincides with spectral regions where the material stack interferes constructively and destructively at the antenna position. The measured directivity is directly affected by the spectral interference as when the directivity peak coincides with a constructive spectral region, the directivity peak can be clearly observed, while when it coincides with a destructive spectral region, a dip in the directivity peak is observed. This is very pronounced for the antennas with a


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205 and 250nm leg length, where the dips at 725nm and 800nm split the antenna’s directivity peak. The red-shifting antenna’s directivity peak is nevertheless clearly visible in the regions with constructive interference.

The plasmonic/photonic hybrid system presented in this paper shows how the directive scattering properties of a plasmonic V-antenna can be implemented to make highly directional waveguide couplers. It demonstrates how the free-space scattering properties of this single element antenna can be transferred to a photonic circuit. Similarly as in the case of the free space antenna, the V-antenna reaches its maximum directionality when the angle is 120°. In addition, we have shown how the underlying dielectric material stack has a large influence on the  incoupling. The destructive interference of the dielectric stack reduces the in-coupling efficiency. However, in the optimum situation where the dielectric stack interferes constructively, directivity up to 25dB can be reached. The high directionality peak at visible frequencies makes the Vantenna an attractive basic component for on-chip photonics, particularly for compact optical sensors. Future work will therefore focus on improving the in-coupling efficiency of such hybrid devices, and investigate how these devices can be implemented for bio-sensing.

METHODS Fabrication The layers needed for the waveguide were evaporated on a bare Si carrier wafer, starting with 2µm of SiO2 deposited using high-density plasma (HDP) chemical vapor deposition (CVD), followed by 180nm of SixNy with a plasma-enhanced CVD process [22]. Subsequently, a 2nm of adhesion layer of Ti and 50 nm of Au was sputtered. The antennas were fabricated from the gold layer using e-beam lithography with negative-tone hydrogen silsesquioxane (HSQ) resist. After


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illumination and development of the resist, unpatterned gold was removed using Xe ion milling. Once the antennas are defined, the waveguides were processed with a second e-beam step. Negative-tone MA-N2403 resist was used for this lithography step and a SF6/C4F8 etch is used to etch the SixNy after development. The remaining MA-N2403 resist was finally removed with O2 plasma. ACKNOWLEDGMENT D.V. acknowledges the financial support for the F.W.O. [Pegasus]2 Marie Skłodowska-Curie Fellowschip. N.V. acknowledges the F.W.O. (Flanders) for financial support. The authors also thank Josine Loo for e-beam assistance. This work has received funding from the FWO and European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska –Curie grant agreement No665501.

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